Thermal Effects of Replacing Solder with Conductive Adhesives

Abstract

An important disadvantage of conducting adhesives is their inferior heat conductivity when compared with soft solder such as Sn60Pb40. Thermal simulations, however, show that, by using thinner layers of adhesive than of solder, the module's thermal resistance does not increase greatly. Test modules with four different silver filled epoxy adhesives and tin/lead solder were manufactured. These test modules contained power diodes, 30 A, 1000 V, die bonded onto Ag/Pt thick film conductors on alumina. The die bond adhesive layer thicknesses were typically 30 or 40 μm. For die bond solder layers the thickness was 90 μm. The alumina substrates were connected to 3 mm thick copper plates with filled epoxy or silicone adhesive. The thickness of these layers was 150 μm or 50 μm, respectively. Thermal resistance of the structures was measured. The results showed that good adhesion between joined surfaces is essential for optimised heat flow. The heat conductivity of an adhesive was only a secondary factor affecting the structure's thermal resistance. When the adhesive joint is of good quality, the replacement of solder with conductive adhesives does not increase the module's thermal resistance any more than as shown by the simulations. It should, however, be remembered that the printing of thin (< 20 μm) uniform layers is not always possible.

Citation

Spalding, O. (1993), "Thermal Effects of Replacing Solder with Conductive Adhesives", Microelectronics International, Vol. 10 No. 2, pp. 26-28. https://doi.org/10.1108/eb044498