FR2479639A1 - Electronic component mounting process for PCB - uses metal bridges in contact with metal tracks to permit substrate flexing without damage to solder joints - Google Patents

Abstract

An electronic component is mounted on a substrate having different mechanical characteristics by providing printed circuit tracks or metallisation of e.g. copper on the substrate. The component is supported in spaced relationship to the substrate by soldering its connector leads to respective bridges, secured in electrical contact with the copper tracks. The tracks are provided by normal printed circuit methods, and after covering the relevant part of the substrate with resist the bridges are deposited as, e.g. nickel coated chrome and the resist then removed chemically. A globule of paste or the like flexibly supports the middle of the component on the substrate and acts as a heat conductor. The mounting allows flexing and expansion/contraction of the substrate without damaging the component solder joints.

Description

 The invention relates to an assembly device between elements of different mechanical characteristics. It relates more precisely to the assembly of electronic and microelectronic components, the concept of assembly encompassing mechanical assembly and the electrical connection of the components. between them and on the substrate that supports them.

The invention also relates to the method for producing the device described.

 The problem of assembly between elements with different mechanical characteristics arises when components of a different nature must be brought together such as a ceramic micro-box for the encapsulation of integrated circuit wafers or ceramic capacitor for example, these components do not thus not having the same behavior with respect to mechanical stresses, in rigidity or in bending, or with respect to thermal stresses, in expansion, which amounts to a mechanical stress between the assembled elements.

 A first case of assembly of elements whose mechanical characteristics are different is given by the soldering of ceramic micro-boxes as they are used in hybrid circuits for the protection of integrated circuit chips, and which are known under the name of chip-carrier, on a substrate of a type such as glass-epoxy, commonly called printed circuit. The chip-carrier made of alumina, or more generally of ceramic, is rigid and brittle, while the printed circuit is flexible compared to the ceramic of the chip-carrier. This results in the risk of breakage when during the handling of the printed circuit it bends, the chip-carrier being endowed with no flexibility.

 This type of chip-carrier assembly on a printed circuit is however extremely interesting. Indeed, hybrid circuits elegantly solve a certain number of problems linked, as their name suggests, to the association of discrete non-integrable components and integrated circuits.

However, the hybrid circuits are limited in their extension and in their applications by the dimensions of the substrates on which they are mounted according to the known art. In fact, these substrates are very generally ceramic or glass plates, the dimensions of which are limited to a few millimeters of coast: larger dimensions would require a prohibitive thickness to avoid the fragility of the substrate. Mounting hybrid circuits on printed circuits is therefore a solution which makes it possible to multiply the integration density and increase the complexity of the electronic devices produced. Indeed, printed circuits can be produced in larger dimensions than the ceramic plates commonly used for hybrid circuits. However, as has just been said, printed circuits have, even if they are called rigid, a certain flexibility relative to the components which are fixed to their surface and which have no flexibility.

 A second example of assembly between elements with different mechanical characteristics can be given by the assembly of a ceramic capacitor, called "chip" that is to say delivered in a monobloc element without wires of output connections, this ceramic capacitor being fixed on a hybrid circuit, the substrate of which is a rigid enamelled sheet. During temperature storage tests that have become compulsory before the delivery of all electronic equipment, or quite simply during the operation of the equipment comprising the hybrid circuit, the temperature released by Joule effect in electronic circuits tends to expand the enamelled sheet substrate more than the ceramic capacitor; it follows the risks of rupture due to the different expansions between the two assembled elements, capacitor and substrate, these risks of rupture resulting in a tearing at the welds. However, enameled sheet substrates are interesting, in the same way as glass and epoxy substrates because of the large dimensions that it is possible to produce in enameled sheet compared to the ceramic substrate and by the low price of such substrate.

 Theoretical models and experiments, undertaken among others by the applicant company, have shown that only a substrate whose mechanical properties, and in particular the coefficient of expansion, the modulus of elasticity and the Poisson's ratio, are adapted to those of the insert could be used. For components of the chip-carrier capacitor and micro-box type, which are with the resistors those of the components which are most frequently used in hybrid circuits, it is not possible to mount them on rigid printed circuits or on sheet metal. enamelled steel: cracks and weld tears are inevitable.

 This problem is solved by the assembly device according to the invention in which the connection between the elements is provided by locally deformable connections which fulfill the dual function of mechanical supports for one of the two components and of electrical connection between the two elements. These locally deformable connections are made by supplying bridges on the conductive layer to the surface of the substrate which are themselves made of a metal different from the conductive metal supported by the substrate: in the conventional case where it is a layer of copper which covers the printed circuits the copper conductor is coated with a metal which attacks chemically less quickly than copper for example chromium or nickel, then one end of the conductor under the chromium or nickel layer is chemically machined to let the chrome form a bridge. The element to be added is welded to the bridge, the deformation of which, depending on mechanical or thermal constraints, prevents the rigid element from breaking relative to the flexible or expandable element.

