Mechanism of fillet lifting in Sn-Bi alloys

The probability of fillet lifting in through-hole solder joints was determined for a series of seven binary Sn-Bi solders with Bi mass contents between 26% and 65%. A 20-pin ceramic dual inline package was soldered to printed wiring boards with three board thicknesses and two pad sizes. A laboratory scale drag soldering process designed to simulate wave soldering was employed. Composition has the largest effect on failure with a peak in lift-off probability near 8%Bi. The presence of intermetallic needles of Cu6Sn5 attached to the pad side of the separation surface, but pulled from the solder side, indicates that fillet lifting occurs during solidification. This conclusion is further supported by real time observation of fillet lifting. The solidification of these alloys is analyzed using a Scheil analysis and the tendency for fillet lifting is related to a hot tearing criterion originally developed for castings.