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GB2521619A - An apparatus and associated methods for flexible carrier substrates - Google Patents

An apparatus and associated methods for flexible carrier substrates Download PDF

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Publication number
GB2521619A
GB2521619A GB1322904.2A GB201322904A GB2521619A GB 2521619 A GB2521619 A GB 2521619A GB 201322904 A GB201322904 A GB 201322904A GB 2521619 A GB2521619 A GB 2521619A
Authority
GB
United Kingdom
Prior art keywords
substrate
component
curved
interconnection
carrier substrate
Prior art date
Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
Withdrawn
Application number
GB1322904.2A
Other versions
GB201322904D0 (en
Inventor
Mark Lee Allen
Darryl Paul James Cotton
Richard White
Surama Malik
Current Assignee (The listed assignees may be inaccurate. Google has not performed a legal analysis and makes no representation or warranty as to the accuracy of the list.)
Nokia Technologies Oy
Original Assignee
Nokia Technologies Oy
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Filing date
Publication date
Application filed by Nokia Technologies Oy filed Critical Nokia Technologies Oy
Priority to GB1322904.2A priority Critical patent/GB2521619A/en
Publication of GB201322904D0 publication Critical patent/GB201322904D0/en
Publication of GB2521619A publication Critical patent/GB2521619A/en
Withdrawn legal-status Critical Current

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    • H—ELECTRICITY
    • H05—ELECTRIC TECHNIQUES NOT OTHERWISE PROVIDED FOR
    • H05K—PRINTED CIRCUITS; CASINGS OR CONSTRUCTIONAL DETAILS OF ELECTRIC APPARATUS; MANUFACTURE OF ASSEMBLAGES OF ELECTRICAL COMPONENTS
    • H05K3/00—Apparatus or processes for manufacturing printed circuits
    • H05K3/30—Assembling printed circuits with electric components, e.g. with resistor
    • H05K3/32—Assembling printed circuits with electric components, e.g. with resistor electrically connecting electric components or wires to printed circuits
    • H05K3/325—Assembling printed circuits with electric components, e.g. with resistor electrically connecting electric components or wires to printed circuits by abutting or pinching, i.e. without alloying process; mechanical auxiliary parts therefor
    • H05K3/326—Assembling printed circuits with electric components, e.g. with resistor electrically connecting electric components or wires to printed circuits by abutting or pinching, i.e. without alloying process; mechanical auxiliary parts therefor the printed circuit having integral resilient or deformable parts, e.g. tabs or parts of flexible circuits
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    • H01L21/02—Manufacture or treatment of semiconductor devices or of parts thereof
    • H01L21/04—Manufacture or treatment of semiconductor devices or of parts thereof the devices having potential barriers, e.g. a PN junction, depletion layer or carrier concentration layer
    • H01L21/48—Manufacture or treatment of parts, e.g. containers, prior to assembly of the devices, using processes not provided for in a single one of the groups H01L21/18 - H01L21/326 or H10D48/04 - H10D48/07
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    • H01L23/498—Leads, i.e. metallisations or lead-frames on insulating substrates, e.g. chip carriers
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    • H05K1/02—Details
    • H05K1/0277—Bendability or stretchability details
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    • H05K2201/10007—Types of components
    • H05K2201/10151—Sensor

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  • Engineering & Computer Science (AREA)
  • Microelectronics & Electronic Packaging (AREA)
  • Computer Hardware Design (AREA)
  • Power Engineering (AREA)
  • Physics & Mathematics (AREA)
  • Condensed Matter Physics & Semiconductors (AREA)
  • General Physics & Mathematics (AREA)
  • Manufacturing & Machinery (AREA)
  • Metallurgy (AREA)
  • Ceramic Engineering (AREA)
  • Structure Of Printed Boards (AREA)
  • Electric Connection Of Electric Components To Printed Circuits (AREA)

Abstract

An apparatus comprising: a flexible carrier substrate 102 comprising a substrate contact pad 104, 106 configured to allow electrical connection of a carried electronic component to one or more other carried electronic components 112; an electronic component 112 carried on the flexible carrier substrate, the electronic component comprising a component contact pad 114, 116 configured to allow electrical connection to the electronic component; and a curved interconnection 126 electrically interconnecting the respective substrate and component contact pads, wherein the curved interconnection is configured such that its curvature allows the interconnection to maintain its connection to the respective contact pads with operational flexing of the flexible carrier substrate. The curved interconnection removes the stress caused by flexing of the substrate.

Description

An apparatus and associated methods for flexible carrier substrates
Technical Field
The present disclosure relates to the field of electronic circuits, associated methods and apparatus, and in particular concerns a flexible substrate for use in flexible electronic applications. Certain disclosed example aspects/embodiments relate to portable electronic devices, in particular, so-called hand-portable electronic devices which may be hand-held in use (although they may be placed in a cradle in use). Such hand-portable electronic devices include so-called Personal Digital Assistants (PDA5) and tablet PCs.
Certain disclosed examples may find applications in packaging, wearable devices/sensors and biosensors.
The portable electronic devices/apparatus according to one or more disclosed example aspects/embodiments may provide one or more audio/text/video communication functions (e.g. tele-communication, video-communication, and/or text transmission, Short Message Service (SMS)/ Multimedia Message Service (MMS)/emailing functions, interactive/non-interactive viewing functions (e.g. web-browsing, navigation, N/program viewing functions), music recording/playing functions (e.g. MP3 or other format and/or (FM/AM) radio broadcast recording/playing), downloading/sending of data functions, image capture function (e.g. using a (e.g. in-built) digital camera), and gaming functions.
Background
It is possible to combine electronic components and flexible substrates to form flexible (including stretchable) electronic apparatus and devices: Different techniques may be used in creating such flexible apparatus/devices, such as roll-to-roll mass-printing of components, and direct-write printing methods. The development of flexible electronics remains limited by the relative inability of materials to withstand mechanical deformation.
In particular, electronic components such as conductors are brittle, fragile, and tend to crack when mechanically deformed.
The listing or discussion of a prior-published document or any background in this specification should not necessarily be taken as an acknowledgement that the document
I
or background is part of the state of the art or is common general knowledge. One or more aspects/embodiments of the present disclosure may or may not address one or
more of the background issues.
Summary
According to a first aspect, there is provided an apparatus comprising: a flexible carrier substrate comprising a substrate contact pad configured to allow electrical connection of a carried electronic component to one or more other carried electronic components; an electronic component carried on the flexible carrier substrate, the electronic component comprising a component contact pad configured to allow electrical connection to the electronic component; and a curved interconnection electrically interconnecting the respective substrate and component contact pads, wherein the curved interconnection is configured such that its curvature allows the interconnection to maintain its connection to the respective contact pads with operational flexing of the flexible carrier substrate, The term "flexible" is used to refer to material which may be bent, stretched, compressed, or otherwise strained. Thus the flexible carrier substrate may be a bendable substrate, a stretchable substrate, and/or a compressible substrate. Accordingly the term "flexed" may be considered to relate to a strairiable substrate and includes one or more of bending, stretching, and compressing.
The term curved" is used to describe an interconnection (or other object such as a supporting medium profile) which is not straight. Thus "curved" may by understood to mean non-straight, nonlinear, curvilinear, arced, bowed, buckled, humped, serpentine and/or arciform, for example. A curved interconnection may have one or more bends or folds, such that it may be C" shaped, "U" shaped, "V' shaped, "S' shaped, or have more than one or two bends/curves, for example. The curved interconnection may allow for flexing of the flexible carrier substrate such that the shape of the curved interconnection may freely change and accommodate for the change in the relative locations of the contact pad and substrate pad to which the curved interconnection is connected without the interconnection breaking.
