Pb-free solders for flip-chip interconnects

A variety of lead-free solder alloys were studied for use as Pip-chip inter connects including Sn-3.5Ag, Sn-0.7Cu, Sn-3.8Ag- 0.7Cu, and eutectic Sn-37P b as a baseline. The reaction behavior and reliability of these solders wer e determined in a Pip-chip configuration using a variety of under-bump meta llurgies (TiW/Cu, electrolytic nickel, and electroless Ni-P/Au). The solder microstructure and intermetallic reaction products and kinetics were deter mined. The Sn-0.7Cu solder has a large grain structure and the Sn-3.5Ag and Sn-3.8Ag-0.7Cu have a fine lamellar two-phase structure of tin and Ag,Sn. The intermetallic compounds were similar for all the lead-free alloys. On N i, Ni,Sn,formed and on copper, Cu6Sn5 Cu3Sn formed. During reflow, the inte rmetallic growth rate was faster for the lead-free alloys, compared to eute ctic tin-lead. In solid-state aging, however, the interfacial intermetallic compounds grew faster with the tin lead solder than for the lead-free allo ys. The reliability tests performed included shear strength and thermomecha nical fatigue. The lower strength Sn-0.7Cu alloy also had the best thermome chanical fatigue behavior. Failures occurred near the solder/intermetallic interface for all the alloys except Sn-0.7Cu, which deformed by grain slidi ng and failed in the center of the joint. Based on this study, the optimal solder alloy for flip-chip applications is identified as eutectic Sn-0.7Cu.