JP2009148832A - Composition for high temperature lead-free solder, method and device - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an improved composition for a solder, particularly, for a lead-free solder, to provide a method for producing the same, and to provide an electronic device using the composition. <P>SOLUTION: The composition for a solder comprises an alloy of Ag in amounts of 2 wt.% to 18 wt.% and Bi in amounts of 98 wt.% to 82 wt.% based on the whole weight of Ag and Bi, and one or more chemical elements in an amount of 10 ppm to 1,000 ppm to the whole weight of Ag and Bi. The chemical element(s) has an oxygen affinity that is higher than the oxygen affinity of the Ag and Bi parts in the alloy. The alloy has a solidus of no lower than 262.5°C and a liquidus of no higher than 400°C. The chemical element is selected from the group consisting of Al, Ba, Ca, Ce, Cs, Hf, Li, Mg, Nd, P, Sc, Sr, Ti, Y and Zr. <P>COPYRIGHT: (C)2009,JPO&INPIT

Description

  The field of the invention is lead-free solder.

  A number of known die bonding methods use high lead solder to bond a semiconductor die to a lead frame in an integrated circuit, mechanically connect the die and the lead frame, and thermal and electrical between them Become conductive. Although most high lead solders are relatively inexpensive and exhibit various desirable physicochemical properties, the use of lead in die joints and other solders has become stricter from an environmental and occupational health perspective. It was. As a result, various approaches have been attempted to replace lead-containing solder with lead-free die bonding compositions.

  For example, in one approach, as described in US Pat. Nos. 5,150,195, 5,195,299, 5,250,600, 5,399,907, and 5,386,000. In addition, a polymer adhesive (eg, epoxy resin or cyanate ester resin) is used to bond the die to the substrate. Polymer adhesives typically cure in a relatively short time, usually below 200 ° C., and retain structural flexibility after curing, as shown in US Pat. No. 5,612,403. In addition, the integrated circuit can be die-bonded on a flexible substrate. However, many polymer adhesives tend to cause resin bleed, which can cause undesirable degradation of die-to-substrate electrical contact, or even partial or complete delamination of the die.

  To avoid at least some of the resin bleed problems, silicone-containing die attach adhesives can be used, as described in Mitani et al. US Pat. No. 5,982,041. Such adhesives tend to improve not only between resin sealants and semiconductor chips, substrates, packages, and / or lead frames, but also wire bonding, but at least some curing processes of these adhesives High energy radiation sources are required, which can add considerable cost to the die bonding process.

  Alternatively, as described in Dietz et al., U.S. Pat. No. 4,459,166, the use of a glass paste containing high lead borosilicate glass typically eliminates the high energy curing step. However, many glass pastes, including high lead borosilicate glass, require temperatures above 425 ° C. to permanently bond the die to the substrate. Furthermore, since the glass paste tends to crystallize during heating and cooling, the adhesive properties of the bonding layer are reduced.

  In yet another approach, various refractory solders are used to bond the die to the substrate or lead frame. There are various advantages to soldering a die to a substrate, including relatively simple processing steps, solventless construction, and relative low cost in some cases. There are various high melting point solders known in the art. However, all or almost all of them have one or more disadvantages. For example, most gold eutectic alloys (eg, Au-20% Sn, Au-3% Si, Au-12% Ge, and Au-25% Sb) are relatively expensive and have ideal mechanical properties Often inferior to anything. Alternatively, Alloy J (Ag-10% Sb-65% Sn, see, eg, Olsen et al. US Pat. No. 4,170,472) can be used for various high melting solder applications. However, the alloy J has a solidus at 228 ° C. and has a problem that the mechanical performance is relatively inferior.

  Various methods and compositions for solder and die bonding compositions are known in the art, all or almost all of which have one or more disadvantages. Accordingly, there remains a need for improved compositions and methods for solder, particularly lead-free solder.

  The present invention relates to a method, composition comprising solder comprising an alloy of Ag and Bi, wherein Ag (silver) is present in an amount of 2 Wt% to 18 Wt% and Bi (bismuth) is present in an amount of 98 wt% to 82 wt%. And for devices. The contemplated solder has a solidus of 262.5 ° C or higher and a liquidus of 400 ° C or lower.

