GB2074791A - Mounting of circuit elements on substrates - Google Patents

Abstract

An electrical component e.g. a chip capacitor 10 has formed thereon conductive epoxy layers 12 about the terminations 15, 17 of the component. The layers 12 are electrically insulated from each other and after curing are each fused to a conductive epoxy local coating formed by screening, painting, dispensing, etc. on a substrate. The bond between the fused jackets of epoxy on the terminations of the chip and the underlying epoxy local coatings disposed on the substrate is electrically conductive and sufficiently mechanically reliable and rigid to withstand impact and thermal shock exposures. Polyamide may be substituted for epoxy for components to operate at high temperatures. <IMAGE>

Description

SPECIFICATION Mounting of circuit elements on substrates This invention relates to a method of mounting a circuit element to a substrate, and to a substrate assembly produced by said method.

For securing ceramic chip capacitors on to subs trates, prior art techniques produce adhesive bonds that frequently fail. It is difficult to maintain sufficient adhesion between the capacitor and substrate parti cularly where significant temperature and change of temperature are encountered, and the bond there between often fails under acceleration. If the capaci tor should become separated from the substrate, its functions are lost to the circuit. The result of indefiniteness of bonding is the risk of failure in the circuit.Efforts at forming adequate and reliable bonding between a ceramic capacitor and the subs trate have proved inadequate such that there is a persistent problem in the art as to how to maintain bonds between terminations, especially dip formed precious metal terminations, of the electrical ele ment and a conductive epoxy coating disposed on the substrate, which will maintain electrical conduc tion and successfully withstand thermal cycles and prevent separation therebetween. One prior art method has applied epoxy to a substrate and placed the precious metal terminations of the electrical elements in the applied epoxy to achieve an electric al/mechanical bond. This technique has exhibited degradation of the electrical conduction and mecha nical adhesion strength of the bond.

It is an object of the present invention to provide an improved method for bonding capacitors and other circuit elements to a substrate by means of an electrically conductive epoxy interface.

Primarily, the present invention provides an im proved method for bonding electrical components (i.e. circuit elements) to substrates through fusible electrically conductive plastics coatings forming termination jackets which are bonded or fused to compatible plastics local coatings formed on the surface of a substrate.

The invention also provides an improved and more economically producible electrical component in which the ends of the component are first subjected to dip coating, such dipped coating form ing electrically insulated spaced apart whole or partial jackets which provide an intermediate for bonding the electrical component to a substrate.

An important feature of the present invention is that electrically conductive coatings insulated one from the other are developed as sleeves or jackets over the opposite ends of the electrical component.

The sleeves serve as mechanical connections for the electrical components and form intermediate bond ings with the plastics local coatings on the substrate.

Another important feature of the present invention is that highly reliable bonds are formed between the jacketed precious metal terminations of the electrical component and the epoxy coatings on the substrate, wherein the jackets are each formed by application or dipping of the terminations of the component.

The single dipped precious metal termination are each coated with an epoxy or polyamide base containing a diffused system of particulate conductor metals such as gold, silver, palladium, platinum and alloys thereof. As a direct consequence of the present invention, there is an epoxy-epoxy interface bonding the circuit element to the substrate wherein the epoxy which forms mechanical and electrical connections with the terminations of the element is bonded to the epoxy on the substrate. The resulting bond is more efficient, is less susceptible to the detrimental effects of heat, and can more readily withstand the mechanical stresses and thermal stresses which are encountered in use.

Further features of the present invention will become apparent from a consideration of the following description which proceeds with reference to the accompanying drawings wherein a selected embodiment of the invention is illustrated by way of example and not by way of limitation.

In the drawings: Figure 1 is a flow chart illustrating the process by which a ceramic capacitor or other electrical component is secured to a substrate; Figure 2 is an enlarged detail view of a chip capacitor having conductive epoxy jackets formed by coating opposite terminations of the capacitor; Figure 3 is an exploded view illustrating the capacitor positioned on a substrate having complimentary local coatings of epoxy on the surface thereof; Figure 4 shows a detail in cross section on lines 4-4 of Figure 3 showing the capacitor mounted upon the substrate after fusion with an epoxy coating on the surface of the substrate; and Figure 5 is a cross sectional view along lines 5-5 of Figure 4 showing the capcitor after it has been joined by fusion with the epoxy coating on the surface of the substrate.

