FOILED UD-PREPREG AND PWB LAMINATE PREPARED THEREFROM

Abstract: The invention relates to a basic material for making a PWB laminate, which is a foiled UD-prepreg comprising a layer of a conductive metal foil bonded to a layer made up of parallel, unidirectionally oriented reinforcing fibres, having a diameter of below 30 mu m, impregnated with not yet fully consolidated matrix resin. The foiled UD-prepreg can be used to manufacture UD-crossply PWB laminates by stacking and pressing them together with, in between, other UD-layers, which may be either non-foiled UD-prepreg layers, or non-flowing UD composite layers.

Description: FOILED UD-PREPREG AND PWB LAMINATE PREPARED THEREFROM The present invention pertains to a basic material for making a PWB laminate comprising at least one layer of parallel, unidirectionally oriented (UD) reinforcing fibres impregnated with not yet fully consolidated matrix resin, i.e., a UD prepreg layer. The invention also pertains to laminates for use in printed wire boards (PWBs) prepared from such a UD prepreg layer. A UD prepreg material for making PWBs is known from US 4,814,945. This disclosure relates to a PWB laminate comprising a matrix resin reinforced with parallel aramid fibres. The laminate is built up from layers of unidirectional aramid tape stacked one on top of the other in crosswise fashion. The aramid tape is formed by arranging a single layer of parallel aramid fibres to form fibre strips, coating the fibre strips with resin, and heating them to a semi-cured or "B"-stage. A problem easily incurred when making UD crossply laminates is disorientation of the UD layers. Retaining proper orientation is necessary for obtaining a laminate having sufficient flatness, which is a property of particular importance to a PWB laminate. Particularly if a still flowable matrix resin is used, e.g., B-stage material, there is a substantial risk of disorientation occurring since, on account of the flow which occurs during lamination, the tension, and hence the orientation of the UD layers, cannot be adequately controlled. Still, it is desired to make UD-reinforced crossply composite laminates on the basis of prepreg. For, making laminates on the basis of prepreg (usually woven glass-fabric prepreg) is customary in the field of PWB laminates, and it may be advantageous to be able to employ resin systems and lamination technology of proven viability in this field. Of course, by virtue of the woven fabric structure the customary prepregs are less prone to disorientation problems. However, the replacement of the woven fabric structure by a UD-crossply structure leads to considerable advantages such as an improved surface area quality, a comparatively low linear thermal coefficient of expansion (TCE) in the x and y directions, the option of incorporating a high content of fibres, and a favourable dimensional stability. In these respects UD-crossply laminates are pre-eminently suitable as PWB substrate. Of course, this holds only if these laminates can be manufactured by means of a method that enables retaining proper orientation. Several such methods are known in the art, e.g., from EP 478051, WO 92/22191, and US 4,943,334. None of these is based on prepreg lamination, whereas the present invention is directed to providing UD crossply laminates on the basis of UD prepreg. A problem associated with the orientation of UD fibres is addressed in DE 3542295. It pertains to photographic shutter materials on the basis of a substrate layer of oriented fibres contained in a matrix resin. It is disclosed that by applying a layer of a heat-shrinkable synthetic foil onto the substrate layer, positional deviations of the parallel fibres during shaping under pressure may be suppressed. The use of a heat-shrinkable foil will cause the UD prepreg to slightly bend in the fibre direction. While this may be desirable in the case of a shutter material, in a PWB laminate it should be avoided entirely. Yet another problem that may be incurred when utilizing UD prepreg, notably if thin laminates are to be made, is that a single layer of UD fibres impregnated with still flowable resin is hard to handle (even if the resin is solid at handling temperature). This problem is enhanced by the fact that UD-layers have a tendency to tear in the longitudinal direction. This is due to the UD layer's lack of strength in the direction perpendicular to that of the fibres. An additional problem is encountered when the not yet fully consolidated matrix resin is a thermally curable resin. In contradistinction to a thermoplastic resin, such a not yet fully consolidated ther oset resin does not yet have its final properties, i.e. all mechnical properties are as yet inferior. This leads to an additional problem in handling the prepreg, since it may easily be damaged. It should be noted that other background art in which UD prepregs are employed pertains to making shaped, round articles such as golf club shafts. Thus JP-Hei-4-329,132 teaches a hybrid prepreg article for use therein. The hybrid prepreg comprises two different kinds of parallel fibres, essentially thick ones having a diameter of from 30 to 500 μm and