  In one aspect of the present inventive subject matter, the silver in the alloy is present in an amount from 2 wt% to 7 wt% and Bi is present in an amount from 98 wt% to 93 wt%, or the silver in the alloy is from 7 wt% to 18 wt%. By weight, Bi is present in an amount of 93 wt% to 82 wt%, or silver in the alloy is present in an amount of 5 wt% to 9 wt%, and Bi is present in an amount of 95 wt% to 91 wt%. The contemplated compositions may further include chemical elements having an oxygen affinity greater than the oxygen affinity of the alloy, with preferred elements being Al, Ba, Ca, Ce, Cs, Hf, Li, Mg, Nd , P, Sc, Sr, Ti, Y, and Zr. Contemplated elemental concentrations typically range between about 10 ppm and about 1000 ppm.

  In another aspect of the present inventive subject matter, the contemplated solder has a thermal conductivity of at least 9 W / mK and a wetting balance after about 1 second with a wetting force on soaked Ag of about 0.2 mN. Indicates. The contemplated composition can be in a variety of shapes, including wires, ribbons, preforms, spheres, or ingots.

  In a further aspect of the present inventive subject matter, an electronic device includes a semiconductor die that is coupled to a surface through the contemplated composition, where the semiconductor die specifically contemplated include silicon, germanium, and gallium arsenide dies. Is included. It is further contemplated that at least a portion of the die or a portion of the surface of such a device can be metallized with silver. In a particularly preferred embodiment, the surface comprises a lead frame that is metallized with silver. In a further aspect, contemplated solder is a semiconductor that serves as an electrical interconnect between a die and a package substrate (commonly known as flip chip) or printed wiring board (commonly known as chip on board). Used in area array electronic packages in the form of a plurality of bumps on the die. Alternatively, the intended solder may be a plurality of solders that connect the package to the substrate (commonly known as a ball grid array, there are many variations) or connect the die to either the substrate or the printed wiring board. It can be used in the form of a ball.

  In yet another aspect of the present inventive subject matter, the method of making a solder composition includes providing bismuth and silver in amounts of 98 wt% to 82 wt% and 2 wt% to 18 wt%, respectively. In a further stage, silver and bismuth are melted at a temperature of at least 960 ° C., resulting in an alloy having a solidus of 262.5 ° C. or higher and a liquidus of 400 ° C. or lower. The contemplated method optionally further includes adding a chemical element having an oxygen affinity greater than the oxygen affinity of the alloy.

  Various objects, features, aspects and advantages of the present invention will become more apparent from the following detailed description of preferred embodiments of the invention, taken in conjunction with the accompanying drawings.

  The inventors have found that the contemplated compositions can be used almost easily and advantageously as an alternative to high lead content solders in various die bonding applications, among other desirable properties. Specifically, contemplated compositions are lead-free alloys having a solidus at 260 ° C or higher (preferably 262.5 ° C or higher) and a liquidus at 400 ° C or lower.

  In a particularly preferred embodiment of the present inventive subject matter, contemplated compositions can be used as solder and are binary alloys comprising silver in an amount of 2 Wt% to 18 wt% and bismuth in an amount of 98 wt% to 82 wt%. It is. In a preferred method of preparing the contemplated composition, a suitably weighed (above) pure metal charge is placed in a refractory or heat-resistant container (eg, a graphite crucible) in a vacuum or Heat between 960 ° C. and 1000 ° C. under an inert atmosphere (eg, nitrogen or helium). The charge is stirred and left at this temperature for a time sufficient to ensure that the two metals are thoroughly mixed and melted. The molten mixture or melt is then quickly poured into a mold, allowed to cool to room temperature and solidified, and then the wire or rectangular slab is first 225 ° C by conventional extrusion techniques including heating the billet to about 190 ° C. Annealing at a temperature between ˜250 ° C. and then secondary processing into ribbons by hot rolling at the same temperature. Alternatively, a ribbon that can later be rolled to a thinner dimension may be extruded. If the resulting slag is removed before pouring the mixture into the mold, the melting step may be carried out in air.

  In another aspect of the present inventive subject matter, and particularly where higher liquidus temperatures are desired, the contemplated composition comprises 7 wt% to 18 wt% Ag and 93 wt% to 82 wt% in the alloy. Bi can be included. On the other hand, if a relatively low liquidus temperature is desired, the contemplated composition can include 2 wt% to 7 wt% Ag in the alloy and 98 wt% to 93 wt% Bi in the alloy. However, for most die bonding applications, it is generally contemplated that compositions can be used in which Ag is present in the alloy in an amount of 5 wt% to 10 wt% and Bi is present in an amount of 95 wt% to 90 wt%.

