Conductive Adhesives

S ince the advent of through-hole printed wiring boards in the 1950s, surface-mount technology has advanced through chip capacitors and resistors to high-density multilayer hybrids and printed circuit boards. Throughout this 40-year span, solder paste has been the accepted technology for component attach.' The present trend toward fine-pitch resolution ( t0.020") and multilayer constructions of dissimilar coefficients of expansion (CTEs) has pushed solder paste technology to its mechanical limits. Furthermore, the need to reflow solder at temperatures above 200°C can damage electrical components and substrates. Mounting environmental awareness of the effects of CFCs on the ozone layer as well as concern over the toxicity of lead have also stimulated interest in solder paste alternatives. The Executive Office has mandated the elimination of CFC usage in component assembly by December 31, 1995. Some manufacturing houses have set even more aggressive timetables?. 3.4, To successfully replace solder in surface-mount applications, a conductive adhesive must meet several criteria. It must be screen printable to fine-pitch resolutions (<0.020") while maintaining a work life of at least 24 hours. Electrical and adhesive properties must remain stable through thermal aging, thermal cycling and humidity exposure. The material must be reworkable, eliminate the need for environmentally harmful cleaning procedures and be curable in the same time frame as a normal solder reflow operation but at temperatures below 200OC.