PRESSURE COMPENSATION BODIES, IN PARTICULAR PRESS PADS FOR EQUIPMENT OF HYDRAULIC SINGLE AND MULTI-STAGE HEATING AND COOLING PRESSES

Description

[0001] The invention relates to a press pad for use in a hydraulic single- or multi-stage heating and cooling press for the production of printed circuit boards, high-pressure laminates or similar plate bodies, the press pad comprising two outer layers arranged on opposite sides of the press pad, each consisting of a film made of a high-temperature-resistant, preferably thermoplastic, polymer with a very low coefficient of friction, a middle layer arranged between the outer layers made of a textile fabric made of fibers and two connecting layers each arranged between the middle layer and the respective outer layer made of a fluoroelastomer, preferably a fluororubber.

[0002] Furthermore, the invention relates to a method for producing a press pad for use in a hydraulic single- or multi-stage heating and cooling press for producing printed circuit boards, high-pressure laminates or similar plate bodies, the press pad comprising two outer layers arranged on opposite sides of the press pad, each consisting of a film made of a high-temperature-resistant, preferably thermoplastic polymer with a very low coefficient of friction, a middle layer arranged between the outer layers made of a textile fabric made of fibers and two connecting layers each arranged between the middle layer and the respective outer layer made of a fluoroelastomer, preferably a fluororubber, where at least a predominant proportion of the fibres, preferably all fibres, of the textile fabric of the middle layer consist of a material with a negative coefficient of thermal expansion.

[0003] For the purposes of the present invention, fluoroelastomers (or synonymously "fluorinated elastomers") are understood to mean not only fluororubbers but also other groups of fluorinated elastomers, such as perfluoro rubber (FFKM), tetrafluoroethylene / propylene rubber (FEPM) or fluorinated silicone rubber (FVMQ), the latter having proven to be less suitable in tests with press pads according to the invention. State of the art

[0004] Press pads of the type described above are used in various hydraulic single- and multi-opening heating and cooling presses and have the task of distributing the pressing pressure as evenly as possible across the entire surface of the products to be manufactured, which can be printed circuit boards for the production of electronic circuits, high-pressure laminates (HPL) consisting of several layers of paper impregnated with melamine and phenolic resin, or similar panel bodies, and transferring it to the products to be manufactured. The requirements placed on such press pads, which are also referred to as pressure compensation bodies, include, on the one hand, the highest possible thermal conductivity that is uniform across the surface in order to keep the process times for heating and cooling short, and to ensure high elasticity and / ora high resilience to return to the initial state after the pressing pressure is released at the end of a pressing cycle, and to do so over the greatest possible number of pressing cycles. This latter requirement also equates to a low so-called "compression set." Furthermore, such pressing pads are required to exhibit the lowest possible static and sliding friction, i.e., in particular, a low static and sliding friction coefficient, especially compared to the metallic heating plates in the press. The demands on the pressing pads used are particularly high in printed circuit board production, as such products are manufactured under very high pressing pressures, long pressing times, and high pressing temperatures.Firstly, the printed circuit boards must have a very narrow thickness tolerance, and secondly, no particulate contamination may be created or adhered to the printed circuit boards during the manufacturing process, as this would significantly increase the risk of defects in the printed electronic circuits subsequently produced from them during further processing. In this context, a low coefficient of static and sliding friction is important to prevent increased abrasion on the surface of the press pad during the expansion and contraction process caused by the temperature changes in the press pad.

[0005] Epoxy, polyamide, polyethylene terephthalate, and polyethylene naphthalate are typically used as substrate materials in printed circuit board manufacturing due to their particularly positive physical and chemical properties. The most commonly used substrate in printed circuit board manufacturing remains a glass fiber-reinforced (fiberglass) epoxy resin with a copper foil bonded on one or both sides. Due to its strong structure and durability, such a fiberglass-reinforced epoxy resin composite structure offers exceptional mechanical strength. Impregnation with epoxy resin also results in an excellent electrical insulator and a flame-retardant material. This latter property is of increasing importance as temperatures tend to rise when certain electronic components are used in printed electronic circuits.

