WO2014023745A1

WO2014023745A1 - Chemical coating unit with low-turbulence flow

Info

Publication number: WO2014023745A1
Application number: PCT/EP2013/066507
Authority: WO
Inventors: Bernhard Wessling
Original assignee: Bwsi Gmbh & Co Kg
Priority date: 2012-08-07
Filing date: 2013-08-06
Publication date: 2014-02-13

Abstract

The present invention relates to a chemical coating unit with low-turbulence flow which is characterized in that it comprises at least one coating module for applying at least one coating material to at least one substrate, in particular a printed circuit board, wherein at least one coating module has at least one propeller device which moves and transports the coating material with a low-turbulence flow, with the result that a sufficient bathing of the substrates to be coated and/or of heating elements, if present, is achieved. The present invention furthermore relates to a corresponding coating method.

Description

Chemical coating unit with low-turbulence flow

The present invention relates to units for the non- electrochemical (i.e.: for chemical) coating of substrates, in particular printed circuit boards, with finishing layers which comprise metals or metal oxides and optionally further constituents such as intrinsically conductive polymers, as well as to coating methods using the units according to the invention, in particular if the coating results from low- viscosity liquids which are not homogeneous solutions, but dispersions or emulsions including microemulsions . These have an average particle or droplet diameter of from about 10 nm to about 1 pm, preferably from about 20 to about 500 nm, as can be determined by means of laser Doppler particle size measurement devices.

At room temperature the viscosity of these liquids lies in the range of from about 0.5 to about 500 mPas such as about 0.5 to about 200, 300 or 400 mPas, in particular about 0.7 to about 100 mPas, such as about 0.7 or 1 to about 50, 60, 70, 80, 90 or 95 mPas, measured e.g. with a suitable cone-and-plate viscometer or with a suitable Ubbelohde viscometer, at room temperature, i.e. as a rule 20°C.

Such finishing layers are deposited using chemical methods, usually oxidatively or reductively, either by applying, beforehand, to the substrate to be coated a reactive layer which reacts with the substance to be deposited (metal, metal oxide) in its dissolved or dispersed/emulsified form in a coating bath (deposition bath) , thus depositing the substance, or by direct reaction with a substance which is a constituent of the substrate to be coated, dissolving out this substance and in return depositing the desired substance (or the substance mixture) that forms the finishing layer (an exchange reaction which usually proceeds in the form of a redox reaction) . One field of use is the application of such layers to printed circuit boards.

It is particularly characteristic of the coating of printed circuit boards that there must be a good bathing of the surfaces and above all throughflow of the holes. For this it is necessary for the coating liquids used to be moved in the coating tanks or modules in a turbulent flow. Laminar flows are not able to guarantee the necessary exchange of liquids on the surfaces and above all in the holes.

Pumps and "flow bars" are therefore used.

When dispersions or emulsions are used for coating it has often been shown that a better deposition rate can be achieved in simple laboratory vessels with simple stirrers, (e.g. blade stirrers or magnetic stirrers) than in production units, however this has always been regarded as normal, explanations for possible causes have not been sought and therefore not found, as the general consensus is that the same ideal conditions could not prevail in large production units (plants) as in laboratory equipment.

Methods as described above are, as a rule, carried out in coating units which have one or more tanks for the coating materials and one or more coating modules in which the coating of the substrates takes place, e.g. in deposition baths in which the substrates to be coated, such as printed circuit boards, are arranged in suitable holding devices. Such units are known per se to a person skilled in the art.

It is crucial for such chemical coating reactions that both the concentrations of the substances involved within the coating bath, in particular at the surface of the substrate to be coated, and the temperature of the bath, in particular at the surface of the substrate to be coated, are constant within the framework of narrow tolerances.

These requirements apply both to so-called horizontal units, in which the printed circuit boards are transported from one coating bath to the next over roller devices (continuous process), and to coating units which have a vertical design and the coating process of which is discontinuous. In such units, the substrates to be coated are usually transported by lifting and transport equipment, such as cranes, from one coating pool to another according to the sequence of coating baths and in each case left in the respective bath for the given time. The above requirements furthermore apply to continuously functioning so-called horizontal units which are, however, suitable only for planar substrates.