Advantageously, the curved interconnection may be formed such that it can maintain the required connection between a component contact pad and a substrate contact pad when the flexible substrate is deformed (e.g., stretched or flexed). If the substrate is flexed the curved interconnection may change its curvature while maintaining a bridging electronic link between the component and substrate contact pads. It may advantageously be possible to design the particular curvature and other properties of the curved interconnection for use in interconnecting particular components and substrates for particular operational use cases.
The curvature of the curved interconnection may be configured to allow for one or more types of flexing of the flexible carrier substrate. Flexing may be considered in some examples to involve bending of the substrate to a given bend radius. Depending on which direction the substrate is bent and on the mechanical properties of the substrate, the interconnection can be on a substrate surface which undergoes tensile strain or on a substrate side which undergoes compressive strain. Flexing may also be considered in some examples to involve stretching. Stretching may be considered to involve uniaxial stretching, biaxial stretching or radial stretching of the substrate. In all of these cases, the interconnection may accommodate the strain and maintain a percolation path I conductive connection path between the component and the substrate contact pads by changing its curvature accordingly.
The apparatus may comprise a low-modulus adhesive between the component and the flexible carrier substrate, the low-modulus adhesive configured to join the component to the substrate and substantially inhibit stresses in the component caused by flexing of the flexible carrier substrate. Thus the low-modulus adhesive may be located between the component and the flexible carrier substrate and configured to substantially mechanically decouple the component* from the flexible carrier substrate. Advantageously, the adhesive may substantially inhibit stress or strain of the flexed substrate passing on to the overlying component, which may in some examples be relatively brittle compared with the substrate. The adhesive may be considered to act as a stress/strain absorbing "cushion", adhering the component to the substrate and reducing forces which may be applied to the component due to flexing of the underlying substrate. The low modulus adhesive may comprise a urethane or silicone based elastomeric adhesive, for example The component may comprise: a rigid packaged electronic component such as a surface-mount (SMD) component (e.g., resistor, capacitor, inductor, diode, transistor, operational amplifier, light-emitting diode, sensor etc.); a rigid component such as a bare silicon-based component or a microchip; a flexible component (e.g., flexible light-emitting diode, or flexible sensor); a packaged component; or a combination of one or more rigid or thin-film based electronic components mounted on a flexible module (smaller than the flexible carrier substrate) which together make up the component with component contact pads.
The flexible component may be formed by: roll-to-roll printing (for example, flexographic printing, gravure printing, and rotary screen printing), sheet-fed printing (for example, screen and stencil printing), direct-write printing (for example, inkjet printing, aerosol jet printing, and dispensing), wet-coating (for example, spin-coating, bar-coating, and blade-coating), vacuum-deposition (for example evaporation, sputtering, chemical vapour deposition). In other examples a component (flexible or rigid) may be formed by transferring a pre-formed component.
The curved interconnectioh may be formed by: roll-to-roll printing (for example, flexographic printing, gravure printing, and rotary screen printing), sheet-fed printing (for example, screen and stencil printing), direct-write printing (for example, inkjet printing, aerosol jet printing, and dispensing), vacuum-deposition (for example, evaporation and sputtering, and chemical vapour deposition, in some examples through a mask), or transferring a pre-formed/pre-moulded curved interconnection.
The curved interconnection may be unsupported between the substrate contact pad and the component contact pad. The curved interconnection may be supported on a supporting structure/medium located between the substrate and component contact pads. Advantageously, in the former case, it may be possible to form a so-called "air-suspended" interconnection which is not supported by a supporting medium in some examples, and it may be, in the latter case, possible to form a supported curved interconnection in other examples.
The curved interconnection may be supported on a supporting structure/medium, and the supporting structure/medium may comprise one or more of a supporting layer configured to form a bridge between the flexible carrier substrate and component contact pads, and a supporting structure/medium configured to fill a space between lateral edges of the substrate and component contact pads. Such a supporting layer may have a thickness of between 30 nm and 50 pm.
The supporting medium may be configured for removal after deposition of the curved interconnection thereupon by one or more of: sublimation, dissolution and etching away.
The supporting medium may comprise trimethylolethane.
The supporting medium may comprise a low elastic modulus material configured to support the curved interconnection during operational flexing of the flexible carrier substrate. Examples of suitable materials include polyurethane, low density polyethylene, polyethylene terephthalate, polyethylene naphthalate, polyimide, polycarbonate, polyaramid, polydimethylsiloxane, acrylic polymers, polyurethane and hydrogels. Such materials may be considered to be reversibly deformable polymeric materials. The Young's modulus of such materials is typically in the range of 1 to 100 MPa. The Young's modulus of hydrogels may be in the range of 0.01 kPa to 10 kPa. The Young's modulus of other materials given above may be in the range of 50 kPa to 100 MPa. The supporting medium may be classed as linear elastic, viscoelastic, or hyperelastic materials.
The flexible carrier substrate may be one of: a polymer film (for example, polyethylene terephthalate, polyethylene naphtbalate, polyimide, polycarbonate, polyethersulfone, polysulfone, polyether etherketone, polyphenylene ether, polyethylene, polypropylene, poly(niethylmethacrylate), a metal foil (for example, thin stainless steel, aluminium foil), a flexible printed circuit (FPC) (for example, polyimide laminate with copper wiring); a flexible printed wiring board (PWB) laminate; a woven or wearable fabric (for example, fabric woven from non-conducting or conducting yarn), an elastomer (for example, polydimethylsiloxane (PDMS), polyurethane, polyvinylchloride, chloroprene rubber, or nitrile rubber), paper, or a stack comprising two or more of these materials laminated into one flexible carrier substrate. Elastomer-based flexible carrier substrates may be particularly suitable for stretchable electronic applications.
The curved interconnection may have one or more of: a substantially symmetrical profile; an asymmetrical profile; a substantially sinusoidal profile; an arc shaped profile; a convex profile which curves away from the flexible carrier substrate above the surface level of the substrate contact pad; a concave profile which curves towards from the flexible carrier substrate below the surface level of the substrate contact pad; and a profile exhibiting a combination of two or more of such above mentioned curved features.
Advantageously, the curved interconnection may be formed with a particular profile depending on the particular arrangement of component and substrate, properties of the substrate and component, and/or an expected operational use of the apparatus.
The component contact pad may be on substantially the same level as the substrate contact pad. The component contact pad may be on substantially a different level as the substrate contact pad.
The curved interconnection may comprise a conducting medium which may be a conductive ink. The conducting medium/curved interconnection may comprise silver, gold or copper, another metal or another conducting material, or a continuous interconnected array of carbon nanotubes, graphene flakes, or silver nanowires, for example. The curved interconnection may have a thickness between 1 nm and 50 pm.
The component may be substantially located in a recess of the flexible carrier substrate.
The apparatus may comprise a gap between the edge of the component and the substrate, wherein the curved interconnection bridges the gap between the substrate and component contact pads. Advantageously, such a gap may be designed according to the particular properties of the component, substrate, and/or curved interconnection to allow the curved interconnection to maintain an electrical connection between component and substrate contact pads during operational flexing of the flexible carrier substrate/ apparatus.
The apparatus may comprise a lamination layer or encapsulation material overlaying at least the curved interconnection configured to protect at least the underlying curved interconnection.
According to a further aspect, there is provided a method comprising: for an electronic component carried on a flexible carrier substrate; the electronic component comprising a component contact pad configured to allow electrical connection to the electronic component and the flexible carrier substrate comprising a substrate contact pad configured to allow electrical connection of a carried electronic component to one or more other carried electronic components; forming a curved interconnection to electrically interconnect the respective substrate and component contact pads, wherein the curved interconnection is configured such that its curvature allows the interconnection to maintain its connection to the respective contact pads with operational flexing of the flexible carrier substrate.
The method may comprise: depositing a supporting medium between the component and substrate contact pads such that the surface of the supporting medium forms a curved shape; and depositing a conducting medium on the supporting medium such that the conducting medium forms the curved interconnection. Depositing the supporting medium may comprise depositing one or more of: trimethylolethane, polyurethane, low density polyethylene, and polydimethylsiloxane.