[0006] Printed circuit boards are typically the core of electronic components and circuits that transmit power, signals, and data from a source to a desired destination. The two main functions of a printed circuit board are to securely mount electronic components and to provide conductive connections between the various components on the board. Electrical current flows between individual electronic components on the board through conductive tracks, or signal paths. These tracks are etched from the copper sheets that form the outer layers of the non-conductive substrate (fiberglass epoxy resin). The printed circuit board must maintain high performance throughout its lifetime to avoid transmission delays or inaccurate data transmission.

[0007] A well-known, simple version of such a circuit board is a so-called single-sided printed circuit board, which has conductor tracks made of copper foil lamination on only one side. As complexity increases, the circuit boards are copper-clad on both sides. If this is not sufficient, a so-called multilayer circuit board is an option. In this case, several thin epoxy resin boards, each equipped with laminated circuit boards, are precisely connected with prepregs. For very complex circuits, such a multilayer circuit board can consist of up to 50 layers with alternating layers of epoxy resin board and copper foil.

[0008] During the manufacturing process, such circuit boards or their "blanks" are assembled in press packs and inserted into the heating and cooling press between press plates, which are mostly made of stainless steel with a smooth surface. To compensate for thickness tolerances in the press system, the press pads described above are inserted between the press plates and the press's heating plates. The individual levels of the press are equipped with heating plates that can be used for both heating and cooling, typically by flowing through a thermal oil. Under high pressure and temperature, the resin used in the pressed part first melts, bonding the conductor tracks to the intermediate layer, for example, epoxy resin, and then cures to form a finished circuit board.Depending on the required condensation and polymerization time, the initial heating process is then interrupted by cooling while maintaining pressure.

[0009] The state of the art in press cushions for press systems used to produce the high-pressure laminates described above still largely consists of press cushions made of paper materials such as wool felt paper or soda kraft paper. In addition, plastic cushions are increasingly being used, either in the form of textile fabrics made of synthetic fibers or of sheet-like or layered plastic materials without an integrated textile fabric in the form of a woven, knitted, or nonwoven fabric.

[0010] Press cushions of the aforementioned type are disclosed in US 2005 / 014437 A1, US 2006 / 014463 A1, and JP 3 197305 U. US 4,461,800 A discloses a press cushion for a molding press, which has a laminated core comprising at least one hard cushion layer arranged between two rigid plates having high thermal conductivity. Furthermore, the known press cushion has two soft cushion layers containing porous elastic layers. The soft cushion layers are bonded to each surface of the laminated core. Furthermore, the aforementioned hard cushion layer comprises at least one porous elastic film impregnated with a binder. The above-described press cushion is complex in its construction, and its service life is comparatively short.

[0011] The press pad described in EP 1 084 821 A includes a layer of felt-like cushioning material having opposing first and second surfaces, and a core member formed on the first surface of the cushioning material, the core member having a first surface in contact with the first surface of the cushioning material and an opposing second surface. Furthermore, the layer of felt-like cushioning material includes a base fabric woven from yarn material and a nonwoven fiber layer integrated by needling. Furthermore, the core member is made of an elastic material, a sponge-like rubber material, or a thermoplastic elastomer, which includes distributed voids in the form of closed, independent cells.

[0012] Furthermore, EP 1 978 528 A discloses a press pad for producing printed circuit boards, which may comprise a fabric, a paper, a film, or a sheet-like structure, each of which is combined as a layer with at least one further layer of a fluoroelastomer. The fluoroelastomer should preferably comprise a fluororubber component of the polyol vulcanization system, a vulcanizing agent, a vulcanization accelerator, and an acid acceptor.

[0013] Furthermore, EP 0 842 764 A discloses a press pad made of a textile yarn which is intended to have an extended service life under high mechanical stress, the textile yarn being made of a flame-retardant melamine resin fibre.

[0014] Furthermore, EP 0 493 630 B1 discloses a press pad made of asbestos-free material and intended for use in high-pressure single-daylight presses for the production of high-pressure laminates.

[0015] Furthermore, DE 103 37 403 A discloses a press pad comprising a fabric containing threads made at least partially of a high-temperature-resistant polymer material. The special feature in this case is that the threads comprising the polymer material contain a gas content of at least 1%.