thin ones of 5 to 30 μm, and a metal layer of 5-100 μm thickness. For PWBs that should meet modern requirements, it is important that any reinforcing fibres be filaments having a diameter below 30 μm, and preferably of from 3 to 15 μm, since thicker fibres are prohibitive for suitable drilling of holes and for obtaining a desirable surface flatness. It is particularly with thin prepregs, having thin reinforcing fibers, that handling problems occur. Other such background art is JP-Hei-6-008,240. It teaches structural composites, the outermost layer of which has been covered with a metal or metal-compound film. While the core of the composite can be reinforced with parallel, unidirectional yarns, the outer layers are reinforced with glass cloth. The disclosure is directed to shaped, round articles such as golf club shafts or antennas. Japanese Patent Application Laid-Open No. 201699/1985 discloses a heat-bondable electric shielding material which comprises a metal foil, a heat-bondable resin layer formed thereon, and a multiplicity of parallel reinforcing wires fixed to the resin layer. The conductive wires, which have a diameter of from 0.03 to 0.5 mm, are spaced apart 10 to 15 cm. It is disclosed that if the metal foil is to be prevented from wrinkling, the shielding material should be wound up together with a cushioning material, e.g. a polyurethane foam sheet. Further background art on PWB laminates includes EP 0372 505. It essentially discloses a fibre-reinforced thermoplastic laminate. The fibre reinforcement can be in any form. The thermoplastic laminate is provided with a metal foil when in the molten state. It generally is a high temperature thermoplastic, which is solid at room temperature. The disclosed laminate is not the type of basic material for making PWBs that the invention is aiming at, as it serves as a PWB laminate itself. Hence, the laminate manufactured according to EP 372 505 is a laminate having the final properties of a PWB substrate. The invention essentially aims at UD prepregs which can be used to make PWBs, but are not suitable as PWB laminates in themselves. Being thin, and having fibre-reinforcement in just a single direction, it will have suitable properties in that direction only. Hence, it is hard to handle, and will easily tear. As indicated above, this problem is even more manifest when a not yet fully consolidated thermally curable resin is used. The invention now seeks to provide a UD prepreg layer that allows further handling and processing without incurring problems such as indicated above, and is of a type essentially suitable for making a PWB laminate. Furthermore, the invention seeks to provide a UD prepreg layer in which it is possible to employ a thermally curable resin as the not yet fully consolidated resin without suffering from the additional problems associated therewith. To this end the invention provides a basic material for making a PWB laminate comprising a UD prepreg layer of the type indicated above, wherein the reinforcing fibres have a diameter of below 30 μm, and a layer of a conductive metal foil, such as copper foil, the layer of conductive metal foil being bonded to the UD prepreg layer. The layer of conductive metal foil makes for a UD prepreg material having sufficient strength perpendicular to the fibres direction to prevent tearing during handling. If the foil is laminated onto the UD prepreg prior to its being cut to size, the problem of handling a thin copper foil is solved too. As indicated above, the invention pertains to a basic material for making a PWB laminate. This basic material comprises a layered structure, the two consecutive layers bonded to each other being a layer of a metal foil, such as copper foil, and a layer of parallel, unidirectionally oriented fibres impregnated with not yet fully consolidated matrix resin. The term "prepreg" is well-known in the art and generally indicates a reinforcing material impregnated with resin and (semi)cured. It usually is still in a tacky stage. The term "not yet fully consolidated matrix material" indicates that the resin can be further cured still. In the case of a thermoset resin, it generally refers to the matrix resin being in the B stage. The several matrix material (matrix resin) stages are customarily identified in the art as the "A", "B", and "C" stages, the A stage indicating unsolidified resin (i.e., in the case of a thermoset resin: the uncured stage), the B stage generally indicating partial solidification (in the case of a thermoset resin: the reaction has proceeded through the formation of longer chains, but not to full network formation), and the C stage indicating a solidified (cured) stage. The terms A stage, B stage and C stage are known to the person of ordinary skill in the art and require no further elucidation here. 96/09158 PC17EP95/03652 The foiled prepreg of the invention can be laminated with other prepreg layers or with layers of a consolidated material. Of course, it is quite possible that such other layers comprise a woven fabric reinforcement, but if the advantages of UD reinforcement are to be enjoyed in full, the other layers should have UD parallel