In both of the above types of unit, the liquids are moved by pumps, arranged outside the storage tanks, which are connected to the tank inside by pipes or tubes. It must be ensured by the design of the liquid supply that the substrates and the heating devices, which are usually arranged in the tanks, in the so-called "sump" in the case of horizontal units, thus beneath the active coating modules, from where pumping occurs "upwards", are sufficiently well and uniformly bathed and that the flow passes through the holes.

In horizontal units, the desired bathing / the required throughflow is guaranteed by special liquid supply elements, so-called "flow bars", with which the liquid is guided uniformly to the substrates being passed through horizontally, such as in particular printed circuit boards, over the whole active width of the respective coating module of the unit. By active width is meant the width inside a coating module of the unit which can be covered with substrates to the maximum. The liquid supply elements ("flow bars") can be nozzles or nozzle^¬ like elements, by which liquid (which contains the coating materials) is conducted through the unit (by pressure produced by the pumps) and which serve to bring liquid containing the coating materials into contact with the substrates to be coated in a suitable manner in a coating module, e.g. in the direction of the top side and / or bottom side of the substrates, such as in particular printed circuit boards.

For the above purpose, the flow bars are filled forcefully with liquid by pumps. On the one hand, the pumps must build up enough pressure and transport sufficient liquid to ensure the desired bathing (the necessary turbulence of the liquid) . On the other hand, they must, in many cases, be stable vis-a-vis aggressive chemicals.

Above all, centrifugal pumps come into consideration as pumps, as they build up sufficient pressure and can transport enough liquid per time unit.

Axial-flow pumps are usually not suitable, as they build up an insufficient flow (too low a pressure) , likewise peristaltic pumps or diaphragm pumps.

In some cases, the use of centrifugal pumps leads to disadvantages, above all in the case of the abrasion of the pump housing and the blades of the impellers as a result of cavitation caused by turbulence, which can result in irregularities in the liquid transportation and bathing of heating devices and substrates, but also in an impairment of the properties of the liquids themselves due to turbulence and cavitation .

JP-A-2003-328142 describes an electroless copper plating unit and method. The unit comprises several vessels, in particular one vessel that contains a plating bath, and another vessel in which a copper solution is produced (from copper hydroxide and copper oxide) . The copper solution is used to supplement the solution in the plating bath. For this it is conveyed, by means of a pump, from the vessel in which it is produced into the vessel that contains the plating bath. In the vessel in which the copper supplementing solution is produced, a stirrer is used to support the formation of the solution. However, this cannot be compared with the design of the present invention, as explained in more detail below, in which one or more propeller devices are used for moving and conveying a coating material.

In WO 2011/009858 A2 a coating unit for coating a workpiece is described, which comprises a dipping bath of a paint (lacquer) and a circulation device for circulating the dipping bath. The circulation device, i.e. the device used to convey the paint, comprises one or more pumps, in order to achieve a circulation volume flow of at least one bath volume of the dipping bath per hour. The paints used in a unit according to WO 2011/009858 are relatively high-viscosity liquids. Furthermore, according to the preferred embodiments of WO 2011/009858 no stirring mechanism arranged within the dipping bath is used. The use of the special pump arrangement used in WO 2011/009858 is aimed in particular at preventing a large proportion of the circulation volume flow from remaining unfiltered, which can result in undesired dirt inclusions in the coatings due to dirt particles in the dipping bath, which has a negative influence on the visual appearance of the coating produced or makes additional treatment steps necessary .

The object of the present invention is therefore to provide a coating unit, in particular for coating printed circuit boards, which avoids the above disadvantages and in particular replaces pumps of the above type with other devices which produce a flow that is as low-turbulence as possible and as free as possible from cavitation i.e. a flow with optimum low turbulence or a flow that is indeed a turbulent flow, but not with turbulence that is supercritical. By "supercritical" is meant here that the turbulence must not be so strong that the coating properties (in particular the deposition rate) are impaired .