[0016] Furthermore, EP 1 386 723 B1 discloses a press pad comprising a fabric whose warp and / or weft each comprise alternating thread types with different elasticity transverse to the thread axis.

[0017] Another press pad for high-pressure applications is described in EP 0 488 071 A. This press pad is intended for pressing pressures between 400 N / cm 2 and 1200 N / cm 2 and temperatures between 160°C and 200°C.

[0018] Finally, DE 200 11432 U discloses a press pad made of asbestos-free material for high-pressure multi-daylight presses or for high-pressure single-daylight short-cycle presses, both for the production of high-pressure laminates. The known press pad comprises a textile fabric containing a yarn made of an aromatic polyamide and metal threads. In particular, the fabric is to be provided, at least on one side, only superficially with a coating made of a heat-resistant and pressure-resistant polymer material. The coating covers the entire surface of the fabric in the form of a continuous layer.

[0019] All of the press pads described above can only partially meet the high requirements that must be met when used in high-pressure press systems for the production of high-pressure laminates, namely a long service life, very good recovery properties, uniform pressure equalization and a very low coefficient of friction. Task

[0020] The invention is based on the object of proposing a press pad for use in single- or multi-daylight heating and cooling presses which better meets the requirements mentioned above in the production of high-pressure laminates. Solution

[0021] Starting from a press pad of the type described at the outset, the above object is achieved according to the invention in that at least a predominant proportion of the fibers, preferably all of the fibers, of the textile fabric of the middle layer consists or consists of a material with a negative thermal expansion coefficient (linear thermal expansion coefficient). The thermal expansion coefficient refers to an expansion in the longitudinal direction of the fibers.

[0022] Because the fibers from which the textile fabric of the middle layer is formed consist entirely or at least predominantly of a material with a negative coefficient of thermal expansion, there is no expansion during the pressing process - i.e. during heating within the press system - contrary to the behavior of known press pads of the type in question here, but rather a shortening of the fibers. Even if the high-temperature-resistant material of the outer layers of the press pad according to the invention and / or the material(s) of the connecting layers, namely a fluoroelastomer, preferably a fluororubber, has orhave, the use of a material with a negative coefficient of thermal expansion in the textile fabric of the middle layer nevertheless surprisingly has a very positive effect, as the overall expansion of the press pad is significantly reduced compared to the prior art. Since the fibers of the textile fabric of the middle layer typically have a much higher modulus of elasticity than the fluoroelastomer, preferably the fluororubber material, of the connecting layers surrounding the middle layer, the longitudinally contracting middle layer forces the two connecting layers, which have quasi-rubber-elastic properties, to contract, creating a greater compressive stress in the connecting layers, even if the material of the connecting layers would have experienced longitudinal expansion with an increase in temperature without the interaction with the contracting middle layer.The same applies - albeit to a somewhat lesser extent - to the two outer layers made of a high-temperature-resistant polymer, since these cannot fully follow their actual expansion tendency due to the pronounced adhesive effect of the connecting layers made of the fluoroelastomer, preferably the fluororubber connecting layers.

[0023] Thus, due to the fibers with a negative thermal linear expansion coefficient processed according to the invention in the middle layer in the form of a textile fabric, the expansion of the press cushion as a whole, i.e. also in the area of ​​its outer layers, can be reduced in an unforeseeable manner. This is of great practical importance because, in particular, large linear expansions of the known press cushions - despite the deliberately very low friction coefficient of the material of the outer layers already chosen in the prior art - lead to undesirable relative movements between the outer layer and the press plate or heating plate coming into contact with it and, as a result, to abrasion of the cushion material, i.e. material of the outer layer, and in this way would cause very detrimental contamination within the press system or the printed circuits produced therewith.As already explained above, contamination on the printed circuit boards can lead to malfunctions in the printed circuits during their further processing into printed circuits.

[0024] Even if, in the case of the press pad according to the invention, a (minor) expansion ultimately occurs on the outer sides as a result of the temperature increase during the pressing process – despite the negative thermal expansion coefficient of the fibers of the middle layer – this is mitigated as best as possible by using a material with a very low coefficient of friction on the outer layer. This low coefficient of friction prevents true adhesion, in the sense of sticking of the outer layer material to the press plate or heating plate. In the event of such adhesion, subsequent forcible removal could easily result in damage to the press pad in the form of torn or detached pieces, which in turn would result in the contamination effect described above and thus the aforementioned loss of quality in the manufactured printed circuit boards.

