WO1999031944A1 - Electric circuit supports - Google Patents

Abstract

The invention concerns a planar electric circuit support comprising a substrate (1) based on a solid material consisting of a polymer matrix (4) wherein are bound glass fibre pieces (3), the proportion by weight of the glass fibre pieces in the polymer matrix being more than 50 %, the glass fibre pieces being advantageously longer than 200 micrometers.

Description

 "Electrical circuit supports"

The present invention relates to electrical circuit supports and, more particularly, to flat supports on which a good quality electrical circuit can be produced, either on one face of the flat support consisting of a composite or plastic structure plate, either on each of the two faces of such a support, these two circuits then being interconnected by means of holes or vias themselves metallized.

The creation of good quality electrical circuits is one of the major technological fields of contemporary industry. The nature of the material in which an electric current flows orients the use of the electric circuits generated.

If the material resists electric current, a significant part of the electric energy transported by this current dissipates in the form of heat in the material and the source of electric energy becomes source of heat, the material then being able to accumulate heat to form a heating element. The materials used are resistive metals and the electrical heating circuits are generally in the form of metallic cables wound on ceramics, very rarely in the form of a "flat" circuit or printed on a flat support.

If, on the other hand, the material does not oppose resistance to electric current, it becomes a good conductor and allows information to be communicated without delaying or distorting it. This electrical conductor can then intervene in the mounting and interconnection of the active elements of any electronic system. Any electrical circuit used in a The electronic system is usually integrated on a flat support (the "printed circuit") and the active elements of the system are individually connected to these circuits. The assembly thus constituted, or "card", offers essential advantages by its functionality, its compactness and its simplified integration in any apparatus comprising electronic servos. After standardization of card manufacturing, these advantages have enabled in particular the emergence and development of electronic data management systems, such as miniaturized computers. Several methods of manufacturing printed circuits have been implemented industrially. Generally speaking, they fall into two types:

- negative or subtractive methods (direct method, total plating, reverse method, selective coating) which incorporate chemical etching to make the metal tracks, or

- positive or additive methods which do not require chemical etching, the tracks being produced directly by specific metal deposition by chemical means.

The screen printing method can take a negative or positive form.

These two types of methods are differentiated first by the number of steps they comprise: negative methods incorporate twice as many steps as positive methods. They also differ in the nature of the planar support used; in the case of a negative method, the support is complex and requires a sophisticated mode of production, while a positive method can accommodate any planar support.

In general, negative methods are therefore necessarily more expensive than positive methods, which should favor the latter. However, the quality of the electrical circuit produced by the subtractive routes remains much higher than that of a circuit obtained by the additive routes proposed to date (porosity of the deposited metal, limited adhesion of the metal layer, sensitivity of the surface of the deposited metal to corrosion). This is the reason why negative methods have developed in electronics and continue to be used massively to cover almost the entire volume of current production of printed circuits for electronics. This trend could only be reversed if the characteristics of the products obtained by the use of a new positive method were to be substantially better than those of the products obtained by known positive methods, and comparable to those of the products obtained by a method. known negative in order to satisfy the specifications imposed by the standardization in force. The current negative technology for manufacturing a printed circuit for electronics is itself very heavy. These difficulties in producing electronic cards are greatly attenuated in the case of flat heating electric circuits. For the latter, in fact, the dimensions of the resistive electrical tracks are much greater than those of the circuits for electronics, thus supporting greater inaccuracies. On the other hand, due to the necessary increase in temperature for extended periods of time, the problems of stress at the interface between insulating plane support and metal tracks require perfect adhesion of the metal with the support material. The positive technique known as "screen printing" can be used. However, the metallic layers which it makes it possible to remain fragile, not very adherent and of high cost price. Only a metallization technique allowing this type of strong adhesion for metal tracks could make it possible to manufacture flat heating elements of sufficient quality and durability. There is therefore a need for innovation in the field of the production of printed circuits, both for electronic applications and for thermal applications. The object of the present invention is to describe simple and economical flat supports of a new type, the characteristics of these flat supports being adapted to the use of a positive and economical metallization process, this process allowing the manufacture of a printed circuit on one or two sides of the support, the characteristics of this circuit equaling or exceeding those of a printed circuit produced by any of the known negative or positive methods.