  It should be particularly understood that the contemplated compositions can be used as lead-free solders that are also free of Sn, a common major component of known lead-free solders. Further, although it is generally contemplated that particularly suitable compositions according to the present inventive subject matter are binary alloys, other compositions may include ternary, quaternary, and higher component alloys. Should be understood.

  For example, another particularly suitable composition may include one or more chemical elements having an oxygen affinity greater than that of the alloy (not including the chemical element). Particularly contemplated chemical elements include Al, Ba, Ca, Ce, Cs, Hf, Li, Mg, Nd, P, Sc, Sr, Ti, Y, and Zr, and such chemical elements It is contemplated that may be present in the alloy at a concentration between about 10 ppm and about 1000 ppm. Without wishing to be bound by any particular theory or theory, elements with higher oxygen affinity than alloys reduce metal oxides known to increase the surface tension of molten or molten solder It is intended. Accordingly, it is contemplated that reducing the amount of metal oxide during soldering will generally increase the wettability of the solder, since it generally reduces the surface tension of the molten solder.

  In a further example, one or more metals may be added to improve the thermomechanical properties (eg, thermal conductivity, coefficient of thermal expansion, hardness, paste range, ductility, etc.) of the lead-free solder. Particularly contemplated metals include indium, tin, antimony, zinc, and nickel. However, various metals other than those described above are also suitable for use in connection with the teachings described herein if such metals improve at least one thermomechanical property. Consequently, further contemplated metals include copper, gold, germanium, and arsenic. Thus, particularly contemplated alloys include Ag in an amount of 2 wt% to 18 wt%, Bi in an amount of 98 wt% to 82 wt%, and a third element in an amount of 0.1 wt% to 5.0 wt%. it can. A third element specifically contemplated includes at least one of Au, Cu, Pt, Sb, In, Sn, Ni, Ge, and / or Zn. Consequently, and depending on the specific amount of the third element, such an alloy may have a solidus line of 230 ° C or higher, more preferably 248 ° C or higher, most preferably 258 ° C or higher, and 400 ° C or lower. Should be confirmed to have a liquidus. Specifically contemplated uses of such alloys include die bonding applications (eg, bonding semiconductor dies to substrates). As a result, it is contemplated that the electronic device will include a semiconductor die coupled to a surface through a material that includes a composition that includes the contemplated three (or more) component alloys. The same considerations apply as outlined above for the production of the intended ternary alloys. In general, it is contemplated that the third element is added in an appropriate amount to the binary alloy or the binary alloy component.

  In addition, the addition of chemical elements or metals to improve one or more physicochemical or thermomechanical properties is such that all components in the alloy are substantially completely (ie, at least 95% of each component). It should be understood that any order can be used as long as it is in the molten state, and it is contemplated that the order of addition is not a limitation on the subject matter of the present invention. Similarly, it is preferred to combine silver and bismuth prior to the melting step, but silver and bismuth may be melted separately, and it is contemplated that the molten silver and the molten bismuth are later combined. Should be understood. A further extended heating step to a temperature above the melting point of silver may be added to ensure substantially complete melting and mixing of the components. It should be particularly understood that the solidus of the contemplated alloy can be lowered when one or more additional elements are included. Thus, contemplated alloys containing such additional elements are in the range of 260-255 ° C, in the range of 255-250 ° C, in the range of 250-245 ° C, in the range of 245-235 ° C, or even lower. May have a certain solidus.

  With respect to the thermal conductivity of the contemplated alloys, it is contemplated that the composition according to the present inventive subject matter has a conductivity of 5 W / mK or higher, more preferably 9 W / mK or higher, most preferably 15 W / mK or higher. Furthermore, a suitable composition is a solder with a wetting power for soaked Ag of greater than 0.1 mN, more preferably greater than 0.2 mN, most preferably greater than 0.3 mN, with a wetting equilibrium after 1 second. It is intended to include. Furthermore, it is also contemplated that the particular shape of the contemplated composition is not critical to the subject matter of the present invention. However, it is preferred if the contemplated composition is shaped into a wire shape, ribbon shape, or sphere (solder bump).