fibres as well, i.e., should be UD prepreg layers or consolidated (non-flowing) UD composite layers such as disclosed in WO 92/22191. The current standard basic materials for printed wire boards are generally manufactured according to the process described in, e.g., C.F. Coombs, Jr.'s Printed Circuits Handbook (McGraw-Hill), which includes the following steps: Woven glass fibres are impregnated with a solution of epoxy resin in MEK. Next, the solvent is evaporated and the resin partially cured up to a so-called B-stage. The resulting prepreg is cut to length and stacked between two copper foils. This package is cured under pressure at elevated temperature in a multidaylight press. The laminate coated with copper on both sides manufactured in this manner is then formed into a printed wire board by etching. PWB laminates on the basis of the UD prepreg according to the invention can be manufactured in essentially analogous manner. Of course, the preparation of the UD prepreg basic material deviates from the process of impregnating and curing a woven fabric. The UD prepreg can be conveniently prepared by coating a copper foil with matrix resin to form a foiled resin layer, heating the foiled resin layer so as to ensure that the resin is sufficiently flowable for impregnation of filaments to occur, and applying parallel filaments onto the resin to form a foiled UD-reinforced resin layer. The impregnation can also be carried through inversely, viz. applying the parallel filaments onto a not necessarily flowable resin layer, and then heating the resin so as to render it sufficiently flowable for impregnation to occur. Depending on the type of resin used, the foiled UD-reinforced resin layer is either further heated or subjected to actinic radiation to effect partial curing of the resin (e.g., to the B-stage) or cooled down in order for the resin to solidify (e.g., with a thermoplastic resin that is solid at room temperature). Surprisingly, the resulting foiled UD prepreg is easier to handle than both the bare copper foil and the corresponding non-foiled UD prepreg. The foiled prepreg is cut to length and ready to be stacked and laminated with other, non- foiled, UD layers. The non-foiled UD layers will form the inner laminae and be sandwiched between two foiled UD prepregs (with the Cu-foil layers on the outer surfaces). In this respect the invention also pertains to a method of making a PWB laminate wherein several layers comprising parallel, unidirectionally oriented reinforcing fibres contained in a resin matrix are stacked and pressed. In this method, the layers forming the outer surfaces of the laminate are formed of a foiled UD-prepreg comprising a layer of a conductive metal foil bonded to a UD-prepreg layer, the conductive metal foil being on the outside of the laminate. In one embodiment, the layers forming the inner laminae of the PWB laminate are prepreg layers comprising parallel, unidirectionally oriented reinforcing fibres impregnated with not yet fully consolidated matrix resin, i.e., non-foiled UD prepreg. In another embodiment, the layers forming the inner laminae of the laminate are formed of non-flowing UD-composite layers or non-flowing UD crossply laminates. The term "non-flowing UD composite" is used to indicate a composite material comprising unidirectionally oriented reinforcing fibres enclosed in a matrix material which has been solidified (consolidated) to the extent that it is not brought to flow again during the 96/09158 PCI7EP95/03652 remainder of the manufacturing process. In general, this means that during storage and processing the non-flowing UD composite is under such conditions of pressure and temperature as to be in a state below its softening point (i.e., below Tg or apparent Tg), or solidified to a stage in which flow no longer can occur. For convenience of storage and processing, it is preferred for the solidification of the non- flowing UD composite to have reached the C stage, or for such resins to be used as those comprising rigid molecular chains in which, under regular storage and processing conditions, a non-flowing state may already be attained at a stage still called the B stage. However, notably when pressing in the laminating zone is conducted under isobaric conditions, also A stage material can be employed. In the embodiments where the inner laminae are formed of non-flowing UD composites, these laminae can be prepared in accordance with WO 92/22191. It is also possible to stack and laminate foiled UD prepreg in accordance with the invention using intermediate substrates such as disclosed in WO 92/22192, which may be coated with adhesive or not. As has long been known, UD crossply laminates preferably are balanced and symmetric. The term "balanced" indicates equal properties in perpendicular directions (e.g., an equal number of filaments in the x and y directions), the term "symmetric" indicates mirror image symmetry over the thickness of the laminate, i.e., the laminate is mid-plane symmetric. The plane of symmetry, which runs through the centre of the laminate and is parallel to the laminate's outer surfaces, is either the boundary between two UD layers or