This particular positive metallization process is described in patent EP 0693 138. According to this patent, a metallization process in two stages of plastic parts is described: a thick and adherent metallic layer is deposited exactly and only on the sites of a plastic support where this layer is necessary, these two steps being followed by a third step intended to improve and stabilize the interface between the deposited metal and the flat support.

This process allows the metallization of parts made of plastic materials containing metal oxide particles. To do this, it uses the beam emitted by an excimer laser source to photochemically degrade the surface of these oxide grains. The actual metallization of the irradiated surface of the plastic is then obtained by immersing the plastic in a bath containing metallic ions of nickel or copper. These are fixed on the apparent surface of the oxide grains present on this irradiated surface. The adhesion of the metal layer which then develops is thus mainly determined by the fact that the oxide grains are partially buried in the plastic. The size, the geometry and the surface dispersion of these grains therefore directly affect the adhesion of the metal layer, in particular in a situation of mechanical stress or thermal. Increasing this adhesion in extreme situations of use can only be done with respect to the metallization process used, by increasing the size and the proportion of these oxide particles. However, the volume proportions (between 0.2 and 30%) and the size (between 0.5 and 50 micrometers) of the oxide grains incorporated in the plastics described and to which the process applies do not allow the use of these plastics to the standards of printed circuits. In particular, plastics which contain 30% of oxides can only operate at temperatures below 245 ° C whereas a printed circuit must be able to withstand an extreme temperature of 300 ° C. For these plastics, the mechanical strength and rigidity are also lower than those imposed on standardized printed circuits. It is therefore essential to provide a flat support of a new type which allows the positive metallization process of this patent EP 0 693 138 to be used while meeting the current standards for the use of a printed circuit.

To this end, according to the invention, the electrical circuit support comprises a substrate based on a solid material consisting of a polymer matrix in which pieces of glass fibers are bonded, the weight proportion of the pieces of glass fibers. in the polymer matrix being greater than 50%.

According to an advantageous embodiment of the invention, the pieces of glass fibers have a length greater than 200 micrometers.

According to a particularly advantageous embodiment of the invention, the length of the pieces of glass fibers is between 200 micrometers and 3 mm and preferably between 200 and 500 micrometers.

Other details and particularities of the invention will emerge from the description of the drawing appended to this memo and which illustrates, by way of nonlimiting example, a particular embodiment of planar support.

The single figure is a sectional view of part of a planar support form of the invention. As can be seen in the figure of the accompanying drawing, the support of the invention comprises a rigid substrate 1 made of a solid material. This rigid substrate 1 is covered on at least one of its faces and preferably, as indicated on its two faces and on certain areas thereof, of a metal layer 2. The rigid part forming the substrate 1 consists of pieces of fibers of glass 3 bonded and held inside a polymer matrix 4 serving as cement. The planar support is said to be "composite". The rigid substrate-forming part can also contain a mixture of a polymer and an inorganic material, such as, for example a mineral material chosen from the group comprising hydrated alumina particles, hydrated magnesia particles, particles of talc and mixtures of two or more of these substances. In this case, the flat support is said to be "plastic". The choice between these two types of support (that is to say composite or plastic) is determined by functional considerations, the composite structure allowing for example temperature resistance higher than that of a plastic structure, or economic, a plastic structure, being easier to realize, is more economical than a composite structure.