It has surprisingly been shown that, with propeller devices or propeller-like devices, which, according to the invention, are also called mixing blades or mixing propellers, and which are arranged in tanks and/or coating modules of the coating units according to the invention, despite high rotational speeds turbulence can be largely avoided or a turbulence range set which allows a sufficient bathing of the substrates to be coated and the heating devices (also called heating elements or heating modules) as well as liquid exchange at the surfaces and throughflow of the holes, without on the other hand damaging the dispersions or emulsions by too-strong turbulence. The disadvantage of axial-flow pumps, that they build up an inadequate flow, is surprisingly removed as a result of this, without damaging turbulence or cavitation being produced.

Therefore pumps are no longer used for moving and transporting (conveying) the coating material within the coating module (s) of the unit according to the invention or the method according to the invention. Instead, the coating material is moved by propeller devices according to the invention, which are arranged within the coating material bath.

Preferred embodiments of the invention are disclosed in the dependent claims. The optimum turbulence range can vary depending on the size and design of the coating unit. With knowledge of the present inventive concept, it is possible for a person skilled in the art to ascertain the turbulence needed for a successful coating taking place as rapidly as possible as well as the maximum tolerable turbulence range. Surprisingly, propellers, suitably designed according to the invention, with suitable geometry, make it possible to operate in this turbulence range. For a 200-litre tank it is possible to use e.g. two three-bladed propellers (in the form of ships' propellers), which are produced from a suitable material such as stainless steel or plastic such as PVDF (polyvinylidene fluoride) or PP (polypropylene) , wherein the plastic is optionally reinforced with fibres (carbon fibres, glass fibres), and which have a diameter of approximately 250 cm. Such propellers are commercially available e.g. from Flux-Gerate GmbH, Maulbronn, Germany, e.g. type BG 112. The propellers can be attached e.g. to the bottom of the tanks.

Propellers within the meaning of the present invention are hydrodynamic flow machines for producing a propulsion (see e.g. the definition in Dubbel, Taschenbuch fur den Maschinenbau, 22nd edition, Springer 2007) . As a rule, they have 2 to 6 blades (vanes), in particular 3 or 4. Suitable propellers according to the invention are optimally designed in terms of flow engineering. This can be ascertained by a person skilled in the art with knowledge of the present invention depending on the structure and size of the coating modules used, optionally by carrying out corresponding tests. In particular they are low-cavitation propellers.

The size (diameter) of the mixing blades (mixing propellers), the number thereof per tank or module and their arrangement and direction of rotation are determined depending on the design of the tanks and modules (length, width, depth, total volume) . This can prove to be very different for the different coating tasks, such as metallization, such as e.g. copper- plating, tin coating, deposition of silver, coating with metal oxides. However, according to the invention, characteristic of all of these embodiments is the fact that waves that run irregularly form at the surface of the modules, i.e. of the coating baths in the modules, without forming large stable vortices. It is likewise characteristic that the wave patterns change, thus no standing waves form.

Small vortices that occur briefly are unproblematic, while larger, stable vortices are a sign of turbulence and cavitation on the mixing blades (mixing propellers) and of an erroneous position or erroneous direction of rotation. It is then necessary to change the direction of rotation of the propellers and/or the position such that the vortices are eliminated .

The mixing blades (mixing propellers) are preferably optimally designed according to knowledge of fluid mechanics theory (fluid dynamics) and practical experience, with the result that turbulence and cavitation can be avoided or set as desired, so far as possible. Figure 1 shows an example, wherein X and Y (or A and B) are adapted to the respective task (tank volume, tank geometry) . The mixing blades (mixing propellers) are preferably made from inert plastics, e.g. from PVDF (polyvinylidene fluoride) .