The method may further comprise removing the supporting medium after deposition of the conducting medium such that the curved interconnection is an unsupported curved interconnection. Removal of the supporting medium may be done by sublimation, dissolution, and/or etching away of the supporting medium.
The supporting medium may be removed by sublimation, by: curing the conduction medium at a temperature below the sublimation temperature of the supporting medium; followed by curing the conduction medium at a temperature above the sublimation temperature of the supporting medium. Curing below the sublimation temperature hardens the overlying conduction medium to form a curved interconnection, while curing above the sublimation temperature removes the underlying supporting material by sublimation.
Depositing the supporting medium may comprise depositing a low elastic modulus material configured to support the curved interconnection during operational flexing of the flexible carrier substrate. Such a supporting medium may remain a feature of the apparatus (that is, it is not removed). The elastic modulus of the supporting medium may be considered low" in the sense that its elastic modulus is in the range of ito 100 MPa whereas that of the curved interconnect is above 1 GPa. The Young's modulus of hydrogel-based low elastic modulus may be in the range of 0.01 kPa to 10 kPa. The Young's modulus of other low elastic modulus materials may be in the range of 50 kPa to MPa.
The method may comprise using a low-modulus adhesive to secure/join the component on the flexible carrier substrate prior to the deposition of the supporting medium and the conducting medium. Such an adhesive may act to secure the component in a desired position and may act as a strain/stress cushion" or layer between a flexible substrate and a potentially less flexible component.
Forming the curved interconnection may comprise: fabricating the curved interconnection using a mould, and transferring the moulded curved interconnection for attaching to the component and substrate contact pads by one or more of: stamping, embossing and transfer printing. In this way the curved interconnection may be fabricated away from the target flexible carrier substrate (using a mould, for example on a different dummy substrate) and transferring the ready-formed curved interconnection to the target substrate for connection by releasing the curved interconnection from the mound/stamp and attaching to the contact pads.
According to a further aspect, there is provided a flexible electronic device comprising an apparatus according to any claimed apparatus or formed using a method according to any claimed method.
According to a further aspect, there is provided a computer program comprising computer code configured to perform any claimed method.
The apparatus may be one or more of an electronic device, a portable electronic device, a telecommunications device, a portable telecommunications device and a module for any of the aforementioned devices.
The steps of any method disclosed herein do not have to be performed in the exact order disclosed, unless explicitly stated or understood by the skilled person.
Corresponding computer programs (which may or may not be recorded on a carrier) for implementing one or more of the methods disclosed herein are also within the present disclosure and encompassed by one or more of the described example embodiments.
The present disclosure includes one or more corresponding aspects, example embodiments or features in isolation or in various combinations whether or not specifically stated (including claimed) in that combination or in isolation. Corresponding means for performing one or more of the discussed functions are also within the present
disclosure.
The above summary is intended to be merely exemplary and non-limiting.
Brief Description of the Figures
A description is now given, by way of example only, with reference to the accompanying drawings, in which:-Figures la-le show example steps to form an apparatus according to the present
disclosure (in cross section);
Figures 2a-2c show example curved interconnections of different shapes according to
the present disclosure (in cross section);
Figures 3a-Se show example steps to form an apparatus according to the present
disclosure (in cross section):
Figures 4a-4c show example curved interconnections of different shapes according to
the present disclosure (in cross section);
Figures Sa-5c show a supporting structure/medium, a supporting layer, and a lamination
layer according to the present disclosure;
Figures 6a-6b show example schematic apparatus comprising interconnected components on a substrate, according to the present disclosure; Figures 7a-7d show photographs of experimental steps to interconnect components on a
substrate according to the present disclosure;
Figure 8 shows a microscope image of an experimental curved interconnection
according to the present disclosure;
Figure 9 shows an example of a flexible electronic device according to the present
disclosure;
Figures ba-lOb show example methods according to the present disclosure; and Figure 11 shows a computer-readable medium comprising a computer program configured to perform a method or methods according to the present disclosure.
Description of Specific Aspects/Embodiments
This disclosure is generally directed to flexible electronics and related manufacturing S methods. Flexible electronics may be fabricated using different methods.
As one example, roll-to-roll mass-printing techniques can be cost-effective for producing simple electronic components in huge numbers. In order to achieve good performance and high yield, system parameters such as substrate surface treatment, print speed, tension of the printing web, curing time and temperature, ambient temperature and humidity all need to be highly optimized for the particular component being printed. In practice, roll-to-roll printing of complete circuits can be an extremely challenging task in terms of combining and fulfilling all the required process parameters on the same printing system.
As another example, direct-write printing methods (e.g. inkjet printing) may be attractive for producing customized circuits and for electronics prototyping. Techniques used for attaching surface-mount components to flexible circuitry include wire bonding, flip-chip bonding, and direct printing of metal ink to form a conductive bridge from the circuitry on the substrate to the pads on the component.
A "modular approach" may be taken, where various mass-manufactured flexible or rigid components are populated and joined up on a "mbther flex" carrier substrate, which in turn can be highly customized. The carrier substrate may be, for example, a polymer foil or a stretchable substrate. This approach may be considered to combine benefits of roll-to-roll mass-manufacturing and the flexibility offered by direct-write printing methods. The "mother flex" used in such an approach may itself be considered to be a modular part in a final product built via, for example, 3D-printing or injection moulding.
Fabricating durable interconnections between components and the carrier substrate is generally recognized to be a technical challenge. It is typically the case that the mechanical properties of the component are different from that of the carrier substrate.
This is the case, for example, with conventional packaged discrete components, and with thinned down bare silicon die. The problem is relevant even in the case that the component is built on a flexible film. Interconnections in such systems may experience significant stress concentrations when the substrate is bent andlor stretched. Repeated bending/stretching can leadto device failure using current interconnect technologies due to the interconnections breaking, for example. The mechanical properties of conventionally packaged components and bare silicon die also mean they cannot S withstand large bending strains without suffering catastrophic brittle fracture.
Flexible printed circuits (FPC) exist in a multitude of existing devices. The standard solution for joining components on a carrier substrate is flip-chip bonding (robotic pick-and-place, where an anisotropic conducting adhesive or isotropic conductive adhesive is used for forming the electrical interconnect). Anisotropic conducting film (ACF) bonding can also be used for interconnecting between flexible circuits.
Methods for packaging ultra-thin chips in flexible polyimide packages may be used in flexible electronic circuits. The resulting components are very thin, flexible and have a copper fan-out structure and exposed electrodes for attachment to a substrate/flex.
Another approach for constructing an electronic system-in-package is to embed chips in a substrate and use inkjet printing to print conductors between the chip pads, as well as dielectric layers, to form a complex integrated module.
Regarding stretchable electronics, one sought after approach is to use rigid islands interconnected with stretchable wiring.
Another approach for constructing flexible electronic circuits is to use microfluidic electronics, by stencil printing a liquid alloy, such as Galinstan, onto a semi-cured polydimethylsiloxane (PDMS) substrate followed by assembly of rigid active components in contact with the liquid alloy by pick-and-place, and finally encapsulation by pouring uncured POMS on top and curing. It may be possible to obtain channel widths and spacings of 200 pm and 100 pm respectively.
It is possible to make air-suspended interconnections using a direct-write method by using a concentrated silver nanoparticle paste with a viscosity in the range of 10-100 Pa-s, which is extruded through a micronozzle enabling spanning arcs to be printed.
There remains a need in the art for further developments and improvements to form durable and tunable electrical interconnections between components for flexible electronic applications.
Examples of the present disclosure may be considered to provide improvements to the fabrication of flexible electronic apparatus and methods for forming flexible electronic apparatus.
Throughout this description, the term "component" is used to refer to a rigid component, such as a microchip or surface-mount component, a packaged component, or another flex/substrate (smaller than the "mother flex"/main flexible carrier substrate) populated with one or several rigid and/or thin-film-based components.