The pieces of glass fibers 3, the length of which is greater than 200 micrometers, preferably between 200 micrometers and 3 mm and advantageously between 200 and 500 micrometers and the diameter between 10 and 20 micrometers, are held together and integrally by a cement-forming matrix consisting of a polymer to form a composite material containing a proportion greater than 50% by weight of pieces of glass fibers. The layer of polymeric material 5, partially covering the rigid part 1, whether this is of a composite nature or not, advantageously consists of a pure, uncharged, high temperature polymer or copolymer. The metal layer 2 and the polymeric material layer 5 of the planar support are shaped as shown by the structure described in the figure. The planar support of the invention generally has a total thickness greater than 0.2 mm.

The metallization of flat supports is generally carried out according to the method taught by patent EP 693 138. In fact, the flat support is subjected by direct contact to a metal mask (for example, nickel) whose design reproduces the negative of the geometry of the metal circuit which must be reproduced on the support. Thus equipped with the mask, the flat support is subjected to the irradiation of a beam emitted by an excimer laser source. The laser irradiation causes the etching of the polymer surface layer of the planar support in the areas of this planar support which are accessible to the beam through the mask. In these same areas, and after stripping of the polymer surface layer, the glass fibers, pieces of glass fibers or various mineral particles (for example talc) which are contained in the rigid part of the planar support and which now are flush with the surface of the pickled support are themselves subjected to irradiation. This irradiation causes photochemical degradation of their surface. Finally, the planar support thus prepared is immersed in a solution containing metal ions (for example nickel or copper) which are fixed on the photochemically degraded surface of solid materials incorporated in the rigid part of the planar support. A metal film then develops (cf. film 2 of the figure) whose thickness is proportional to the time of immersion in the solution and will depend on the content of metal ions in the solution. The desired thickness of the metal having been reached, the metallized flat support is then subjected to a heat treatment at the highest possible temperature taking into account the choice of materials used for its manufacture (in an oven or on a hot plate, for example) . This heat treatment has several objectives:

- homogenize the thickness of the metal layer,

- eliminate the liquid or volatile products contained in the metallic layer or in the material of the plane support proper,

- homogenize the interface between the metallic film and the underlying material (composite or polymer) by densification of the polymer and migration of the materials in contact.

In particular, the printed circuit can advantageously be brought during this heat treatment to a temperature higher than the extreme operating temperature of the printed circuit for a predetermined time. This heat treatment can also and just as advantageously be integrated into the process of implementing the electrical circuit which consists, for a circuit intended for electronics, of mounting by soldering or other soldering means ("wave" for example) the active elements ("components") of this circuit on the metal tracks of the electrical circuit. In this final stage of functionalization of the latter, the tracks are in fact brought either to the temperature of the molten metal binder (tin-lead alloy, for example), or to a temperature of the order of 180 ° C. usually. During this thermal type assembly, the objectives listed above are achieved ensuring the proper functioning of the entire electrical circuit and components.

The main advantage of the flat supports of the invention over the supports usually used resides in the simplicity of their structure, which also allows them to be metallized in excellent conditions thanks to the use of the process described in the cited EP patent. These supports are therefore much less expensive.

Substrates having a rigid part made up of a mixture of pieces of glass fibers (in proportion greater than 50% for composites or less than 50% for plastics) and a polymer have three characteristics which give them a particular advantage on other media:

1) after irradiation by the laser, the pickling of the pure polymer surface film reveals on the pickled surface the pieces of glass fibers which are randomly oriented with respect to each other while remaining partially "buried" in the polymer material that cements them (see Figure 2). Simultaneously with this partial exposure, these pieces of fiber are photochemically altered. Consequently, during the immersion in the solution containing the metal ions, the emerging parts of these pieces of fibers are selectively covered with metal, the rest of these pieces of fibers which is "buried" in the polymer ensuring adhesion. of the metal layer which develops while ensuring the metal bridging between the pieces of fibers. This type of adhesion is superior to that obtained with mineral particles of the talc, hydrated alumina and other types;

2) the etching of the part irradiated by the laser induces a metallization "buried", at least partially, in the non-irradiated material, giving the metal layer resistance to increased mechanical wear;

3) this type of material also has the advantage of being able to be pierced much more easily than supports containing only glass fibers coated or not with epoxy resin. As in all the supports described here, the holes then obtained can be metallized according to the metallization technique described, simultaneously nement to the metallization of the isolated tracks during irradiation by the laser.

