FR2523397A1 - Mounting method for chip carriers on substrate - uses intermediate ceramic carrier plate which is glued to substrate and allows differential thermal strains - Google Patents

Abstract

The component mounting system provides a means of connecting a multi-terminal device such as a chip-carrier, to a circuit substrate where the coefficients of expansivity of the component package and substrate (10) may be very different. The component (11) is mounted on an intermediate ceramic plate (16) which is slightly larger than the component and has a number of pre-tinned contact pads (17) around its periphery. The component has its own terminal contacts (15) soldered to the pads. Flexible wave leads (18) from the pads to the substrate circuit terminating pads (13). The substrate has metal ground plane (21). A metal stud (20) is fixed to the component and fits into a hole through the intermediate layer and substrate. It gives the mounting extra mechanical solidity and provides heat dissipation. The intermediate layer is glued to the substrate surface using a polyurethane adhesive which allows dilation relative to the substrate. After all the solder connections have been made, the whole circuit assembly is covered with a layer of insulating varnish.

Description

Method for mounting electronic components on a support and product achievable by the method
The present invention relates to a method for mounting, on a support carrying a circuit, electronic components to be fixed flat and provided, on their underside, with solderable contacts, and for connecting them to the circuit, the component having a coefficient of thermal expansion not very compatible with that of the support.

 The method finds a particularly important application, although not exclusive, constituted by the assembly of micro-components called "chip carriers" or "chip carriers", deprived of housing and fixing lugs, whose size is much less than that of conventional housings. However, it is also applicable to any other component whose substrate or envelope is ill-suited, from the point of view of thermal expansion coefficient, to the support intended to receive it. This is the case, for example, of various passive circuits (resistance networks in particular) on a ceramic substrate provided at the periphery with metallized connection notches.

 We know that "chip holders" have a ceramic envelope provided on its underside with contacts allowing it to be fixed flat on a support. However, the difference between the expansion coefficients of the ceramic and the glass-epoxy or glass-polyimide laminate which forms the basis of printed circuits leads to assemblies incapable of supporting thermal cyclayes.

 The present invention aims in particular to allow the mounting of components to be fixed flat on any type of support provided with a circuit to be electrically connected to the component, such as a printed circuit board (possibly multilayer), thick-film circuit or circuit with thin layers.

 To this end, the invention proposes in particular a method according to which the component is fixed flat to the support by mechanical connection means capable of tolerating differential expansions and the contacts of the component are electrically connected to the circuit carried by the support by through sections of wires ensuring mechanical decoupling.

 In a particular and particularly advantageous embodiment of the invention, the mechanical fixing is carried out by means of an interlayer of plan dimensions slightly greater than those of the component, of material thermally compatible with the component; the electrical connections are made by welding (this term to be interpreted broadly and extending in particular to soldering) of the contact pads of the component on a conductive network provided on the interlayer and projecting from the component and by fixing the sections of wires between the network and the circuit.

 The interlayer will generally be formed by a ceramic plate having a coefficient of expansion close to that of the component, secured to the support by an adhesive having sufficient flexibility to protect the differential expansions. Polyurethane adhesives can in particular be used in general. The conductive network may be formed by screen printing, using a technology close to that of manufacturing thick layers. The network will then be tinned, for example by application of a soldering cream by screen printing. The components can then be soldered or welded to the spacers by reflow techniques on a heating mat or in the vapor phase. The conductive network can be in the form of divergent short tracks, so that the outlets to which the wires are fixed have a spacing greater than that of the contact pads of the component. Pawns integral with the spacers, for example by fitting, can help the mechanical retention of the spacers. They can also participate in heat dissipation, especially when the support in which these pins are fitted has a metallic ground plane in its thickness.

 In an alternative embodiment of the invention, intended for mounting a chip holder or microcomponent having a comparable constitution, that is to say having a casing having a ceramic bottom face provided with contact pads and a insulating cover, the component is fixed flat by its cover (that is to say upside down) directly on the support using an adhesive having sufficient flexibility to withstand differential expansions. Electrical connections are made using sections of wire welded to the contact pads of the component and to the circuit.

To ensure the necessary mechanical decoupling, the electrical connecting wires must not be stretched. A particularly convenient solution for performing junctions using sections of tension-free wires is described in the Certificate of Utility FR 2 191 399 and the patents
FR 2 327 019 and 2 327 025 from the applicant company.

The automation of junctions is made particularly convenient in that it is possible to provide, on the support, contact zones arranged at equal intervals and such that the junctions are carried out using sections of wires of the same length and parallel each other.