Referring now to Figures 1-5, a ceramic chip capacitor designated generally by reference numeral 10 has an epoxy coating 12 forming a jacket 14 one at each of the opposite terminations 16, 18 at the ends 15, 17 of the capacitor, respectively. The coating may be formed by dipping or the like, and the jackets so formed are electrically conductive.

While the specific composition of the epoxy coating is not a part of the present invention, one suitable epoxy composition for producing said coating is obtainable from an air dry thermoset epoxy composition containing conductive silver and supplied by DuPont under the identificaton 9960, being more fully described in "Electronic Materials" Bulletin E23581 published October 1978. The composition can be screen printed, dispensed, dipped, sprayed, or brushed. DuPont 9960 contains 60% silver and a thinner of methyl cellosolve acetate. It has a viscosity at 25 Cof 1.4-1.8 Pa.s(kcp) and a sheet resistivity of less than 0.2 ohms/sq. at 25 iim (1 mil) film thickness. The coverage of such product is approximately 83cm2/g at 50 ism (2 mil) wet film thickness.

Another possible epoxy composition is obtainable from Epoxy Technology Inc. and is known as EPO-TEKH31,assetforth in a publication published by Epoxy Technology Inc. 65 Grove Street, Watertown, Massachusetts 02172. The present invention is not, however, limited to these epoxy compositions.

Polyamide electrically conductive materials are also contemplated in the present invention. The composition of one suitable polyamide is setforth in a publication designated Ablebond 71-1 and published by Ablestik Laboratories, 833 West 182nd Street, Gardenia, California. The polyamide is usable in lieu of the epoxy for particularly high operating temperatures.

Once the desired thickness of the epoxy on the terminations has been developed, the capacitor is then joined to a substrate 20 having brushed, dispensed, or deposited thereon epoxy (or polyamide) local coatings 22 which are likewise electrically conductive and are electrically connected to a printed conductive pad 24 (Figure 3). The coatings 22 are fused with confronting portions of the coatings 12 forming the jackets 14 on the outer surface of terminations 16, 18. The capacitor 10 thus forms an epoxy-epoxy bond with the substrate 20 (Figures 4 and 5). Generally, it is preferred to use the same epoxy composition for the dip as the epoxy coatings on the substrate 20 in order to ensure reliable bonding.

It is a characteristic of the epoxy that, to achieve fused bonding, the epoxy coatings both on the capacitor and substrate should be cured sufficiently to removed all of the volatile constituents, thereby also minimising the effects of outgassing which would otherwise have a degrading effect on the semi-conductors also mounted on the surface of the substrate. The substrate coatings, as previously mentioned, can be screened, dispensed, brushed or otherwise suitably formed on the surface of the substrate, in one or more layers. The resulting connection provides a good mechanical connection through adhesion and one which is both electrically and mechanically stable under elevated temperatures, and which will not readily release the capacitor.That is, the fused interfacial connection (Figure 5) between the dip formed jackets on the opposite terminations of the capacitor and the confronting local coatings on the substrate is one which is strongly resistant to shear forces and will reliably retain the capacitor in its operative position notwithstanding changes in temperature and pressure, whether or not accompanied by external mechanical forces tending to dislodge the capacitor. As a result, the capacitor function is seldom lost to the system and the mechanical connection is a reliable one characterised by non-temperature sensitivity. The bonding described serves as a tenacious mechanical connection and additionally provides an electrical connection between the capacitor and the conductive coatings on the surface of the substrate.

It is readily possible to apply careful control on the dipping or other coating operation on the capacitor, since this electrical component can be pre-prepared and can originally be provided with a single epoxy layer over the precious metal terminations and later additional layers can be added to the terminations according to any particular design requirement. Part of the reason that the resulting bond between the capacitor and substrate is highly reliable is that the epoxy jacket is a mechanical and electrical connection forming a tightly held enclosure of epoxy around each termination which enables the electrical element to be retained, with the adhesive bonding between the jackets and the coating on the substrate forming a secure fused bond retaining electrical and mechanical connection.