[0025] For the purposes of this application, a very low coefficient of friction of the polymer material of the outer layers is understood to mean a coefficient of friction of less than 0.06, preferably less than 0.05, more preferably less than 0.04.

[0026] The fibers within the meaning of the invention disclosed here can be filaments, namely monofilaments or multifilaments, for example also (twisted) multifilaments, which are then processed into a textile fabric using conventional textile processing methods (weaving, knitting, warp-knitting, nonwoven production). The fibers according to the present application can be staple fibers or continuous fibers, for example spun fibers, which are processed into a nonwoven material, which then represents the textile fabric of the middle layer.

[0027] The term "high-temperature-resistant polymer" of the outer layers in the sense of the present application is to be understood as meaning a temperature resistance up to at least 240 °C, preferably up to at least 260 °C, even more preferably up to 300 °C.

[0028] The fibers of the textile fabric of the middle layer preferably comprise para-aramid and / or meta-aramid and / or carbon and / or glass. It is preferred that the fibers consist exclusively of one of the aforementioned materials or of a fiber blend of several of the aforementioned materials. In addition to a negative coefficient of thermal expansion, the aforementioned fibers also possess the high tensile strength necessary to ensure the required stabilizing function of the middle layer.

[0029] According to the invention, it is further provided that the middle layer comprises a woven fabric and / or a knitted fabric and / or a knitted fabric and / or a nonwoven material and / or a felt material, preferably a needle felt.

[0030] According to a particularly preferred embodiment of the press pad according to the application, the textile fabric of the middle layer is provided on its surface, preferably on both opposite sides, with needled short fibers, preferably consisting of the same material as the textile fabric or of a different material than the fibers of the textile fabric. The needled short fibers, which predominantly extend in a direction perpendicular to the opposite surface(s), not only stabilize tensile stress but also provide an additional cushioning effect due to their vertical orientation.

[0031] In a further development of the invention, the films of the outer layers are made of polytetrafluoroethylene (PTFE), ethylene-tetrafluoroethylene (ETFE), perfluoroalkoxy polymer (PFA), tetrafluoroethylene-hexafluoropropylene copolymer (FEP), or polychlorotrifluorotriethylene (PCTFE). All of these materials are characterized by high temperature resistance and a low coefficient of friction.

[0032] In order to achieve a strong and permanent bond between the outer layer and the connecting layer, a surface of the film of the outer layer facing the connecting layer can be prepared to enhance adhesion, preferably over its entire surface, i.e. etched with liquid ammonia or subjected to a treatment with a sodium naphthalene solution or a plasma treatment, in particular chemically etched or subjected to an ionization treatment, preferably by means of a low-pressure plasma.

[0033] According to the invention, the (initially partially crosslinked) fluoroelastomer, preferably fluororubber polymer, can be the copolymer of vinylidene fluoride (VDF) and hexafluoropropylene (HFP) or the terpolymer of vinylidene fluoride (VDF), hexafluoropropylene (HFP) and tetrafluoroethylene (TFE) or a polymer of vinylidene fluoride (VDF), hexafluoropropylene (HFP), tetrafluoroethylene (TFE) and perfluoromethylvinylether (PMVE) or a polymer of vinylidene fluoride (VDF), hexafluoropropylene (HFP), tetrafluoroethylene (TFE), perfluoromethylvinylether (PMVE) and ethene With regard to the crosslinking of the components of the (initially partially crosslinked) fluoroelastomer, preferably fluororubber polymer, peroxide, diamine, or bisphenolic crosslinking are generally possible. Of these three crosslinking mechanisms, diamine crosslinking is the oldest. Blocked diamines are used as crosslinkers and, in particular, achieve good adhesion between the elastomer and metals, for example. The second type of crosslinking is the bisphenolic mechanism, also known as the dihydroxy mechanism. Bisphenolic crosslinking exhibits better resistance to hydrolysis and higher temperatures, as well as an improvement in compression set.