A particular advantage of glass fiber supports lies in their possible miniaturization. Since metallization operates on the surface of the glass fibers, it is indeed possible to reduce the thickness of the fiber fabric to its lower limit (25 micrometers). The support is thus mechanically resistant but flexible and easily cut to facilitate its integration. The circuit which is produced on such a support using the technique described in the cited EP patent is also flexible with excellent adhesion of the metal to the fibers. In addition, each of the two faces of a support can receive an electrical circuit independently of the other. The metallization process is maintained long enough to allow the metal layer to "bridge" the interstices between the fibers or between the fiber bundles, without ever passing through the glass fiber fabric. The support can then be "encapsulated" between two polymer films (in polyethylene, for example). It can then be mounted by rolling on a rigid plastic support (thickness greater than 0.2 mm, for example) to form a single or double-sided card of small size. Using the same laser beam, it is still possible to strip the polymer layer superimposed on the face of the support containing the printed circuit in certain areas of this layer in order to free specific areas of this circuit from their polymer protection. The integration of a multilayer circuit is then carried out by association of individually fabricated layers ("associative" integration). A flexible multilayer assembly can then be formed by laminating such supports produced separately then superimposed, each having a specific electrical circuit, and being isolated from the others by encapsulation, except in the areas previously stripped where the electrical connection between superposed circuits is then made to form vias, the assembly being hot rolled to ensure the integrity and homogeneity of the interfaces between the different layers, then optionally mounted by rolling or hot gluing on a rigid support.

Another advantage of the supports in pieces of glass fibers relates to the manufacture of "flat" heating elements. For this type of application, the resistive electrical circuit is produced on a flat support consisting of a polymer or copolymer containing pieces of glass fibers. During the passage of electric current, the circuit rises in temperature and transmits heat to the support. A quantity of heat is dissipated on the opposite side of this support. This quantity is lower if the thickness of the support increases, and simultaneously the inertia of the assembly also increases. It is therefore necessary to optimize the thickness of the support, reducing it as much as possible while maintaining sufficient electrical insulation. This optimization essentially depends on the electrical power that one wishes to dissipate, that is to say on the application. Once having made the optimized support, this fabric is metallized according to the process described, then dressed with a polymerized epoxy resin with high thermal resistance, then heat treated to ensure the best homogeneity at the metal / fiber interfaces. In another embodiment, still according to the method described in patent EP693138, one of the two faces of such a planar support can be previously entirely metallized and then mounted by brazing on a metal plate with high thermal conductivity. The second face of the planar support is then equipped with an electric circuit as above in which a high amperage electric current flows, the assembly forming a heating plate of low inertia, compact, economical and easy to produce.

Claims

1. An electrical circuit support comprising a substrate based on a solid material consisting of a polymer matrix in which pieces of glass fibers are bonded, characterized in that the weight proportion of the pieces of glass fibers in the polymer matrix is greater than 50%. 2. Support according to claim 1, characterized in that the pieces of glass fibers have a length greater than 200 micrometers. 3. Support according to claim 1, characterized in that the length of the pieces of glass fibers is between 200 micrometers and 3 mm. 4. Support according to claim 3, characterized in that the length of the pieces of glass fibers is between 200 and 500 micrometers. 5. Support according to any one of claims 1 to 4, characterized in that the solid material is a mixture of a polymer and an inorganic material chosen from the group comprising hydrated alumina particles, particles of magnesia hydrated, talc particles and mixtures thereof. 6. Support according to any one of claims 1 to 5, characterized in that the pieces of glass fibers have a diameter between 10 and 20 micrometers. 7. Support according to any one of claims 1 to 6, characterized in that it has a total thickness of at least 0.2 mm.