The propellers according to the invention can for example be operated at rotational speeds of from about 1000 to about 1500 rpm. They can for example have a circulation rate in the range of from about 10 to about 500 m³/h, such as about 100 or 200 to 300 or 400 m³/h. The propellers can be used in production-scale units (plants) . In such units the coating modules have bath volumes of up to several hundred litres, such as for example about 10, 20, 50, 100, 200 1 or more to about 300, 400, 500 or 1000 1 and more.

In particular, each tank or each module can have one or more propeller devices.

The unit can also be designed such that a spatial separation between separate tanks and coating modules is dispensed with.

The mixing propellers can optionally, depending on the practical requirements, be introduced into the tanks or coating unit modules from the top, wherein the drive units (motors) are then arranged above the tanks or modules.

Alternatively, the mixing blades (mixing propellers) can also be introduced from the side through the tank/module walls, wherein the respective feedthrough is then sealed off with suitable O-rings.

Likewise, it is possible to arrange the propellers inside the tanks and/or coating modules without connecting them to a motor by means of axles, but to drive them without mechanical connection, e.g. by rotating magnets attached outside the coating modules or tanks, for which the internally rotating propeller then has a corresponding magnetic component, e.g. a bar magnet. The power transmission thus takes place in this case e.g. via a field acting into the module or tank.

In particular the coating unit according to the invention or the coating method according to the invention is characterized in that the area of the unit in which the dispersions or emulsions and the substrates to be coated are contained has no pumps. This area of the unit as a rule comprises the coating modules .

In a further embodiment of the invention, in particular no "flow bars" are also used there.

The unit according to the invention can be charged with coating material that comprises metals and/or metal oxides, in particular Fe, Sn, Cu and alloys and oxides thereof as well as compounds or mixtures with intrinsically conductive polymers, in particular polyaniline. The coating material is present in particular in the form of a dispersion or emulsion. At room temperature (i.e. approximately 20°C) the dispersion or emulsion generally has a viscosity in the range of from about 0.5 to 500 mPas, such as about 0.5 to 200, 300 or 400 mPas, in particular about 0.7 to about 100 mPas, such as about 0.7 or 1 to about 50, 60, 70, 80, 90 or 95 mPas, measured e.g. with a suitable cone-and-plate viscometer or with a suitable Ubbelohde viscosimeter , at room temperature, i.e. as a rule 20°C.

When using a dispersion or emulsion, the solids content thereof is generally from about 0.1 to about 50 wt.-%, in particular about 0.2 to about 45 wt.-%, such as about 0.2 to about 40 wt.-%.

Aqueous liquids, in particular water, are generally used as dispersants, optionally with a proportion of up to 15% of water-miscible organic substances which are preferably low- volatile.

The dispersions or emulsions have an average particle or droplet diameter in the range of from about 10 nm to about 1 pm, preferably from about 20 to about 500 nm, as can be determined by means of laser Doppler particle size measurement devices .

The surprising result in the case of a unit according to the invention designed as disclosed above is that the coating results are stably good in critical or sensitive coating processes. Thus, in the case of the coating of printed circuit boards using the method known as "chemical tin plating" according to EP 1 002 322, see in particular Example 4, it was possible to keep the deposition speed stable over a longer period of time than when pumps are used, wherein the deposition rate was above that which is usual for production units which use pumps of the type named at the beginning, and lay in a range typical of laboratory units.

Thus, at a bath temperature of from about 65 to about 69°C, approximately 1 pm of tin can now be applied within from about 9 to about 13 minutes, for which in existing production units, about 17 to about 25 minutes, in many cases even more, are usually required.

Furthermore, in the case of the coating of printed circuit boards with a nanoscopic coating according to the teaching of WO 2008/031492, it was possible to keep the coating baths stable, with the result that the coated copper surfaces did not change colour during assembly with electronic components under reflow and no soldering problems occurred in the 2^nd and 3^rd assembly steps.