More specifically, the invention relates to an assembly device between electronic components of different mechanical characteristics, with a view to ensuring their mechanical and electrical connection by means of their metallic connections, characterized in that:
- first, the components are separated by a spacing, which allows the movement of one component relative to the other, under the effect of mechanical stresses;
- secondly, the connection between components is provided by bridges, locally deformable metal strips, one end of which is welded to a connection of a first component and the other end partially covers a connection of a second component, connection of which the thickness of the metal layer provides the bridge with a spacing relative to the second component, which allows the deformation of the bridge under the effect of the stresses imposed by the components.

The invention will be better understood from the explanations which follow, which are based on a practical embodiment and on figures which represent:
- Figure I the classic mounting of a chip carrier on a substrate, seen in section;
- Figure 2: mounting a chip carrier on a substrate according to the invention, seen in section;
- Figure 3: mounting a chip carrier on a substrate according to the invention, seen in plan;
- Figure 4, 5 and 6: the different stages of the manufacturing process;
- Figure 7: a first improvement to the invention;
- Figure 8 a second improvement to the invention;
- Figure 9: a third improvement to the invention.

 In order to simplify the disclosure and to clarify the drawings, the invention will be explained by relying on the case of assembling a chip-carrier ceramic micro-box on a substrate.

 Figure 1 shows the known art in the field of fixing a chip carrier on a substrate.

 A ceramic or glass substrate 1 supports copper metallization strips 2 on which are directly soldered connection outputs 3 of a micro-housing 4. The solder wets in 5 the output connections 3 and the metallization strips 2 .

 The scale adopted to represent this mounting of micro-housing on a substrate, and especially the thickness, reinforced on the drawing, of the copper metallizations 2 does not make it easy to realize that the micro-housing is in fact very close of substrate 1: it is only separated therefrom by the thickness of the metallizations of the order of 30 microns. Consequently, if the substrate was a flexible substrate - which is not the case since it has been specified that in the known part it is a ceramic or glass substrate - the microbatter could not follow the substrate in its movements and the welds would be irremediably torn off by shearing either following a bending of the substrate, or following a different expansion between substrate and micro-box.

 Figure 2 shows the fixing device between the elements according to the invention.

 The substrate 6 has the particularity of being endowed with mechanical and thermal characteristics inter alia different from those of the element 4 which must be assembled to it. This substrate 6 comprises, as in the example of the known art, copper metallization strips 2. The chip-carrier ceramic micro-housing 4 or the element which is to be mounted on the circuit is joined to the metallization strips 2 by via locally deformable connections 7 which are made of nickel-coated chrome These connections 7 are integral with the metallization strips 2 in region 8, but on the other hand they are isolated from the substrate 6 in region 9, region in which the copper of the metallization strip 2 has been etched chemically after depositing the chromium layer 7. The outlet connections 3 of the component 4 are welded to the deformable connections 7 and wetted by a drop of solder at 5.

 It is convenient, and this greatly facilitates the adaptation of the component 4 to the deformations of the substrate 6, to start by depositing a drop of glue 10 in the center of the component 4. This drop of glue acts as a fixed point on either side of which component 4 can tip over: the drop of glue 10 therefore makes it easier to maintain connections 7 when they are subjected to the soldering process (wave, hardened, etc.). Furthermore, it provides thermal bonding with the substrate if necessary. Indeed, the drop of glue takes part in the evacuation of the calories released by the element supported by the chip carrier when it is under tension. In fact, due to the suspended position of the chip carrier, calories, if there was no drop of glue, could only be evacuated through the connection bridges 7. Finally, the drop of glue s opposes the vibrations of the chip carrier, which could destroy the connections, or the welds.

 FIG. 3 represents the mounting of a component on a substrate according to the invention according to a plan view. In 6 is represented a fraction of the substrate, which extends well beyond what is represented and which supports the metallic connections, generally in copper 2. Beyond the ends of these connections in copper 2, the bridges have been made to grow in chromium 7 onto which the component 4 is then welded, with an offset relative to the plane of the substrate 6. FIG. 3 represents the case of a chip-carrier provided with output connections on two faces; it is obvious that the invention applies to the case of components having only two output connections, or being provided with output connections on four sides.

 Figures 4, 5 and 6 show the manufacturing process of the assembly device according to the invention.

 In FIG. 4, it can be seen that a substrate 6 is covered with a conductive layer of copper 2. On this conductive surface, and by suitable means known to those skilled in the art, that is to say by photomasking, a photoresist film 11 is deposited which delimits the future locations of the connections 7 to be made. After the photomask operation, the connections 7 made of a material different from copper, for example chromium, are grown by electrolysis or sputtering processes. It is these connections 7 which will later connect the element to be welded to the substrate; then a layer of a weldable element, for example nickel, gold, etc. is deposited using the same methods.

