Lift-off phenomenon in wave soldering

The mechanism of lift-off, which is the serious defect formation that the solder fillet is peeled off from a Cu land pad on a printed wiring board in wave soldering, has been examined primarily by using basic Sn-Bi alloys (Sn at 2-5 wt%) and solidification simulation. The probability of lift-off increases with increasing Bi content. Lift-off results from the segregation of Bi, which diffuses in a short distance, in the interfacial region between a Sn-Bi alloy and a Cu land pad during dendritic structure formation. A higher fillet results in a more severe lift-off-the top side of a printed wiring board, where the fillet is low, seldom shows lift-off. The solidification simulation indicates that the solidification of the solder fillet rapidly propagates from a lead wire to a fillet edge. A Cu land acts as a heat sink, with heat flowing from the inner through-hole to the land pad. Both rapid cooling and annealing at high temperature can effectively prevent lift-off. (C) 2000 Acta Metallurgica Inc. Published by Elsevier Science Ltd. All rights reserved.