Pb Whisker Formation in the Au-Pb System

Metal whiskers are fine protrusions (approximately 1 – 10 microns in diameter and which can be millimeters long) that have been known to cause electrical failures due to shorting of electrical connections or even allowing metal vapor plasmas to form with the application of sufficient voltage and current in vacuum. Although metal whiskers are a known phenomenon that occur most commonly in Sn, Zn, and Cd whiskers of pure Pb have also been observed. Sines (1960) first showed that Pb whiskers could be extruded when large compressive forces are applied by a ring clamp to a Pb-plated surface. We have also found that Pb whiskers, approximately 0.1 microns in diameter and approximately 50 microns long, can evolve after high temperature exposure (1000 hours at 150°C) when a Pb-containing solder is reflowed on a Au-plated surface. The growth of these whiskers is related to the formation of intermetallic compound (IMC) phases on the solder surface. It is hypothesized that, similar to Sn whiskers, Pb whiskers can form due to the relief of stresses caused by IMC growth, including AuPb2 and AuPb3 IMCs. Alternatively, the decomposition of the metastable AuPb3 IMC to AuPb2 is also a possible but, perhaps, less likely cause. The conditions that may lead to Pb whisker formation as well as the risk of Pb whiskers causing electrical failure in different applications will be discussed.