 The first solution is particularly advantageous when the heat dissipation of the components is relatively high; it does not bring any noticeable complication, since the only additional component necessary is constituted by an interlayer, identical for all the circuits having the same distribution of contacts: in practice, at the moment it is enough a dozen types of '' standard spacers, designed for components with 16 to 92 weld ranges. The solution can be directly transferred to components with a much greater number of electrical output ranges.

 The second solution is particularly suitable for mounting components having moderate heat dissipation.

The invention will be better understood on reading the following description of various modes of implementation, given by way of nonlimiting examples. The description refers to the accompanying drawings, in which
- Figure 1 is a plan view of a fraction of a product resulting from the implementation of the method, comprising components of the type called "chip-carrier" or "chip carrier" on a support that will be assumed to be a circuit board
- Figure 2 is a sectional view along the line II-II of Figure 1 ;;
- {es Figures 3 and 4, similar to Figure 2, but in which the electrical connections are not shown, show variants of attachment of a component
- Figure 5, similar to Figure 2, shows an alternative embodiment of the invention involving placing the component upside down.

 FIG. 1 shows a product which comprises, in a conventional manner, a support 10 and chip carriers 11. This support is not, however, a conventional printed circuit where all the connections are provided by metallized tracks, but a circuit on which welding pads 13 are simply printed between which the equipotential connections are made by welding sections 14 of insulated wire (typically copper wire of diameter between 50 and 250 microns, covered with a polyurethane / polyamide heat-sealable enamel). The equipotential links can be produced by other methods, for example that known under the name "multi-wire".

 In all cases, the connecting wires 14 can be secured to the support, either by gluing during fixing, or by subsequent deposition of a varnish trapping the wires. In the embodiment of the invention shown in FIGS. 1 and 2, intended to weld components having, on their underside, weldable contact pads 15, use is made of a ceramic insert 16 on which is serigraphed a divergent conductive network 17 adapted to the distribution of the contact pads provided on the underside of the casing of the component 11.

 The steps for implementing the method are then as follows.

 It is first of all necessary to have the necessary intermediate plates 16. In practice, a few types of standardized pads will be used, allowing the usual components to be received.

 These components will generally be chip holders having 16 to 92 solder pads. Other dividers will obviously be expected when chip carriers with a greater number of contact pads will be available.

 The plates are made of ceramic material having a coefficient of thermal expansion compatible with that of the component to be received. On the wafers, the conductive network 17 is screen printed using the conventional technology for manufacturing circuits with thick layers. Then, in the embodiment illustrated in FIG. 1, the network is partially covered by a strip 19 of protective glass. which fulfills several roles. This strip increases the thermal dissipation from an element of the network 17 on which wire sections are welded and therefore limits the risk of contact melting between the circuit element and the corresponding range of component 11. This same strip precisely delineates the spread areas, as will be seen later.

 Next, on the network 17, a tin-lead soldering cream is applied, generally by screen printing.

This soldering cream does not wet the glass, the presence of the strip 19 avoids pre-tinning the conductive network elsewhere than in the areas where junctions are to be made.

 Each component 11 is then fixed to its intermediate plate 16 by brazing. This operation can be carried out by remelting the heating mat at a temperature of the order of 3500 ° C. (a flux must then be provided in the welding cream). It is also possible to use vapor phase brazing, at a slightly lower temperature, which makes it possible to operate without flux given the absence of risk of oxidation in the vapor phase.

 The intermediate plates 16 are then fixed on the support 10. This fixing will generally be carried out using a polyurethane adhesive with rapid setting. In the case illustrated in FIG. 2, the intermediate plate 16 is also held by a pin 20 glued into a hole in the plate 16, fitted and glued in the support 10. This pin 20 also improves the cooling of the component 11, with which it is in contact. When the support 10 comprises a copper ground plane 21, the pin even carries out a direct heat evacuation path from the. component base on plan 21.

As an example of an embodiment, it can be indicated that dividers 0.6 mm thick have been made to receive chip holders with 40 outlets. The plan dimensions of the inserts were roughly double those of the chip carriers. The pin 20 had a diameter of the order of 3 mm.

 In the alternative embodiment shown in FIG. 3 (where the electrical junctions are not shown), the pin 20 consists of a core of heat-conducting glue, generally comprising a filler (alumina for example) bringing its coefficient of thermal expansion of that of ceramics. This core constitutes, with the intermediate plate 16, a heat evacuation path towards a heat dissipation plate 22. In the case shown in FIG. 4, the component 11 is associated with two cores 20 which pass through the intermediate plate 16 and attach directly to the component. Other arrangements are obviously still possible.