A further effect according to the invention is that the coating units, in particular horizontal units, can be designed substantially more compact and thus more cost effectively. The provision of heating modules (so-called "sump") which are connected to the actual coating modules by complicated pipeworks and flow bars can be dispensed with. The units can be designed substantially shorter, it is possible to save several meters in the length of units, or existing units can be operated at higher speed, i.e. higher productivity, following modification.

Claims

1. Chemical coating unit with low-turbulence flow,

characterized in that the unit comprises at least one coating module for applying at least one coating material to at least one substrate, wherein at least one coating module has at least one propeller device which moves and transports the coating material with a low-turbulence flow, with the result that a sufficient bathing of the substrates to be coated and/or of heating elements, if present, is achieved.

2. Coating unit according to claim 1, characterized in that

the unit is charged with coating material which comprises metals and/or metal oxides, in particular Fe, Sn, Cu and alloys and oxides thereof as well as compounds or mixtures thereof with intrinsically conductive polymers, in particular polyaniline, wherein the coating material is present in particular in the form of a dispersion or emulsion.

3. Coating unit according to claim 1, characterized in that

the average particle or droplet diameter of the dispersed phase lies in the range of from about 10 nm to about 1 pm, preferably from about 20 to about 500 nm, as can be determined by means of laser Doppler particle size measurement devices.

4. Coating unit according to one of the previous claims,

characterized in that the unit is charged with one or more substrates and in that the substrate to be coated comprises or is a printed circuit board. Coating unit according to one of the previous claims, characterized in that the propeller device (s) of the at least one coating module is or are introduced into the coating module from the top or the side.

Coating unit according to one of the previous claims, characterized in that the coating unit comprises at least one tank and in that the propeller device (s) is or are also or alternatively arranged in the tank(s), wherein it or they is or are introduced into the tanks in particular from the top or the side.

Coating unit according to one of the previous claims, characterized in that the propeller device (s) is or are arranged inside the at least one coating module and/or tank and driven without mechanical connection towards the outside of the coating module or tank, in particular by a drive unit arranged outside the coating module or tank .

Coating unit according to one of the previous claims, characterized in that the area of the unit (coating module ( s )) , in which the dispersions or emulsions and the substrates are contained, has no pumps.

Chemical coating method with low-turbulence flow, characterized in that at least one coating module for applying at least one coating material to at least one substrate is provided, wherein at least one coating module has at least one propeller device which moves and transports the coating material with a low-turbulence flow, with the result that a sufficient bathing of the substrates to be coated and/or of heating elements, if present, is achieved, and coating material is applied.

10. Coating method according to claim 9, characterized in that coating material comprises metals and/or metal oxides, in particular Fe, Sn, Cu and alloys and oxides thereof, as well as compounds or mixtures thereof with intrinsically conductive polymers, in particular polyaniline, wherein the coating material is present in particular in the form of a dispersion or emulsion.

11. Coating method according to claim 10, characterized in that the average particle or droplet diameter of the dispersed phase lies in the range of from about 10 nm to about 1 pm, preferably from about 20 to about 500 nm, as can be determined by means of laser Doppler particle size measurement devices.

Coating method according to one of the previous claims 9 to 11, characterized in that the substrate to be coated comprises or is a printed circuit board.

Coating method according to one of the previous claims 9 to 12, characterized in that the propeller device (s) of the at least one coating module is or are introduced into the coating module from the top or the side.

Coating method according to one of the previous claims 9 to 13, characterized in that the coating unit comprises at least one tank and in that the propeller device (s) is or are also or alternatively arranged in the tank(s), wherein it or they is or are introduced into the tanks in particular from the top or the side.

15. Coating method according to one of the previous claims 9 to 14, characterized in that the propeller device (s) is or are arranged inside the at least one coating module and/or tank and driven without mechanical connection towards the outside of the coating module or tank, in particular by a drive unit arranged outside the coating module or tank.

16. Coating method according to one of the previous claims 9 to 15, characterized in that in the area of the unit (coating module ( s )) , in which the dispersions or emulsions of the coating material and the substrates are contained, no pump(s) are used for transporting the coating material.