Effects of Flux and Reflow Parameters on Lead-Free Flip Chip Assembly

The melting temperatures of most lead-free solder alloys are somewhat higher than that of eutectic Sn/Pb solder, and many of the alloys tend to wet typical contact pads less readily. This tends to narrow down the fluxing and mass reflow process windows for assembly onto typical organic substrates and may enhance requirements on placement accuracy. Flip chip assembly here poses some unique challenges. The small dimensions provide for particular sensitivities to wetting and solder joint collapse, and underfilling does not reduce the demands on the intermetallic bond strength. Rather, the need to underfill lead to additional concerns in terms of underfill process control and reliability. Relatively little can here be learned from work on regular SMT components, BGAs or CSPs. The present paper addresses the effects of reflow profile, pad metallurgy, and amount of flux on the assembly of flip chips with lead-free solder bumps onto organic substrates. Special attention was paid to the 85.9Sn/3.1Ag/10In/1.0Cu alloy, which has a relatively low melting point of 197C. Good wetting and a robust collapse could, however, not be achieved with flip chip relevant no-clean fluxes developed for eutectic Sn/Pb, even with peak reflow temperatures approaching 250C. In fact, both 95.5Sn/3.5Ag/1.0Cu and 95Sn/5Sb were seen to perform much better at such temperatures. Given the relatively high melting point of the latter, a Sn/Ag/Cu alloy would seem to offer the more acceptable process windows.