Modeling joining materials for microelectronics packaging

Modeling of solder-substrate interactions together with careful experimental work can provide a good basis for developing new materials such as conductive adhesives and Pb-free solders as well as fluxless soldering processes for microelectronics packaging. The modeling of the solder-substrate interactions will in effect lead to a rationalization of the trial and error methods commonly employed and hence minimize the number of experiments required. It provides useful information on the chemical reactions, stabilities of various microstructures and growth rates of reaction products during joining or in use of electronic devices. This is of particular importance in the case of small solder volumes, 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 investigated in the Sn-Bi/Cu and Sn-Bi-Zn/Cu systems with and without adhesives. The solder-substrate-environment interactions under high purity reducing gases have been studied also with the meniscograph. The reduction of surface oxides, formation and the stability of the intermetallic 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 explanation concerning the strong dewetting effect of zinc-containing solders found experimentally is given. An emphasis was placed also on the solder-substrate-environment interaction by studying the effect of humidity on chemical stability of microjoints in an epoxy-based adhesive