 The electrical connections between the conductive network 17 carried by the intermediate plate 16. and the circuit carried by the support 10 must not be subjected to any mechanical stress in the event of differential expansion (that is to say be of the "stress" type relief "in Anglo-Saxon terminology). This result is achieved by constituting the connections using sections of fine wire 18 (diameter between 80 and 100 microns). These sections or "strapps" have a regular shape and arrangement, so that their mounting can be easily automated.

One can in particular use copper wires covered with polyurethane / polyamide enamel and fix them by the method described in patent FR 75 31128 already mentioned.

 These sections of wires (of which only a few are shown in FIG. 1) are mounted between the tinned ends of the elements of the conductive network 17 and of the corresponding pads 13, also pre-tinned, provided on the support 10. These wires being not stretched do not exert any mechanical action on the contacts.

 Once the sections 18 have been put in place, as well as the equipotential links 14, all of the wires can be immobilized by a varnish. In particular, it is possible to use a weldable polyurethane varnish sprayed in the form of an aerosol through the meshes of a racket of wire coated with teflon, which effectively controls the entire network of wires. The assembly can finally be encapsulated in a polyurethane resin: this type of f-ixation leaves sufficient flexibility at the junctions to avoid constraints.

 In the alternative embodiment shown in FIG. 5 (where the members corresponding to those of FIG. 1 are designated by the same reference number), the component is fixed upside down on the support 10. The insulating cover is fixed by bonding: on the one hand, the adhesive can be flexible enough to absorb differential thermal expansion; on the other hand, the temperature differences remain smaller than in the previous case, the hood heating up less than the bottom. On the other hand, the heat dissipation path is less effective than in the previous case.

 We find, in the case illustrated in FIG. 5, the electrical junctions by wire sections 18 between the contact pads 15 on the bottom of the components 11 and of the pads 13 provided on the support 10.

 The invention is susceptible of numerous variant embodiments and, in particular, applies to very numerous types of cabling on the support 10.

Claims (11)

 1. Method for mounting, on a support (10) carrying a circuit, electronic components (11) to be fixed flat and provided, on their underside, with solderable contacts (15) and for connecting these components to the circuit, characterized in that the component is fixed flat to the support by mechanical connection means capable of tolerating differential expansions and that the contacts of the component are connected to the circuit carried by the support via sections of wires (18 ) ensuring mechanical decoupling.  2. Method according to claim 1, characterized in that the component (11) is mechanically fixed to the support by means of an intermediate plate (16) of plan dimensions slightly greater than those of the component, of a compatible material thermally with the component and in that the electrical connections are made by welding the contact pads (15) of the component on a conductive network (17) provided on the intermediate plate and projecting from the component and by fixing said sections of wires (18 ) between the network and the circuit.  3. Method according to claim 2, characterized in that the intermediate plate is made of ceramic having a coefficient of expansion close to that of the component and is secured to the support (10) by an adhesive, such as a polyurethane adhesive, having flexibility sufficient to tolerate differential expansions.  4. Method according to claim 2 (, u 3, characterized in that the conductive network is constituted by divergent tracks screen-printed and pre-tinned.  5. Method according to claim 4, characterized in that partially covers the network pa a strip (19) of protective glass, before pre-tinning.  6. Method according to any one of claims 2 to 5, characterized by at least one pin (20) associated with each interlayer (16), glued, molded or force fitted into the support, intended to improve the joining of the intermediate and possibly to increase heat transfers.  7. Method according to claim 6, characterized in that the support is a printed circuit having a metallic ground plane, said pin being in contact with the ground plane.  8. Method according to claim 1, characterized in that, the component having an envelope with a lower face provided with contact pads and an insulating cover, the component is fixed flat by its cover directly on the support using '' an adhesive having sufficient flexibility to withstand thermal expansion and in that the electrical connections are made using sections of wires (18) welded on the contact pads (15) of the component (10) and on the circuit.  9. Method according to any one of the preceding claims, characterized in that the son sections (18) are arranged in a regular distribution and welded automatically on pre-tinned pellets provided on the support.  10. Method according to any one of the preceding claims, characterized in that the equipotential connections of said circuit are formed by welding of the enameled wires, all the connections then being immobilized by depositing an insulating varnish.  11. Product comprising a support, such as a printed circuit, and electronic components provided, on their underside, with solderable contacts, characterized in that the components (11) are fixed flat to the support (10) by means mechanical link capable of tolerating differential expansion and in that the component contacts are connected to the circuit carried by the support by means of welded wire sections (18) ensuring mechanical decoupling.