Figures la-le show example steps for fabricating a flexible electronic apparatus comprising air-suspended curved interconnections. Figure la shows a flexible carrier substrate 102 with two conducting substrate contact pads 104, 106. The flexible carrier substrate 102 may be a flexible printed circuit, a flexible printed wiring board, a flexible printed circuit board, a woven or wearable fabric, or paper-based, for example. Examples of materials used to form the flexible carrier substrate include polyethylene terephthalate, polyethylene naphthalate, polyimide, polycarbonate, polyethersulfone, polysulfone, polyether etherketone, polyphenylene ether, polyethylene, polypropylene, poly(methylmethacrylate), flexible printed circuit board (copper on polyimide laminate), printed wiring board, thin stainless steel, aluminium foil, fabric woven from non-conducting or conducting yarn, or a stack comprising two or more of these materials laminated into one flexible substrate. The thickness of such flexible substrate may be from 0.01 mm to 5 mm. The degree of operational flexing of the flexible substrate may be, at a maximum, to a bend radius equivalent to twice the thickness of the flexible substrate.
Examples of materials used to form a stretchable carrier substrate include elastomers such as: polydimethylsiloxane (PDMS), polyurethane, polyvinylchloride, chloroprene rubber, and nitrile rubber. The degree of operational stretching of the stretchable substrate may be, at a maximum, 600% tensile strain, but more practically may be up to 20% tensile strain, and may be, at a maximum, 600% compressive strain, but more practically may be up to 20% compressive strain.
In this example a low elastic modulus adhesive 110 is deposited in a recess 108 in the substrate 102. The flexible, low modulus adhesive 110 is configured to inhibit strain in a component positioned on top of the adhesive 110 component caused by flexing of the underlying flexible carrier substrate 102. Accordingly, adhesion to the substrate 102 using a flex-absorbing adhesive 110 acts to mechanically decouple the bending or stretching substrate 102 from the component, helping to avoid catastrophic brittle fracture of the component and allowing for thicker flexible devices to be used. Examples of low-modulus adhesive include urethane or silicone based elastomeric adhesive, for example, which have Young's modulus in the range of ito 100 MPa.
In figure lb, a component 112 with two component contact pads 114, 116 facing away from the substrate 102 surface is substantially located in the recess 108 of the flexible carrier substrate 102, on top of the low modulus, flex-absorbing adhesive 110. The adhesive 110 attaches the component 112 to the substrate 102. The component 112 may be rigid or flexible. Two gaps 118, 120 are located at either side/edge of the component 112 in the recess 108.
Figure ic shows that supporting structures/media 122, 124 are deposited across the gaps 118, 120 in the carrier substrate 102 at either side of the component 112, between corresponding component contact pads 114, 116 and substrate contact pads 104, 106.
The supporting structures/media 122. 124 each have a protruding convex surface profile extending away from the surface of the substrate and component contact pads 104, 106, 114,116.
Figure id shows that a conductor layer 126, 128 is deposited across each supporting structure 122, 124 and over the corresponding substrate and component contact pads 104, 106, 114, 116. The conductor layers 126, 128 adopt substantially the same shape as the surfaces of the supporting structures/media 122, 124. Once hardened/cured, they form the curved interconnections 126. 128 between the corresponding component and substrate contact pads 104, 106, 114, 116. The curved interconnections 126, 128 each bridge the corresponding gaps 118, 120 between the edges of the substrate and component contact pads 104, 116, 114, 116.
In figure le the supporting structures 122, 124 are removed. The apparatus 100 is heated to cause the supporting structures 122, 124 to sublime (through available apertures, not shown), leaving an empty space 130, 132 under each curved interconnection 126, 128. The empty spaces 130, 132 may be air gaps. The curved interconnections 126, 128 in this example may be considered to be air-suspended interconnections, and they are unsupported between the substrate and component contact pads 104, 106, 114, 116. The material used to form the supporting structures may be trimethylolethane (TME), which starts to sublime at approximately 100 °C and sublimes rapidly at 150°C and thus may be removed by sublimation. In other example.
the material used to form the supporting structures may be removed by dissolution, or etching away, for example.
In other examples, if the supporting medium is not to be removed after deposition of the curved interconnections, then reversibly deformable polymeric materials such as polyurethane, low density polyethylene, and polydimethylsiloxane may be used to form the supporting structure.
While a range of different materials such as those noted above could be used to form the supporting structures, the specific materials used will depend on whether or not the supporting structures are to be removed after deposition of the curved interconnections, the expected level of operational flexing/ deformation required if the supporting structures are not removed, the process used to form the supporting structures, and/or the process used to remove the supporting structures.
The resulting apparatus of figure le comprises a flexible carrier substrate 102 comprising a substrate contact pad 104, 106 configured to allow electrical connection of a carried electronic component 112 to one or more other carried electronic components; an electronic component 112 carried on the flexible carrier substrate 102, the electronic component 112 comprising a component contact pad 114, 116 configured to allow electrical connection to the electronic component 112; and a curved interconnection 126, 128 electrically interconnecting the respective substrate and component contact pads 104, 106, 114, 116, wherein the curved interconnection 126, 128 is configured such that its curvature allows the interconnection 126, 18 to maintain its connection to the respective contact pads 114, 116 with operational flexing of the flexible carrier substrate 102.
The curvature of the curved interconnections 126, 128 are configured to allow for flexing of the flexible carrier substrate 102 while maintaining the electrical connections between the component contact pads 114, 116 and the substrate contact pads 104, 106. Flexing the carrier substrate 102 can be considered to include the case where the substrate is bent around an axis perpendicular to the cross-section shown in Figure 1. Depending on the direction of bending, the gaps 118 and 120 will either increase or decrease under tensile or compressive strain, respectively. "Flexing" may also be considered to include stretching the carrier substrate 102 to cause tensile strain resulting in an increase in gap size 118, 120. Flexing" may also be considered to include compressing the carrier substrate, wherein compressing the substrate 102 causes compressive strain and results in a decrease in the gap size list 120.
In this example, the curved interconnections 126, 128 are each symmetrical because the contact surfaces of the substrate and contact pads 104, 106, 114, 116 to which each curved interconnection 126, 128 is connected are at substantially the same level. Each curved interconnection 126, 128 also has an arc-shaped profile formed by the deposited conductor layers (which form the curved interconnections 126, 128) substantially adopting the surface profile of the supporting medium 122, 124. In this example the curved interconnections 126, 128 each also have a convex profile which curves away from the flexible carrier substrate 102 above the surface level of the substrate contact pads 104, 106, due to the supporting structure having a protruding surface above the level of the substrate surface/substrate contact pad surface.
Figures 2a-2c each show differently shaped air-suspended curved interconnections which curve away from the flexible carrier substrate above the surface level of the substrate contact pad. Each figure shows a substrate 202a, 202b, 202c with a substrate contact pad 204a, 204b, 204c, a component 206a, 206b, 206c with a component contact pad 208a, 208b, 208c, adhesive 210a, 210b, 210c between the component and the substrate, and a curved interconnection 212a, 2i2b, 212c between the substrate and component contact pads. The thickness of the component 206a, 206b, 206c may be between 10 pm and 500 pm. The lateral spacing between the component contact pad 208a, 208b, 208c, and the substrate contact pad 204a, 204b, 204c may be assumed to be larger than the height of the component 206 206b, 206c plus the height of the adhesive 210a, 210b, 210c, denoted heomp in figure 2a.
In figure 2a, there is no recess in the substrate 202a in which to position the component 206a, and the component 206a rests on an adhesive layer 210a applied to the surface of the substrate 202a. The surface level of the component contact pad 208a is above the surface level of the substrate contact pad 204a as illustrated. Thus, the curved interconnection 212a protrudes out of the plane of the carrier substrate 202a, forming an asymmetric arc having a peak height ha,j above the surface level of the component contact pad 208a and a larger peak height h81,2 above the surface level of the substrate contact pad 204a.
