Lead-Indium for Controlled-Collapse Chip Joining

Lead-indium for flip-chip solder ("controlled-collapse") connections has been investigated as a possible alternative to the commonly used 5-95 tin-lead. In thermal cycle tests, lead-indium was substantially mote resistant than tin-lead to fatigue resulting from chipsubstrate mismatch. The lead-indium that has been studied most extensively, 50-50 by weight, gives a 3:1 improvement in lifetime over 595 and is compatible with standard chip and substrate processing. Evaporated above a Cr-Cu-Au pad on the chip surface, the solder is eventually joined to tinned Cr--Cu lands on the ceramic substate. Because of its lower melting point, lead-indium can be joined at a lower temperature than 5-95 tin-lead. In its reactions with copper and gold, which are present in chip and/or substrate metallizations, indium resembles tin. Gold, present only as the protective top film of the chip metallization, dissolves completely during chip joining, whereas copper is present in sufficient quantity to exceed its solubility limit, and thus forms copper-indium intermetallic layers at both interfaces. To ensure terminal strength and reliability, these layers should be stable and adherent both to the solder and to the underlying metnllizations. Lead-indium has been found to be more susceptible than lead-tin to corrosion in high-humidity and hostile environments. This susceptibility must be borne in mind when lead-indium is considered for nonhermetic applications.