Referring to Figure 1 for the process of manufacture, the procedure is to position capacitors in a series or row with one end of each capacitor exposed, all the capacitors being held jointly, and then simultaneously dip the exposed ends in a solution of conductive epoxy as indicated by the dipping step (reference numeral 26 in Figure 1).

Excess epoxy is then removed by blotting, step 28, and the coating thereafter air dried as designated by step 30. Steps 26, 28 and 30 are then repeated as steps 261, 281 and 301 for the other ends of the capacitors. Each capacitor termination may be dipped more than once if desired.

The jackets are then cured at a higher temperature according to step 32 and the finished product is then overlayed in relation to a substrate 20 bearing printed local coatings 22 of electrically conductive epoxy, these coatings of one or more layers being integrated into the remainder of the printed circuit on the base of the substrate 20. The capacitor having cured jackets 14 is mounted on the coatings 22 and the capacitor and substrate are fused, step 34. The combination is heated to fusing temperature by step 34to form a bond between the confronting surfaces of the epoxy jackets 14 and the epoxy coatings 22.

In practice, a plurality of such combinations may be simultaneously fused. The bonding or fusing occurs at temperatures above 150-1 750C for from 30 to 60 minutes. The resulting combination exhibits excellentthermostability at temperatures as high as 250 C.

Where polyamide coatings are employed, the cure is effected at 150 C for approximately ten minutes.

The resulting polyamide combination will withstand continuous operation at 3500C and intermittent exposure to 5000C without deteriorating the bond securement between the capacitor and the substrate.

Although the present invention has been illustrated and described in connection with the single example embodiment, it will be understood that this is illustrative of the invention and that various modifications thereof are possible within the scope of the invention as defined by the following claims.

Claims (10)

1. A method for mounting an electrical compo nent including the steps of coating one end of the electrical component having a conductive termination with a conductive epoxy or polyamide to form a first electrically conductive jacket over the termination, coating the other end of the component having a conductive termination with a conductive epoxy or polyamide to form a second electrically conductive jacket which is electrically insulated from the first electrically conductive jacket, applying conductive epoxy or polyamide local coatings on a substrate, and then fusing the jackets to the substrate coatings.

2. The method in accordance with claim 1 in which said coatings applied on the substrate are developed by screening or painting or dispensing or stamping.

3. The method in accordance with claim 1 or claim 2, wherein said epoxy or polyamide is made electrically conductive by containing dispersed particles of silver or palladium or gold or alloys thereof.

4. The method in accordance with any of claims 1,2 or 3 wherein the first and second electrically conductive jackets are cured prior to fusion with the coating on said substrate.

5. The method in accordance with any of claims 1 to 4 including the step of removing excess conductive epoxy or polyamide by blotting from the component or substrate to which it is applied.

6. A method of connecting an electrical component to a substrate substantially as hereinbefore described with reference to the accompanying drawings.

7. An electrical component having an electrical termination coated by a jacket about said electrical termination, said jacket consisting of conductive epoxy or polyamide whereby the conductive jacket may be bonded to a conductive epoxy or polyamide coating disposed on a substrate to connect the electrical component mechanically and electrically to the substrate and coating respectively.

8. The electrical component in accordance with claim 7 wherein spaced apart electrical terminations disposed at the ends of the electrical component are each coated by a jacket of conductive epoxy or polyamide and a corresponding complementary set of local coatings are disposed on said substrate whereby the jackets may be bonded to said coatings.

9. The electrical component in accordance with claim 7 or claim 8 wherein the conductive epoxy or polyamide jacket is cured and thereafter fused with the epoxy or polyamide coating(s) on said substrate.

10. A substrate assembly produced by fusing the electrical component of any of claims 7 to 9 to a substrate, in accordance with the method of any of claims 1 to 6.