In figure 2b, the component 206b is located within a recess of height hmce carved into the surface of the substrate 202b, such that the surface level of the component 206b is level with the surface level of the substrate 202b (that is, h,,,,, = hrecoss). The curved interconnection 212b forms a symmetric arc across the gap of width gap. The curved interconnection 212b has a peak height ham above the surface levels of the component contact pad 208a and substrate contact pad 204a. Figure 2c is similar to figure 2b except the curved interconnection 212c has a substantially sinusoidal profile shape.
In the case of having the component sitting in a recess as in figures 2b and 2c, the gap between the lateral edge of the component 206b, 206c and the lateral edge of the recess, gap may typically range from the order of 10 pm up to 500 pm. The peak height of the curved interconnect, ham, depends on the maximum required strain the curved interconnection is required to accommodate for, which in turn is a function of the degree of bending or stretching of the carrier substrate 202a, 202b, 202c, and may also be affected by the dimensions of gap.
Figures 3a-3e show other example steps for fabricating a flexible electronic apparatus comprising air-suspended curved interconnections 326, 328. In this example, the air-suspended curved interconnections 326, 328 protrude into grooves 318, 320 patterned into the flexible carrier substrate 302.
Figure 3a shows a flexible carrier substrate 302 with two conducting substrate contact pads 304, 306. The substrate 302 has a main recess 308 for a component to sit in, and two grooves 318, 320 located at either side of the main recess 308. The grooves 318, 320 may be patterned into the substrate 302, for example by hot-embossing or laser ablation.
In figure 3b, a component 312 with two component contact pads 314, 316 facing away from the substrate 302 surface is substantially located in the main recess 308 on top of a layer of low modulus flex-absorbing adhesive 310 as in figure lb. The two grooves 318, 320 remain exposed at either side of the component 312.
Figure 3c shows that supporting structures 322, 324 are formed by depositing supporting structure material (a supporting medium) into the grooves 318. 320 between corresponding component contact pads 314, 316 and substrate contact pads 304, 306.
The supporting structures/media 322, 324 line the walls and floor of the grooves 318, 320 and reach to the substrate and component contact pads 304, 306, 314, 316. The supporting structures/media 322, 324 each have a concave surface profile sunken in from the surface of the substrate and component contact pads 304, 306, 314, 316. The supporting structures/media 322, 324 may be formed, for example, by direct printing, or by capillary flow of a liquid supporting structure material, such as TME ink, in the grooves 318, 320. Exploiting the capillary flow of a liquid supporting structure material/medium in the grooves 318, 320 can provide an ideal cross-profile" by means of an optimal match between the surface energy of the liquid supporting structure material/medium and the surface tension of the substrate in the grooves 318, 320.
Figure 3d shows that a conductor layer 326, 328 is deposited across each supporting structure/medium 322, 324 and over the corresponding substrate and component contact pads 304, 306, 314, 316. The conductor layers 326, 328 adopt substantially the same shape as the surface of the supporting structures/media 322, 324 and form the curved interconnections between the corresponding component and substrate contact pads 304, 306, 314, 316 once hardenedfcured. The curved interconnections 326, 328 each bridge the corresponding gaps 318, 320 between the substrate and component contact pads 304, 306, 314, 316. In figure 3e the supporting structures 322, 324 are removed (through apertures, not shown) by sublimation as described in relation to figure le.
The conductor layer 326, 328 may be deposited as a thin film on top of the supporting structures/medium 322, 324, for example using sputtering, evaporation1 chemical vapour deposition, spin-coating, spray-coating, screen printing, inkjet printing, or aerosol deposition. In sortie instances it may be necessary to heat the conduction material after deposition to cure the material and form curved interconnections. The conductor material used to form the curved interconnections may comprise, for example, silver, gold, copper, or another metal or conductive material.
Figures 4a-4c each show differently shaped air-suspended curved interconnections which curve Into a groove towards the flexible carrier substrate below the surface level of the substrate contact pad (the curved interconnections may be considered to be "downwards-arcing"). Each figure shows a substrate 402a, 402b, 402c with a substrate contact pad 404a, 404b, 404c, a component 406a, 406b, 406c with a component contact pad 408a, 408b, 408c, adhesive 410a, 410b, 410c between the component and the substrate, and a curved interconnection 412a, 412b, 412c between the substrate and component contact pads within a groove 414a, 414b, 414c. The thickness of the component 406a, 406b, 406c may be between 10 pm and 500 pm. The height of the component 406a, 406b, 406c plus the height of the adhesive 410a, 410b, 410c is denoted hcomp.
In figure 4a, the groove 414a is formed by the edge of the component 406a and the wall of the substrate, and has a recess in which the component 406a is located of height hgmove. The surface level of the component contact pad 408a is the same as the surface level of the substrate contact pad 404a, as the height of the component plus underlying adhesive, hmp = hgroove. The curved interconnection 412a sinks into the groove 414a and forms a symmetrical curve having an arc depth ham below the surface level of the component contact pad 408a and substrate contact pad 404a (ham C h9roova).
In figure 4b, the component 406b is adhered to a base/platform 416b with a groove 414b located between the base 416b and the substrate contact pad 404b. The base/platform 416b has a height hb from the bottom of the groove 414b to the top of the base/platform 416b (i.e., bottom of the adhesive layer 410b). In this example hcomp c hgroove, such that, with the component 406b adhered to the base 416b, of height h0, the surface level of the component 406b is level with the surface level of the substrate 402b (and the surface level of the component contact pad 408a is level with the surface level of the substrate contact pad 404a). Thus, hgmov. = hbaso + hcomp. The curved interconnection 412b forms a symmetric curve with an arc depth ham below the surface level of the component contact pad 408a and substrate contact pad 404a (ham C hgmovo and ham> hcornp) in this example.
In figure 4c, the component 406c is adhered to a base/platform 416c with a groove 414c located between the base 416c and the substrate contact pad 404c. The base 416c has a height and the groove 414c has a depth hgmove. ht may or may not equal If iThasa = hgroovo, the base of the adhesive layer 41Cc is level with the surface of the substrate 402c underlying the substrate contact pad 404c and only the groove 414c (no recess) has been patterned into the surface of the substrate 402c. The surface of the component contact pad 408c is above the surface of the substrate contact pad 404c as illustrated. The curved interconnection 412c forms an asymmetric curve with a peak arc depth harc,i below the surface level of the component contact pad 408c and a smaller peak arc depth below the surface level of the substrate contact pad 404c.
As in relation to figures 2a-2c, the peak depth of the curved interconnect, ham, depends on the maximum required strain the curved interconnection is required to withstand, which in turn is a function of the degree of flexing or stretching of the carrier substrate 402a, 402b, 402c.
Figures 5a-5b show examples of a supporting element/medium remaining as part of the apparatus to support a curved interconnection. Figure Sc illustrates a lamination layer for protecting a curved interconnection. Each of figures 5a-5c show a substrate 502 with a recess housing a component 506 adhered to the substrate 502 with a low modulus flex-absorbing adhesive 510. A component contact pad 508 of the component 508 is shown connected by a curved interconnection 512 to a substrate contact pad 504 of the substrate 502.
In figure 58, the curved interconnection 512 is supported on a supporting structure/medium 514 located between the substrate and component contact pads 504, 508. The supporting structure/medium 514 fills the space between the lateral edges of the substrate and component contact pads 504, 508. Such a supporting structure/medium, which is not sacrificed after deposition of the curved interconnection 512 may be, for example, formed using a low modulus plastic or elastomeric material.
The supporting structure/medium 514 may comprise material having a Young's modulus which is at least two orders of magnitude lower than the Young's modulus of the overlying curved interconnection 512.
