Influence of alloy composition on fillet-lifting phenomenon in tin binary alloys

The influence of alloy composition on the fillet-lifting phenomenon was investigated for Sn−Bi, Sn−Pb, and Sn−In binary alloys. Fillet-lifting occurs in Sn-(1–30%)Bi, Sn-(1–5%)Pb, Sn-(2–15%)In, but does not occur in pure Sn, Sn-(40–62%)Bi, Sn-(10–45%)Pb, and Sn-43 In. Fillet-lifting does not correlate with the formation of a Bi-concentrated layer at the solder/Cu land interface, previously thought to cause fillet-lifting. The solidification temperature range also does not necessarily correlate with fillet-lifting. Fillet-lifting was found to be related to the retention time (t_r) for which solidus temperature (the final solidification temperature) remains by latent heat released during eutectic solidification in the cooling curve. The released latent heat contributes to the alleviation of the temperature gradient causing fillet-fifting in the solder joint. A fillet-fifting resistant index (FRI) is proposed as a new index for fillet-lifting, and correlates well with fillet-lifting occurrence rate in each of the studied tin binary alloys.