 The first layer of photoresist 11 is dissolved and replaced by a second layer of photoresist 12 which no longer conceals the regions of copper 2 which must be attacked and which partially covers the connection 7 in the area where this connection must remain integral with the copper layer 2 which it covers. The copper parts left bare by the photoresist 12 are then attacked by conventional routes based on acid dissolution.

 FIG. 6 represents the structure obtained at the end of the manufacturing operations. We see that on the substrate 6 a connection 2 partially supports a connection 7 made of a different material, such as for example chrome and nickel The connection 7 is integral with the connection 2 in the region 8 but is free and can deform relative to to the substrate 6 in the region 9. It is this region which provides flexibility and allows the free expansion of the component, expansion absorbed by deformation of the connection 7 cantilevered above the substrate.

 FIG. 7 represents a first improvement made to the invention. By comparison with FIG. 6 we see that a varnish, a photoresist or more generally a generally organic flexible and deformable material, 13, was included between the bridge 7 and the substrate 6. The filling of the space 9- by a flexible material 13 has the function of locating the tinning which is operated subsequently at the end of the bridge and of preventing the welding alloy from being deposited around the bridge which would then make it rigid.

However, it has been clearly specified that the layer 13 is flexible and that consequently it does not oppose the deformations of the connection 7 when the latter is subjected to a stress.

 FIG. 8 represents another improvement to the invention. 2 being the end of a copper connection supported by the substrate, seen in plan, the chromium connection 7 is advantageously pierced with holes 14. This operation is fairly simple to perform by photomasking during the etching operation copper 2 which has been reported on the occasion of FIG. 5. The presence of holes, which can also be replaced by notches on the edges of the connection 7, makes it possible to reduce and facilitate the duration of the lateral attack copper: this operation is actually quite delicate since the connections are much wider than the distance which separates them from the substrate and therefore the copper is attacked from the side. The presence of holes or notches in the connection 7 also makes it possible to give greater flexibility to this connection.

 FIG. 9 represents a third possible improvement to the invention. It is seen in plan like the previous one and shows that the deformable connection 7 can advantageously be assigned curvilinear shapes, which are not linear like those which have been shown previously to simplify the drawings. Thus if the deformable connections are drawn in S for example this structure makes it possible to better adapt the assembly to the difference of the mechanical stresses which are opposed there by the two assembled parts.

 The present invention is not limited to the embodiments which have been explicitly described above; it includes the various variants and generalizations thereof included in the field of claims below.

Claims (8)

 1. Assembly device between electronic components of different mechanical characteristics, with a view to ensuring their mechanical and electrical connection by means of their metallic connections, characterized in that:  - First, the components (4 and 6) are separated by a spacing, which allows the movement of one component relative to the other, under the effect of mechanical stresses;  - secondly, the connection between components is ensured by bridges (7), locally deformable metal strips, one end of which is welded to a connection (3) of a first component (4) and the other end partially covers ( in 8) a connection (2) of a second component (6), connection (2) whose thickness of the metal layer provides the bridge (7) with a spacing (in 9) relative to the second component (6), which allows the deformation of the bridge (7) under the effect of the constraints imposed by the components.  2. An assembly device according to claim 1, characterized in that the bridges (7) are made of a metal different from that which constitutes the connections (2) that they partially cover, and insoluble in the chemical solutions of attack on said connections (2).  3. An assembly device according to claims 1 and 2 characterized in that the bridges (7) are constituted by rectilinear metal strips.  4. Device according to claims 1 and 2, characterized in that the bridges (7) are constituted by curvilinear metal strips.  5. Device according to any one of claims 3 or 4, characterized in that orifices (14) formed in the metal strips of the bridges (7) promote the deformation of the bridges and the chemical operations involved.  6. Device according to any one of claims 1 to fi, characterized in that a drop of glue (10) placed between a first (4) and a second (6) component serves as a tipping point of the components, the one over the other.  7. Device according to any one of claims 1 to 5, characterized in that a layer of flexible material, of organic material (13) introduced under the bridges (7), in the space (9) provided for their deformation, is opposed to this spacing being filled by welding during the welding operations of the first component (4) on the bridges (7).  8. Method for producing an assembly device between electronic components, characterized in that it comprises the following operations:  a) masking by photosensitive resin (11) on a component (6) provided with a first metallic layer (2) leaving bare the locations of future bridges (7).  b) deposition, by known means, of the second metal layer, constituting the bridges (7) in the locations provided by the first mask (11).  d) partial dissolution by chemical means of the first metallic layer, including in its part (9) underlying the second metallic layer in which it undergoes lateral dissolution.  c) dissolving the first layer of resin (11) and masking with a second layer of photosensitive resin (12) which delimits the tracing of the connections in the first metallic layer (2) as well as the part (8) of the second metallic layer which covers the first metallic layer  e) tinning of the bridge (7) left cantilevered by the dissolution of the part (9) of the first metallic layer.  f) welding, by the connections, of the other component deposited on the deformable bridges (7).