In figure 5b, the curved interconnection 512 is supported on a supporting structure/medium 516 which is a thin layer 516 bridging the substrate and component contact pads 504, 508 and underlying the curved interconnection 512. The supporting structure 516 may be a thin polymer layer which is printed down and cured prior to printing of the conductor as a curved interconnection 512. The supporting polymer layer 516 may, for example, be a thermally curable polymer such as polyimide or poly(methylmethacrylate) (PMMA). or a UV-curable polymer. A benefit of using a UV curable polymer is that it can be fully cured without unintentionally removing a sacrificial supporting structure. If using a thermally curable material to create a supporting layer/medium 516, the curing temperature must comply with other temperature restraints, such as the sublimation temperature of a sacrificial supporting material.
The thickness of the supporting layer/medium 516 may range from 3Onm to SOpm depending on the geometrical dimensions of the component 506, the size of the gap between the substrate and component contact pads 504, 508, and the targeted stretchability of the interconnection 512 which is further dependant on the Young's modulus of the supporting layer/medium 516 as well as the thickness and Young's modulus of the curved interconnection conductor 512. The supporting structure/medium 516 may comprise material having a Young's modulus which is at least two orders of magnitude lower than the Young's modulus of the overlying curved interconnection 512.
In figure Sc, a laminating layer 518 overlies the curved interconnection 512. In other examples the laminating layer 518 may overlie the curved interconnection 512 and overlie a portion of one or more of the substrate 502, substrate contact pad 504, component 506 and component contact pad 508. Such laminating layers may be suitable for protecting downwards-curving/sunken interconnections as shown. The laminating layer 518 may be considered to act as a protective film encapsulating the curved interconnections 512. As another example, an elastomer or other type of potting material may be cast/coated over the curved interconnectIon 512 as a laminating layer 518 to protect the interconnections. The modulus of the laminating layer 518 should be low enough to allow the curved interconnection 512 to move without causing large stress concentrations within the interconnection 512.
In the above examples, the supporting material/medium has been described as being deposited after positioning/adhering the component to the substrate. In other examples, the process steps may be carried out in a different order, such that the supporting material/medium is deposited onto the carrier substrate prior to placing the component.
When the component is placed on the carrier substrate, the component may "push out" the (e.g. sacrificial) supporting material from underneath the component, thereby forming a supporting structure/medium spanning a gap between the component and substrate contact pads.
In the above examples, curved interconnections may be formed, for example, by deposition of a conduction material via inkjet printing, screen printing, evaporation, sputtering, chemical vapour deposition, spin-coating, spray-coating, or by aerosol deposition. In some instances it may be necessary to heat the conduction material after deposition to cure the curved interconnections. The conductor material used to form the curved interconnections may comprise, for example, silver, gold, copper, or another metal or conductive material or formed from a contihuous interconnected array of carbon nanotubes, graphene flakes, or silver nanowires.
Curved interconnections described herein may each have a thickness between 1 nm and pm. The thickness of the curved interconnections may be tuned depending on factors including, for example, the geometrical dimensions of the component, the dimensions of the gaps and the targeted stretchability of the curved interconnection which is further dependent on the Young's modulus of the material used to form the curved interconnection.
In these examples, the conducting curved interconnections (which may be thin and/or air suspended or supported) provide mechanically robust stretchable interconnections between component and substrate contact pads. Gaps between the substrate contact pads and the component contact pads are allowed to extend and retract in response to mechanical deformation of the flexible carrier substrate, such as flexing and stretching.
Figures 6a and 6b provide a visualization of a "modular approach" to constructing a flexible electronic apparatus. The rectangles 602, 652, 654, 656, 658 represent various components, such as roll-to-roll mass-manufactured sensors, organic light emitting diodes (OLEDs), memory modules, silicon-based chips for computation, and other discrete components. The larger carrier substrate blocks 600, 650 have different shapes in figures 6a and 6b. In figure 6b, the lines 660, 662, 664, 666 represent circuit wiring on the surface of the carrier substrate 650, which connect to the components 652, 654, 656, 658 via air-suspended durable interconnections as described herein. Locations of the interconnections are marked as points 668, 670, 672, 674, 676, 678.
An experiment demonstrating an example of interconnection fabrication will now be discussed. Figures 7a-7d show microscope images at different stages in the production of a flexible electronic apparatus.
A mother flex laminate 700 with dummy components 702 inserted therein is shown in Figure 7a. Firstly, to prepare a flexible carrier substrate 700 (the "mother flex"), a film of polyethylene naphthalate (PEN) (Dupont Teijin PEN Q65FA) of 50 pm thickness was employed as a first substrate. A 3M Optically Clear Adhesives (OCA) 8211 film of 25 pm thickness was then laminated onto the first PEN substrate. A second PEN film (Dupont Teijin PEN Q65FA), also 50 pm thick, was mounted onto the cutting mat of a Silhouette electronic cutting tool. An array of rectangular windows holes of size 10 mm x 16 mm were cut out of the film. Thereafter, the second PEN flex with window holes was laminated onto the OCA adhesive side of the first flex. The resulting structure comprises two PEN flex films laminated together, where the upper flex has rectangular window holes cut out at which locations the adhesive OCA film remains exposed.
Next follows the placement of dummy components 702 onto the "mother flex" substrate 700. The cut out pieces (dummy flex components 702) of the second PEN flex were "pick-and-placed" into the window holes of the mother flex 700. The OCA film exposed at the window locations held the dummy components 702 in place. The resulting mother flex 700 populated with the dummy components 702 was passed through a laminator to obtain a smooth structure with a roughly 20-100 pm gap 704 between the mother flex 700 and each dummy component 702.
Figure 7b shows deposited supporting structures/media 706. As a sacrificial supporting material, trimethylolethane (TME) purchased as pure granules from GEO Specialty Chemicals was dissolved into a solvent consisting of hexylene glycol, ethanol and deionized water all at equal weight ratios (25 wt% of each component) and mixed for an hour at 50°C on a stirrer hotplate. The resulting ink was deposited over the gap between the mother flex and the component by pipetting. The IME structure 706 was solidified by drying on a hotplate at 100°C for approximately five minutes. The resulting TME structures 706 are shown in figure 7b as bumps bridging the gaps 704 between the dummy components 702 and the mother flex 700.
Next the conducting curved interconnections 708 were printed as bridge structures bridging the gap between the dummy components 102 and the mother flex 700 as shown in figure 7c. Each bridge structure/interconnection 708 has a "dog bone" shaped pattern.
The interconnections 708 were screen printed using Dupont Silver paste 5064H across the gap 704 housing the TME sacrificial supporting structure 706. After printing, the ink was dried by placing the sample on a hotplate at 100°C for approximately 15 minutes, resulting in the interconnection structures 708 shown.
The next stage was to remove the TME sacrificial supporting structures 706 by sublimation as shown in figure 7d. The TME structures 706 were sublimed by placing the sample on a hotplate at 150°C for approximately 10 minutes. When removed from the hotplate, there was no trace of the TME material left below the interconnection bridge 708, and hence the screen printed bridge structure 708 can be considered to be fully air-suspended across the gap between the mother flex 700 and the inserted dummy flex component 702. The resulting interconnection air-bridge 708 shown in figure 7d, has a curved profile 710 over the gap 704 where the TME sacrificial supporting structure had been deposited.
Figure 8 shows a microscope image of the plan view of a screen printed air-suspended interconnection 802 across a gap 804 approximately 50 pm deep and approximately 100 pm in length.
The resistances of air-suspended bridges fabricated this way were measured to be between 1.7 0 and 2.5 0, whilst the resistance of reference dog bone structures 708 screen printed on the same PEN flex without a gap was of the order of 1 0. All measurements were carried out with a standard multimeter (2-wire measurement).The increased resistance of the air-suspended interconnections with respect to the reference structures is presumed to result from a non-optimized experimental fabrication procedure. The electrical and mechanical properties of the interconnections may be improved by printing a structural supporting layer (e.g. a polymer ink) before printing the conductor layer, as discussed in relation to figure Sb.