[0034] In addition, fluororubbers can also be crosslinked peroxide-based (triacin process) using free radicals. Tests with a silicone elastomer (as an alternative to fluororubber) have shown a significantly shorter service life and lower stability and dimensional stability.

[0035] With regard to the method described at the outset for producing a press pad for use in a hydraulic single- or multi-stage heating and cooling press, the underlying problem is solved in that the fluororubber material is introduced in a pre-crosslinked state between one of the two outer layers and the middle layer and is distributed there evenly and over the entire surface, and in that in a multi-layer composite formed in this way, the fluororubber material is converted into a finally crosslinked state under full-surface pressure and elevated temperature, and the multi-layer composite is thereby permanently bonded to the press pad.

[0036] In this manufacturing method, a multilayer composite is first produced using the pre-crosslinked, and thus not fully crosslinked, fluororubber material, which already possesses sufficient cohesion for further handling. To achieve a multilayer composite suitable for use as a high-pressure press pad with a permanent and very tight bond between the individual layers, the precursor multilayer composite is then finally crosslinked under increased pressure and elevated temperature.

[0037] During this final crosslinking process, the temperature is typically between 130°C and 160°C, and the pressure is typically between 0.5 N / mm 2 and 1.0 N / mm 2 . Various known systems can be used to produce the press cushion from the multi-layer composite, which is initially held together solely by the pre-crosslinked fluorinated rubber, preferably fluororubber. For example, a simple press system can be used in which the press cushion remains stationary during the pressing process. One variant is an endless belt system with a cooling device, through which the press cushion is passed during the final crosslinking process. The pressing time is controlled by the belt speed. Another option is the use of a roller laminating system, also known as an "AUMA laminating system."Here, a circulating steel belt is guided over a partially heatable drum and the pressing pressure is regulated by the contact forces of the steel belt against the drum. Example

[0038] The invention is explained in more detail below using an embodiment of a press cushion shown in the drawing.

[0039] It shows: Figure 1: A partial cross-section through the press cushion and Figure 2: an enlarged section of the middle layer of the press cushion according to Figure 1 .

[0040] A press pad 1 has a structure symmetrical to a central plane 2 and comprises a central layer 3, a connecting layer 4 arranged on both sides of the central layer and two outer layers 5 forming the outer surfaces of the press pad 1.

[0041] The middle layer comprises a textile fabric 6 consisting of a single- or multi-layer fabric with para-aramid short fibers 8 needled onto both sides. The short fibers are perpendicular to the surface of the fabric with their longitudinal direction. The composite of fabric and needled short fibers 8 forms a so-called needle felt.

[0042] Both outer layers 5 are each made of a PTFE film with a thickness of approximately 200 µm. PTFE is a high-temperature-resistant material with high abrasion resistance and a very low coefficient of friction. The surface of the outer layer 5 facing the middle layer 3 is chemically treated by etching, specifically etching with ammonia.

[0043] A multi-layer composite with very close adhesion or connection of the individual layers is created using an initially only partially cross-linked or pre-cross-linked fluororubber, which consists, for example, of the monomers vinylidene fluoride and hexafluoropropylene. The connecting layer 4 formed from the fluororubber has a thickness of approximately 800 µm, whereby the partially cross-linked fluororubber has been modified using an adhesion promoter so that it has particularly good bonding properties. On the one hand, the fluororubber is in contact with the etched surface of the outer layer 5 and, on the other hand, with the needled short fibers 8 of the textile fabric 6 of the middle layer 3. Due to the initially comparatively low consistency of the pre-cross-linked fluororubber, it penetrates deeply into the areas between the short fibers 8 of the textile fabric 6, which is why the bond is particularly close and durable.

[0044] In the present case, the multilayer composite as described above was finally crosslinked in a roll calendering system under pressure (0.5 N / mm 2< ) and temperature (150 °C) for the purpose of final crosslinking of the fluororubber.