an imaginary plane running through one UD layer, depending on the number and order of UD layers over the thickness of the laminate. A major advantage of such a balanced and mid-plane symmetric laminate provided with crosswise applied UD-reinforced layers consists in the isomorphism of its properties in the x and y directions (i.e., the two fibre directions perpendicular to each other). More particular preference is given to the laminate being so composed that the UD-reinforced layers are oriented as specified in one of the following models, with 0° and 90° standing for orthogonal orientational directions and the relative thickness of the layers being indicated by repeating the given orientation where necessary: 0°/90°90°/0° 0°/90 o 90°/0 o 0°/90 o 90°/0°. In general, for utilisation in PWBs the UD-reinforced layers in the laminate according to the invention will each have a thickness in the range of 6 to 800 μm, preferably of about 12.5 to 400 μm. The outer layers of the crossply laminate will be formed by a foiled UD prepreg in accordance with the present invention, i.e., a layered structure having a layer of metal foil (say; Cu) and a UD layer (say; 0°) . In the above example, in which the inner UD layers have a double thickness as compared with the outer UD layers, the inner layers can be built up of a UD prepreg. Of course, in that case the aforementioned danger of disorientation applies, but the UD layers of double thickness do not display the same handling problems as a UD layer of single thickness (which problem is solved in accordance with the invention by applying metal foil). In this embodiment it is preferred that lamination be conducted in an isobaric press, so as to avoid a driving force for flow. Preferably, though, the inner layers are the above-identified non-flowing UD composite layers in accordance with WO 92/22191. As is clear from the above, it is preferred that the stack of non- foiled UD layers sandwiched between two foiled UD prepregs (with the Cu-foil layers on the outer surfaces) is such that the UD-reinforced layers are oriented as specified in one of the above models, i.e., Cu0°/90 o 90 o /0°Cu, or Cu0°/90 o 90 o /0 o 0 o /90°90 o /0°Cu. The lamination may be conducted in a multidaylight press, an autoclave, a vacuum press, a double belt press, or in any other suitable apparatus. The PWB laminates made on the basis of the foiled UD-prepreg in acccordance with the present invention are suitable to be used in multilayer PWBs (MLBs), e.g., as disclosed in WO 92/22192. The materials employed in carrying through the present invention are not especially critical. Preferably, use is made of the materials discussed hereinafter. The matrix material is a thermoplastic or a ther osetting polymer, preference being given to thermosetting resins. More preferred is the use of an epoxy resin based matrix material, but other resins are also useful in principle. Examples include cyanate esters, unsaturated polyester (UP) resins, vinyl ester resins, acrylate resins, BT epoxy resin, bismaleimide resin (BMI), polyi ide (PI), phenol resins, triazines, polyurethanes, silicone resin, biscitraconic resin (BCI). Alternatively, combinations of said resins may be employed, and it is also possible to mix the aforementioned resins with certain appropriate thermoplasts, such as PPO, PES, PSU, and PEI among others. Also interpenetrating polymer networks (IPNs) may be suitable. It is of advantage to incorporate compounds into the matrix material to render it flame resistant, such as phosphorus or halogen-(particularly bromine-) containing compounds. A particular matrix material which is preferred for its favourable flow and curing properties comprises about 100 parts by weight of Epikote ® 828 EL, about 73 parts by weight of Epikote ® 5050, and about 4 parts by weight of a complex of boron trifluoride and onoethyl amine. While the preferred reinforcing material consists of filament yarns, non-continuous fibres may also be employed. According to the invention, the reinforcing yarns are preferably selected from the following group of materials: glass, e.g., E-glass, A-glass, C-glass, D-glass, AR-glass, R-glass, Sl-glass, and S2-glass, and various ceramic materials, such as alumina and silicon carbide. Also suited to be used are polymer based fibres, more particularly so-called liquid- crystalline polymers, such as paraphenylene terephthalamide (PPDT), polybenzobisoxazole (PBO), polybenzobisthiazole (PBT), and polybenzoimidazole (PBI), as are fibres based on polyethylene naphthalate (PEN), polyethylene terephthalate (PET), and polyphenylene sulphide (PPS). The fibres (filaments) should have a diameter of below 30 μm, e.g. 20 μm. Typical diameters more particularly range from 3 to 15 μm, and preferably are of from 5 to 13 μm. In general, the fibre content in the matrix is about 10-90 vol .%, preferably in the range of about 40 to about 70 vol.%. A fibre volume fraction of about 50 vol .