Figure 9 shows an example of an apparatus 900 comprising a flexible electronic apparatus 908 as described herein. The apparatus 900 also comprises a processor 902 and a storage medium 904 which are electrically connected to one another by a data bus 906. The apparatus 900 may be one or more of an electronic device, a portable electronic device, a telecommunications device, a portable telecommunications device and a module for any of the aforementioned devices.
In this example, the flexible electronic apparatus 908 comprises a flexible substrate configured to carry and support electrical connections between various electronic components 910, 912. In this example, the flexible electronic apparatus 908 comprises two electronic components 910, 912 electrically connected by a curved interconnection 914 as described herein. The electronic components 910, 912 may be discrete components, silicon-based microchips or surface-mount components, for example. The flexible electronic apparatus 906 may not necessarily be housed within the apparatus 900 as shown, and the apparatus 900 may be connected as an external component to an apparatus 900, for example if the flexible electronic apparatus 908 is comprised in a wearable fabric to which the apparatus 900 is connected.
The processor 902 is configured for general operation of the apparatus 900 by providing signalling to, and receiving signalling from, the other components to manage their operation. The processor 902 may be a microprocessor, such as an Application Specific Integrated Circuit (ASIC).
The storage medium 904 is configured to store computer code configured to perform, control or enable operation of the apparatus 900. The storage medium 904 may also be configured to store settings for the other components. The storage medium 904 may comprise read-only memory (ROM), for example for storage of computer code/computer readable instructions, and random-access memory (RAM), for example for executing stored computer code/computer readable instructions. The processor 902 may access the storage medium 904 to retrieve the component settings in order to manage the operation of the other components. The storage medium 904 may be a temporary storage medium such as a volatile random access memory. In other examples the storage medium 904 may be a permanent storage medium such as a hard disk drive, a flash memory, or a non-volatile random access memory.
Figure ba illustrates an example method 1000 for an electronic component carried on a flexible carrier substrate. The electronic component comprises a component contact pad configured to allow electrical connection to the electronic component. The flexible carrier substrate comprises a substrate contact pad configured to allow electrical connection of a carried electronic component to one or more other carried electronic components. The method comprises forming a curved interconnection to electrically interconnect the respective substrate and component contact pads. The curved interconnection is configured such that its curvature allows the interconnection to maintain its connection to the respective contact pads with operational flexing of the flexible carrier substrate.
Figure lOb illustrates another example method 1002, comprising the steps of depositing a supporting medium between a component and substrate contact pads such that the surface of the supporting medium forms a curved shape 1004, depositing a conducting medium on the supporting medium such that the conducting medium forms the curved interconnection 1006, and (in some circumstances) removing the supporting medium after deposition of the conducting medium such that the curved interconnection is an unsupported curved interconnection 1008.
In a method comprising a step of removing a supporting medium by sublimation, the removal may comprise the steps of curing the conduction medium at a temperature below the sublimation temperature of the supporting medium, followed by curing the conduction medium at a temperature above the sublimation temperature of the supporting medium.
In some methods depositing the supporting medium may comprise depositing a low elastic modulus material configured to support the curved interconnection during operational flexing of the flexible carrier substrate.
Another example method may comprise the step of using a flex-absorbing adhesive to secure the component on the flexible carrier substrate prior to the deposition of the supporting medium and the conducting medium. Such an adhesive may be a low modulus adhesive configured to absorb any strainsistress cause by deformation of the underlying flexible carrier substrate such that such strains/stresses are not passed on to the overlying component.
The supporting material/medium may be deposited as an ink, via, for example, inkjet printing, screen printing, gravure printing, flexographic printing, pad printing, bar-coating, spin-coating, blade-coating, spray-coating, another printing or coating method, aerosol deposition, chemical vapour deposition, sputtering, or evaporation. In some instances it may be necessary to heat the supporting medium after deposition to cure it prior to deposition of a conduction material to form a curved interconnection. An example composition of supporting material ink may be a mixture of pure TME granules dissolved into a solvent consisting of hexylene glycol, ethanol and deionized water, where the weight ratios of each of the four components is equal.
Overall, curved interconnections as described herein may provide several advantages.
For example, due in part to the curved shape of the interconnections, they provide durable conductive links between and component and substrate contact pads, and may be able to withstand significantly more strain (due to bending or stretching of the flexible carrier substrate) than conventional interconnections (e.g., by changes in the curvature of the interconnections under operational flexing). The interconnections can be manufactured cost-effectively via printing methods. Also in the case of the grooved architecture of figures 3a-3e and 4a-4c, wherein the curve of the interconnection is directed towards the substrate, cost-effective methods for groove formation such as embossing or ablation can be used.
Methods described herein may enable the curved interconnections to be formed at low temperature and without the use of any aggressive solvents. Hence such methods are compatible with low-cost substrates such as PET or even paper. Attaching the component or bare silicon die to the carrier substrate with a low modulus adhesive may increase the amount of flexibility the device can withstand overall before failure as the low modulus adhesive takes the strain of a flexed substrate rather than the strain passing onto the overlying component.
Certain examples of the described interconnection architectures advantageously act to decouple the mechanical and electrical aspects of component integration on the carrier substrate so that each aspect can be optimized accordingly. The electronic properties of a device comprising a flexible substrate, components and curved interconnections as described herein may be tuned with greater freedom. This is because fewer restrictions are imposed on the interconnections by the mechanical properties of the substrate and components due to the mechanical decoupling of the components from the substrate, via use of the flexible/stretchable curved interconnections and by adhesion of the component to the substrate using a low modulus adhesive.
The use of flexible substrates, and curved interconnections as described herein, may be for flexible/stretchable circuit boards which extend across the hinge between a keyboard and display of a laptop computer, for example.
Figure 11 illustrates schematically a computer/processor readable medium 1100 providing an example computer program configured to carry out any of the methods described herein (e.g., computer control of external equipment configured to prepare an apparatus according to a method as disclosed herein). In this example, the computer/processor readable medium 1100 is a disc such as a digital versatile disc (DVD) or a compact disc (CD). In other embodiments, the computer/processor readable medium 1100 may be any medium that has been programmed in such a way as to carry out an inventive function. The computer/processor readable medium 1100 may be a removable memory device such as a memory stick or memory card (SD, mini SD, micro SD or nano SD).
Other embodiments depicted in the figures have been provided with reference numerals that correspond to similar features of earlier described embodiments. For example, feature number 1 can also correspond to numbers 101, 201, 301 etc. These numbered features may appear in the figures but may not have been directly referred to within the description of these particular embodiments. These have still been provided in the figures to aid understanding of the further embodiments, particularly in relation to the features of similar earlier described embodiments.
It will be appreciated to the skilled reader that any mentioned apparatus/device and/or other features of particular mentioned apparatus/device may be provided by apparatus arranged such that they become configured to carry out the desired operations only when enabled, e.g. switched on, or the like. In such cases, they may not necessarily have the appropriate software loaded into the active memory in the nan-enabled (e.g. switched off state) and only load the appropriate software in the enabled (e.g. on state).
The apparatus may comprise hardware circuitry and/or firmware. The apparatus may comprise software loaded onto memory. Such software/computer programs may be recorded on the same memory/processor/functional units andfor on one or more memories/processors/functional units.
In some embodiments, a particular mentioned apparatus/device may be pre-programmed with the appropriate software to cany out desired operations, and wherein the appropriate software can be enabled for use: by a user downloading a key", for example, to unlock/enable the software and its associated functionality. Advantages associated with such embodiments can include a reduced requirement to download data when further functionality is required for a device, and this can be useful in examples where a device is perceived to have sufficient capacity to store such pre-programmed software for functionality that may not be enabled by a user.
It will be appreciated that any mentioned apparatus/circuitry/elements/processor may have other functions in addition to the mentioned functions, and that these functions may be performed by the same apparatus/circuitry/elements/processor. One or more disclosed aspects may encompass the electronic distribution of associated computer programs and computer programs (which may be source/transport encoded) recorded on an appropriate carrier (e.g. memory, signal).