[0045] In tests with a press pad 1 as described above, excellent properties were observed. When used in a press system for the production of multilayer printed circuit boards, the press pad 1 demonstrated an enormously high number of possible press cycles compared to previously known press pads. The dimensional accuracy of the press pad 1 was almost completely maintained at the end of the test. Even after a very long period of use, a sufficiently high residual recovery was still observed after the release of the pressing pressure. The surfaces of the outer layers 5, consisting of PTFE film, showed no changes in terms of abrasion or damage. List of reference symbols

[0046] 1Press pad 2Midplane 3Middle layer 4Connecting layer 5Outer layer 6Textile fabric 8Short fiber

Claims

1. A press pad a press pad for a hydraulic single-level or multi-level heating and cooling press for producing circuit boards, high pressure laminates or similar plate material, the press pad comprising: two outer layers arranged on two opposite sides of the press pad and respectively made from a foil made from a high temperature resistant thermoplastic polymer with a very low friction coefficient; a center layer made from a flat fiber contexture and arranged between the two outer layers; and two connection layers made from a fluor elastomeric material or a fluor rubber and respectively arranged between the center layer and the two outer layers, wherein at least a major portion of the fibers, or all the fibers of the flat contexture of the center layer are made from a material with a negative thermal expansion coefficient.

2. The press pad according to claim 1, wherein fibers of the flat contexture of the center layer are made from para-aramid and / or meta-aramid and / or or carbon and / or glass, wherein the fibers of the flat contexture are made from one or plural materials including para-aramid and / or meta aramid and / or or carbon and / or glass.

3. The press pad according to claim 1, wherein the center layer includes a woven material, and / or a knitted material, and / or a fleece material, and / or a fleece material, and / or a needle fleece material.

4. The press pad according to claim 1, wherein the flat contexture of the center layer includes needled on short fibers on both opposite sides of the flat contexture, and wherein the needled on short fibers are made from the same material as the fibers of the center layer or from a different material.

5. The press pad according to claim 1, wherein the foils of the outer layers are made from polytetrafluorethylene (PTFE), ethylene tetrafluorethylene (ETFE), perfluoralkoxy polymer (PFA) tetrafluoroethylene hexafluoropropylene copolymer (FEP) or polychlorotrifluorotriethylene (PCTFE).

6. The press pad according to claim 5, wherein an entire surface of the foil of the outer layer oriented towards the connection layer is adhesion enhanced by chemical etching or by ionization treatment by a low-pressure plasma.

7. The press pad according to claim 1, wherein the partially crosslinked fluor rubber material is made from a co-polymer of vinylidene fluoride (VDF) and hexafluoropropylene (HFP), or a terpolymer of vinylidene fluoride (VDF), hexafluoropropylene (HFP) and tetrafluoroethylene (TFE), or a polymerized material made from vinylidene fluoride (VDF), hexafluoropropylene (HFP), tetrafluoroethylene (TFE) and perfluoromethylvinyl ether (PMVE), or a polymerized material made from vinylidene fluoride (VDF), hexafluoropropylene (HFP), tetrafluoroethylene (TFE) and perfluoromethylvinyl ether (PMVE) and ethene.

8. The press pad according to claim, wherein components of the partially crosslinked fluoro rubber polymerized material are cross linked in a peroxidic manner or a diaminic manner or in a bisphenolic manner.

9. A method for producing a press pad for a hydraulic single-level or multi-level heating and cooling press for producing circuit boards, high pressure laminates or similar plate material, the press pad comprising: two outer layers arranged on two opposite sides of the press pad and respectively made from a foil made from a high temperature resistant thermoplastic polymer with a very low friction coefficient; a center layer made from a flat fiber contexture and arranged between the two outer layers; and two connection layers made from a fluor elastomeric material or a fluor rubber and respectively arranged between the center layer and the two outer layers, wherein at least a major portion of the fibers, advantageously all fibers of the flat contexture of the center layer are made from a material with a negative thermal expansion coefficient, wherein fluor elastomeric material or the fluor rubber material is introduced in a pre-crosslinked condition between one of the two outer layers and the center layer and distributed evenly over an entire surface area and the fluor elastomeric material of the fluor rubber material is transferred into a completely cross linked condition in the multi-level composite thus formed under pressure over the entire surface area and elevated temperature so that the multi-level composite is permanently glued together with the press pad.