Lead-free solders for high-reliability applications: high-cycle fatigue studies

摘要

The use of lead (Pb) in solders for electronic connections is now extensively restricted in Europe, with its use likely to be phased out completely in the medium term. Although Pb-free solders have been the subject of much research, little investigation has been carried out into their reliability for applications exposed to vibration in service. Aerospace applications, which have service lives measured in decades, are of particular pertinence. The present work shows the development and validation of a method for testing small, model solder joints in high-cycle fatigue. The tests are conducted using common equipment yet provide fast results and objective comparisons between solders without the influence of PCBs or components, which typically obscure the solders’ intrinsic contribution. S-N diagrams are presented which compare the performance of traditional Sn-Pb solder to that of Pb-free alloys at room and high temperatures and with copper and nickel substrates. It is found that in all situations the Pb-free alloys offer lower lifetimes to failure than the traditional Sn-Pb, an unexpected result when considering the inferior mechanical properties of the latter. The large disparity at room temperatures and with copper substrates is significantly reduced by elevated temperatures and by soldering to nickel substrates. In order to investigate these results, a number of techniques are employed. In addition to extensive fractography, the damping capacity of the solders is investigated and a scanning acoustic microscope is used in conjunction with resonant decay tracking of specimens to study the crack propagation paths prior to complete failure. The analysis of results focuses on the possible causes for this performance difference, drawing on existing soldering literature and wider engineering principles. It is concluded that the overall pattern of results presents contradictory evidence for the contribution of various factors, such as yield strength or interfacial adhesion, which are hard to reconcile. It is thought likely that more numerous fatigue initiation sites in the Pb-free alloys are responsible to some degree for their lower cycles to failure, although more research into the effect of substrate and interfacial intermetallics is necessary to determine the mechanism by which these influence the results, in the absence of relevant fractographic evidence.