It will be appreciated that any "computer" describ!d herein can comprise a collection of one or more individual processors/processing elements that may or may not be located on the same circuit board, or the same region/position of a circuit board or even the same device. In some embodiments one or more of any mentioned processors may be distributed over a plurality of devices, The same or different processor/processing elements may perform one or more functions described herein.
It will be appreciated that the term "signalling" may refer to one or more signals transmitted as a series of transmitted and/or received signals, The series of signals may comprise one, two, three, four or even more individual signal components or distinct signals to make up said signalling. Some or all of these individual signals may be transmitted/received simultaneously, in sequence, and/or such that they temporally overlap one another.
With reference to any discussion of any mentioned computer and/or processor and memory (e.g. including ROM, CD-ROM etc), these may comprise a computer processor, Application Specific Integrated Circuit (ASIC), field-programmable gate array (FPGA), and/or other hardware components that have been programmed in such a way to carry out the inventive function.
The applicant hereby discloses in isolation each individual feature described herein and any combination of two or more such features, to the extent that such features or combinations are capable of being carried out based on the present specification as a whole, in the light of the common general knowledge of a person skilled in the art, irrespective of whether such features or combinations of features solve any problems disclosed herein, and without limitation to the scope of the claims. The applicant indicates that the disclosed aspects/embodiments may consist of any such individual feature or combination of features. In view of the foregoing description it will be evident to a person skilled in the art that various modifications may be made within the scope of the
disclosure.
While there have been shown and described and pointed out fundamental novel features as applied to different embodiments thereof, it will be understood that various omissions and substitutions and changes in the form and details of the devices and methods described may be made by those skilled in the art without departing from the spirit of the invention. For example, it is expressly intended that all combinations of those elements and/or method steps which perform substantially the same function in substantially the same way to achieve the same results are within the scope of the invention. Moreover, it should be recognized that structures and/or elements and/or method steps shown and/or described in connection with any disclosed form or embodiment may be incorporated in any other disclosed or described or suggested form or embodiment as a general matter of design choice. Furthermore, in the claims means-plus-function clauses are intended to cover the structures described herein as performing the recited function and not only structural equivalents, but also equivalent structures. Thus although a nail and a screw may not be structural equivalents in that a nail employs a cylindrical surface to secure wooden parts together, whereas a screw employs a helical surface, in the environment of fastening wooden parts, a nail and a screw may be equivalent structures.

Claims (23)

  1. Claims 1. An apparatus comprising: a flexible carrier substrate comprising a substrate contact pad configured to allow electrical connection of a carried electronic component to one or more other carried electronic components; an electronic component carried on the flexible carrier substrate, the electronic component comprising a component contact pad configured to allow electrical connection to the electronic component; and a curved interconnection electrically interconnecting the respective substrate and component contact pads, wherein the curved interconnection is configured such that its curvature allows the interconnection to maintain its connection to the respective contact pads with operational flexing of the flexible carrier substrate.
  2. 2. The apparatus of claim 1, wherein the curvature of the curved interconnection is configured to allow for one or more types of flexing of the flexible carrier substrate.
  3. 3. The apparatus of claim 1 or claim 2. comprising a low-modulus adhesive between the component and the flexible carrier substrate, the low-modulus adhesive configured to join the component to the substrate and substantially inhibit stresses in the component caused by flexing of the flexible carrier substrate.
  4. 4. The apparatus of any preceding claim, wherein one or more of the electronic component and the curved interconnection are formed by one or more of: roll-to-roll printing, sheet fed printing, direct-write printing, wet-coating, vacuum deposition, and transferring a pre-formed electronic component or curved interconnection.
  5. 5. The apparatus of any preceding claim, wherein the curved interconnection is: unsupported between the substrate and component contact pads; or supported on a supporting medium located between the substrate and component contact pads.
  6. 6. The apparatus of claim 5, wherein the curved interconnection is supported on a supporting medium, and the supporting medium comprises one or more of: a supporting layer configured to form a bridge between the flexible carrier substrate and component contact pads; and a supporting medium configured to fill a space between lateral edges of the substrate and component contact pads.
  7. 7. The apparatus of claim 6, wherein the supporting medium is configured for removal after deposition of the curved interconnection thereupon by one or more of: sublimation, dissolution and etching away.
  8. 6. The apparatus of claim 6, wherein the supporting medium comprises a low elastic modulus material configured to support the curved interconnection during operational flexing of the flexible carrier substrate.
  9. 9. The apparatus of any preceding claim, wherein the flexible carrier substrate is one of: a polymer film, a metal foil, a flexible printed circuit board; a flexible printed wiring board laminate; a woven or wearable fabric, an elastomer; and paper, or a stack comprising two or more of these materials laminated into one flexible carrier substrate.
  10. 10. The apparatus of any preceding claim, wherein the curved interconnection has one or more of: a substantially symmetrical profile; an asymmetrical profile; a substantially sinusoidal profile; an arc shaped profile; a convex profile which curves away from the flexible carrier substrate above the surface level of the substrate contact pad; and a concave profile which curves towards from the flexible carrier substrate below the surface level of the substrate contact pad.a profile exhibiting a combination of two or more of such above mentioned curved features.
  11. 11. The apparatus of any preceding claim, wherein the curved interconnection has a thickness between 1 nm and 50 pm.
  12. 12. The apparatus of any preceding claim, wherein the component is substantially located in a recess of the flexible carrier substrate.
  13. 13. The apparatus of any preceding claim, comprising a gap between the edge of the component and the flexible carrier substrate, wherein the curved interconnection bridges the gap between the substrate and component contact pads.
  14. 14. The apparatus of any preceding claim, comprising a lamination layer or encapsulation material overlaying at least the curved interconnection configured to protect at least the underlying curved interconnection.
  15. 15. A method comprising: for an electronic component carried on a flexible carrier substrate; the electronic component comprising a component contact pad configured to allow electrical connection to the electronic component and the flexible carrier substrate comprising a substrate contact pad configured to allow electrical connection of a carried electronic component to one or more other carried electronic components; forming a curved interconnection to electrically interconnect the respective substrate and component contact pads, wherein the curved interconnection is configured such that its curvature allows the interconnection to maintain its connection to the respective contact pads with operational flexing of the flexible carrier substrate.
  16. 16. The method of claim 15, wherein the method comprises: depositing a supporting medium between the component and substrate contact pads such that the surface of the supporting medium forms a curved shape; and depositing a conducting medium on the supporting medium such that the conducting medium forms the curved interconnection.
  17. 17. The method of claim 15 or 16, wherein the method further comprises: removing the supporting medium after deposition of the conducting medium such that the curved interconnection is an unsupported curved interconnection.
  18. 18. The method of any of claims 15 to 17, wherein the supporting medium is removed by sublimation, by: curing the conduction medium at a temperature below the sublimation temperature of the supporting medium; followed by curing the conduction medium at a temperature above the sublimation temperature of the supporting medium.
  19. 19. The method of any of claims 15 to 18, wherein depositing the supporting medium comprises depositing a low elastic modulus material configured to support the curved interconnection during operational flexing of the flexible carrier substrate.
  20. 20. The method of claim 16, comprising using a low-modulus adhesive to secure the component on the flexible carrier substrate prior to the deposition of the supporting medium and the conducting medium.
  21. 21. The method of any of claims 15 to 20, wherein forming the curved interconnection comprises: fabricating the curved interconnection using a mould, and transferring the moulded curved interconnection for attaching to the component and substrate contact pads by one or more of: stamping, embossing and transfer printing.
  22. 22. A flexible electronic device comprising the apparatus of any of claims 1-14 or formed according to the method of any of claims 15-21.
  23. 23. A computer program comprising computer code configured to perform the method of any of claims 15-21.
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