  Among other various uses, a contemplated compound (eg, in the form of a wire) can be used to bond the first material to the second material. For example, as shown in FIG. 1, compositions (and contemplated) are contemplated for bonding a semiconductor die (eg, silicon, germanium, or gallium arsenide die) to a lead frame in an electronic device. Materials comprising the composition) can be utilized. Here, the electronic device 100 includes a lead frame 110 metallized with a silver layer 112. A second silver layer 122 is deposited on the semiconductor die 120 (eg, by backside silver metallization). The die and lead frame include a contemplated composition 130 (wherein, for example, solder includes an alloy containing 2 wt% to 18 wt% Ag and 98 wt% to 82 wt% Bi, which alloy is 262 With a solidus above 5.degree. C. and a liquidus below 400.degree. C.) through their individual silver layers. In an optimal die bonding process, the contemplated composition is heated for 15 seconds to exceed the liquidus of the particular alloy by about 40 ° C., and preferably below 430 ° C. for 30 seconds or less. Soldering may be performed under a reducing atmosphere (for example, hydrogen or forming gas).

  In yet another aspect, it is contemplated that the composite according to the present inventive subject matter can be used in numerous soldering processes other than die bonding applications. In fact, contemplated compositions may be particularly useful in all or almost all steps of soldering, where a subsequent soldering step is performed at a temperature below the melting point of the contemplated composition. In addition, high lead solders need to be replaced with lead free solders, and the contemplated compositions can be used as solder in applications where a solidus temperature greater than about 260 ° C. is desirable. Another particularly preferred use involves the use of the intended solder to join the components of the heat exchanger as an infusible stand-off sphere or electrical / thermal interconnect.

  Due to the difference in thermal expansion coefficients of various materials, a shear load is often applied to the solder joint. Therefore, it is particularly desirable that the alloy connecting such materials has a low shear modulus and therefore also has excellent resistance to thermomechanical fatigue. For example, in die bonding applications, low shear modulus and excellent thermomechanical fatigue can help prevent die cracking. This is especially true when relatively large dies are connected to the solid support.

  Based on the known elastic modulus of pure metals, the fact that Ag and Bi exhibit partial solid miscibility, and the fact that the Ag-Bi system does not contain any intermetallic or intermediate phase, The contemplated room temperature shear modulus of the Ag-Bi alloy is contemplated to be in the range of 13-16 GPa (additional to room temperature shear modulus, ie, according to the composite law Assuming). The shear modulus at room temperature in the range of 13-16 GPa for the contemplated alloys is 25 GPa for both Au-25% Sb and Au-20% Sn alloys (calculated in the same way, with the same assumptions) , And 21 GPa for alloy J (Ag-10% Sb-65% Sn) (the measured value of alloy J is 22.3 GPa).

  A test assembly consisting of a silicon die bonded to a lead frame with an Ag-89% Bi alloy had no visible signs of failure after 1500 thermal aging, which is intended This further confirms that the calculated and measured shear modulus of the Ag-Bi alloy is low.

  In test assemblies and various other die bonding applications, the solder is typically made as a thin sheet that is placed between the die and the substrate to which it is soldered. Subsequent heating causes the solder to melt and form a joint. Alternatively, the substrate is heated and later placed in a thin sheet, wire, melted solder, or other form of solder on the heated substrate where the semiconductor die is placed to form a bond. Solder droplets may be generated.

  In an area array package, the intended solder can be placed in a sphere, small preform, paste made from solder powder, or other form to produce multiple solder joints commonly used in this application. it can. Alternatively, in processes including plating in a plating bath, evaporation from a solid or liquid state, printing with nozzles such as inkjet printers, or sputtering to create an array of solder bumps used to create joints The intended solder may be used.

  In a preferred method, the spheres are placed in pads on the package using either flux or solder paste (solder powder in a liquid medium) and the spheres are in place until they are heated and bonded to the package. Hold on. The temperature at which the solder sphere melts may be used, or may be lower than the melting point of the solder when a solder paste having a low melting composition is used. Next, the attached package with solder balls is arranged in an area array on the substrate using either flux or solder paste, and heated to form a joint.

  The preferred method of bonding a semiconductor die to a package or printed wiring board is to create solder bumps by printing solder paste through the mask, depositing solder through the mask, or plating the solder onto an array of conductive pads. including. The bumps or columns created by such techniques have a uniform composition so that all the bumps or columns melt when heated to form the junction, or only a portion of the bumps or columns melt. In addition, it may be non-uniform in a direction perpendicular to the surface of the semiconductor die.

  Thus, specific embodiments and applications of lead-free solder have been disclosed. However, it should be apparent to those skilled in the art that many more modifications besides those already described are possible without departing from the basic idea of the invention herein. Accordingly, the subject matter of the present invention should not be limited except in the spirit of the appended claims. Further, in interpreting both the specification and the claims, all terms should be interpreted in the broadest possible usage to the extent that they do not contradict the contents. In particular, the terms “comprising” and “including” should be construed as a reference in a non-limiting manner of elements, components, or steps, where the referenced elements, components, or steps are explicitly indicated. Indicates that other elements, components, or steps that are not referenced to may be present, utilized, or combined.