% is highly satisfactory. In contradistinction to woven fabric-reinforced laminates, the composite laminates manufactured using the process according to the invention are also suited to be used in a flexible panel or laminate and in rigid-flex laminates. When used in a flexible panel, woven fabrics undergo cracking at the junctions of warp and weft fibres, due to the fact that fibres oriented in the bending direction are interwoven with fibres perpendicular to the bending direction, this adverse effect being enhanced by the high fibre concentration at these junctions, which leads to cracking at a relatively low degree of bending. Such cracks cause a high concentration of stress in the conductive traces present on the flexible laminate, and consequently a high risk of cracking, which leads to circuit breakage. In a flexible laminate (or in the flexible portion of a rigid-flex laminate) the orientation of the outer UD layers preferably parallels the desired bending direction. In addition, the present UD crossply laminates are pre-eminently suited to be used as supporting material in devices with various integrated circuits provided thereon (multichip modules). This is notably due to the favourable TCEs, which mostly are the result of the high fibre volume fraction that can be obtained when crossply laminates are used and may be close to the TCEs of electronic components (chips) used in conjunction with PWBs, more particularly MLBs. Such components may be provided on top of an MLB (chip-on-board) or else be embedded in a substrate such as an intermediate substrate according to WO 92/22192 (chip-in-board) . The build-up of laminates made on the basis of the foiled UD-prepreg of the present invention is further illustrated in the schematic drawings. Fig. 1 shows a foiled UD prepreg (1) in accordance with the invention. Indicated in the Figure, which shows cross-sections in x and y direction, are copper foil (2), which is applied onto a UD prepreg layer (3) made up of parallel, unidirectionally oriented reinforcing fibres (4) impregnated with not yet fully consolidated matrix resin (5). Fig. 2 shows a non-flowing UD composite (6) in accordance with WO 92/22191. Indicated in the Figure (x and y cross-sections) are two layers made up of UD fibres (7) impregnated with non-flowing matrix resin (8). Fig. 3 shows a CuO°/90°90°/0°Cu PWB laminate made by stacking and laminating the non-flowing UD composite (6), with two foiled UD prepregs (1).

Claims: 158 PC17EP95/03652 13 Claims: A basic material for making a PWB laminate comprising a UD prepreg layer made up of parallel, unidirectionally oriented reinforcing fibres impregnated with not yet fully consolidated matrix resin, characterized in that the basic material is a foiled UD prepreg comprising a layer of a conductive metal foil bonded to the UD prepreg layer, the reinforcing fibres in the UD prepreg layer having a diameter of below 30 μm A basic material for making a PWB laminate according to claim 1, characterized in that the metal foil is a copper foil having a thickness of below 18 μm. A basic material for making a PWB laminate according to claim 1 or 2, characterized in that the matrix resin is a thermally curable resin. A method of making a UD prepreg layer wherein filaments not bonded in the form of a woven fabric are laid parallel, in a single direction, characterized in that the parallel filaments are impregnated with a flowable matrix resin bonded to and supported by a conductive metal foil layer. A PWB laminate having layers of a conductive metal foil on the outer surfaces, the laminate being built up of a plurality of UD layers, each UD layer comprising a resin matrix reinforced with unidirectionally oriented parallel fibers, the UD layers being arranged in crosswise fashion, and the laminate having a plane of symmetry parallel to the outer surfaces, characterized in that the conductive metal foil and the adjacent outer UD layers are formed of a foiled UD prepreg in accordance with any of claims 1-3. 158 PCI7EP95/03652 14 A method of making a PWB laminate wherein several prepreg layers are stacked and pressed, the prepreg layers comprising parallel, unidirectionally oriented reinforcing fibres impregnated with not yet fully consolidated matrix resin, characterized in that the prepreg layers forming the outer surfaces of the laminate are formed of a foiled UD prepreg comprising a layer of a conductive metal foil bonded to a UD prepreg layer, the conductive metal foil being on the outside of the laminate. A method of making a PWB laminate wherein several layers comprising parallel, unidirectionally oriented reinforcing fibres contained in a resin matrix are stacked and pressed, characterized in that the layers forming the inner laminae of the laminate are formed of non-flowing UD composite layers, and the layers forming the outer surfaces of the laminate are formed of a foiled UD prepreg comprising a layer of a conductive metal foil bonded to a layer comprising parallel, unidirectionally oriented reinforcing fibres impregnated with not yet fully consolidated matrix resin, the conductive metal foil being on the outside of the laminate.

Inventors: MIDDELMAN ERIK, MIDDELMAN, ERIK
Applicants: AMP AKZO LINLAM VOF, MIDDELMAN ERIK, AMP-AKZO LINLAM VOF, MIDDELMAN, ERIK
Application year: 1995
Main classification: B29C70
IPC codes: B29C70, B32B15, B32B17, B32B37, C08J5, H05K1