MODELING JOINING MATERIALS FOR MICROELECTRONICS PACKAGING

Modeling of solder-substrate interactions together with careful experi mental work can provide a good basis for developing new materials such as conductive adhesives and Pb-free solders as wed as fluxless solder ing processes for microelectronics packaging. The modeling of the sold er-substrate interactions will in effect lead to a rationalization of the trial and error methods commonly employed and hence minimize the n umber of experiments required. It provides useful information on the c hemical reactions, stabilities of various microstructures and growth r ates of reaction products during joining or in use of electronic devic es. This is of particular importance in the case of small solder volum es, since the compositions and microstructures of solder alloys can be entirely altered by the solder-substrate reactions during the joining . As specific examples the solder-substrate reactions have been invest igated in the Sn-Bi/Cu and Sn-Bi-Zn/Cu systems with and without adhesi ves. The solder-substrate-environment interactions under high purity r educing gases have been studied also with the meniscograph. The reduct ion of surface oxides, formation and the stability of the intermetalli c layers, Cu3Sn and Cu6Sn5, and the growth of brittle Bi layer in the microjoints due to the change of the composition of the solder filler was studied both theoretically and experimentally. Moreover, an explan ation concerning the strong dewetting effect of zinc-containing solder s found experimentally is given. An emphasis was placed also on the so lder-substrate-environment interaction by studying the effect of humid ity on chemical stability of microjoints in an epoxy-based adhesive.