INTERMETALLIC GROWTH IN LEAD-FREE SOLDERS ON PCB SUBSTRATES

摘要

The morphology and thickness of the intermetallics that form at the interface between solder and the joint substrate, factors that can affect the reliability of the joint, are influenced by the composition of the solder and the surface finish. The change to lead-free forced by the EU RoHS directive means that the electronics industry is dealing with some new combinations of solder alloys and substrate finishes. Lead does not form intermetallics with copper and tin remains a constituent of lead-free solders so that the basic composition of the interfacial intermetallic remains basically the same in lead-free assembly as that in tin-lead assemblies, mainly Cu6Sn5. However, some changes in morphology and growth rates can be expected. The widespread adoption of a nickelmodified tin-copper eutectic alloy as both a lead-free solder and a HASL finish introduces a new element with a reported effect on intermetallic behaviour into the system. In this paper a study of the thickness, morphology and rate of growth in thermal cycling of the intermetallic at the interface between some widely used solders and finishes is reported with particular attention being paid to any effect of the nickel addition in the basic tin-copper eutectic.