FIG. 3 schematically illustrates a vertical cross section of an exemplary electronic device.

Claims (33)

  A solder comprising an alloy comprising Ag in an amount of 2 wt% to 18 wt% and Bi in an amount of 98 wt% to 82 wt%, the alloy having a solidus at or above 262.5 ° C. and a liquidus at or below 400 ° C. Composition.   The composition of claim 1, wherein Ag is present in the alloy in an amount of 2 wt% to 7 wt% and Bi in an amount of 98 wt% to 93 wt%.   The composition of claim 1, wherein Ag is present in the alloy in an amount of 7 wt% to 18 wt% and Bi in an amount of 93 wt% to 82 wt%.   The composition of claim 1, wherein Ag is present in the alloy in an amount of 5 wt% to 10 wt% and Bi in an amount of 95 wt% to 90 wt%.   The composition according to claim 1, wherein the solder has a thermal conductivity of 9 W / mK or more.   The composition of claim 1, wherein the solder has a wetting force of about 0.2 mN for soaked Ag with a wetting equilibrium after 1 second.   The composition of claim 1, further comprising a chemical element having an oxygen affinity greater than the oxygen affinity of the alloy.   8. The composition of claim 7, wherein the chemical element is selected from the group consisting of Al, Ba, Ca, Ce, Cs, Hf, Li, Mg, Nd, P, Sc, Sr, Ti, Y, and Zr. .   9. The composition of claim 8, wherein the chemical element is present at a concentration between 10 ppm and 1000 ppm.   The composition of claim 1, wherein the alloy is formed into at least one of a wire, ribbon, preform, anode, sphere, paste, and evaporative slag.   An electronic device comprising a semiconductor die coupled to a surface through a material comprising the composition of claim 1.   The electronic device of claim 11, wherein at least a portion of the semiconductor die is metallized with Ag.   The electronic device of claim 11, wherein at least a portion of the surface is metallized with Ag.   The electronic device of claim 11, wherein the surface comprises a lead metallized with silver. Providing Ag and Bi, wherein Ag is present in an amount of 2 wt% to 18 wt% of the total weight of Ag and Bi, and Bi is present in an amount of 98 wt% to 82 wt%; and Ag and Bi of at least 960 ° C Melting at temperature to produce an alloy having a solidus of 262.5 ° C. or higher and a liquidus of 400 ° C. or lower;
The manufacturing method of the solder composition containing this.   16. The method of claim 15, wherein there is a step of combining Ag and Bi prior to the step of melting Ag and Bi.   The method of claim 15, further comprising adding a chemical element having an oxygen affinity greater than the oxygen affinity of the alloy.   The method according to claim 15, wherein Ag is present in an amount of 2 wt% to 7 wt% and Bi is present in an amount of 98 wt% to 93 wt%.   16. The method of claim 15, wherein Ag is present in an amount from 7 wt% to 18 wt% and Bi is present in an amount from 93 wt% to 82 wt%.   The method of claim 15, wherein Ag is present in an amount of 5 wt% to 10 wt% and Bi is present in an amount of 95 wt% to 90 wt%.   A third element comprising solder comprising an alloy comprising Ag in an amount of 2 wt% to 18 wt%, Bi in an amount of 98 wt% to 82 wt%, and a third element in an amount of 0.1 wt% to 5.0 wt%; Is selected from the group consisting of Au, Cu, Pt, Sb, Zn, In, Sn, Ni, and Ge, and the alloy has a solidus at 230 ° C. or higher and a liquidus at 400 ° C. or lower.   The composition of claim 21, wherein the alloy has a solidus of 248 ° C. or higher.   The composition of claim 21, wherein the alloy has a solidus of 258 ° C. or higher.   The composition according to claim 21, wherein the third element is Au.   The composition according to claim 21, wherein the third element is Cu.   The composition according to claim 21, wherein the third element is Pt.   The composition according to claim 21, wherein the third element is Sb.   The composition according to claim 21, wherein the third element is Zn.   The composition according to claim 21, wherein the third element is In.   The composition according to claim 21, wherein the third element is Sn.   The composition according to claim 21, wherein the third element is Ni.   The composition according to claim 21, wherein the third element is Ge.   An electronic device comprising a semiconductor die connected to a surface through a material comprising the composition of claim 21.

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