Adaptlive Solder Paste Composites Smart Solder Pastes

Abstract

111 the ficld of surfacc mounting. thc nicchanical propcrtics of thc soldcr paste alloy pla! n fundan1cnlal rolc. In addition 10 cicctric conncction. the allo? also mcchanicalll sccurcs the component 10 thc interconnection mcdiuni It should also bc noted that Illcoretical approachcs for determining thermomcchanical constraints arc rcali~ed b? assuming that . (1) elastic constants do not v a n uith temperature due to the lack of data on how thesc constants vary with tempcraturc: and (2) thc materials arc assumed to behavc clastically. This I S substantiated bj the fact that solder joints start cracking when subjected to more than 1.000 rapid temperature qclings. This is tmc even when the packages and the substrate have the same mechanical properties (e.g.. ceramic chip carrier on alumina substrate). Moreover. Boeing demonstrated that thc cracking mechanism arose from the cxpansion of the solder in the Z-axis. As a result. France's Ministp of Research and Indusq funded a key study in 1989 covering this topic. In this paper. we present heretofore unpubltshed results on this study. In pan I. thc results obtained from thermomechanical measurements raken in the elastic and plastic domains on two alloys. Sn Pb and Sn Pb Ag. are discussed. In pan 11. the innovative part. focus is on the realization of a solder paste composite composed of a solder paste standard and inclusions such as nickel and nickel-coated graphite Properlies of the new solder composite alloy were measured on standard samples and are reponed here. The process is still not industrialized. Perspectives look good for realizing families of smart solder pastes tailored to specific applications primarily with the use of shape memory alloys (SMA) as inclusions.