Impact of Lead Contamination on Reliability of Lead Free Alloys

摘要

Meeting the RoHS directive will require the transition from the historical tin-lead based system of materials to one that does not contain lead. This is of course is not straight forward as it is impossible to control 100% component material compositions. It is therefore entirely possible that components with lead finishes will find their way into Lead Free processes. The concern is that lead contamination occurring in this way results in 1 to 10% level of lead, and that this lead is not uniformly distributed, but segregates to the grain boundaries and joint interfaces. But what is the effect of this on thermal fatigue. The work reported here will describe experiments where the lead level in solder joints was controlled by altering the plating on component terminations and using controlled solder compositions. Microstructural examination verifies the segregation of lead. The built assemblies were then thermally cycled between -55 and 125°C for 2000 cycles to assess this effect on reliability. Hot peel tests were also run to simulate problems that may occur is secondary wave operations where the fillet strength collapses and components can detach with little force at temperatures above 180°C. The use of indicator test kits to detect lead were also evaluated on this set of reference samples. This paper will discuss these results and the likely impact on the industry and the necessary precautions.