The Spontaneous Growth of Pb Whisker from the Pb60Sn40 Solder Layer on the Au Coating

In a recent failure analysis case of relay, the pure Pb whisker growth is found on the Pb60Sn40 solder layer on the Au coating of the spring leaf. The content analysis by the EDX shows that the Pb whisker grows from the Pb grain, which is surrounded by the AuSn intermetallic compound. It is thought that the elastic force of the spring leaf and the compressive stress from the intermetallic compound contributes to the Pb whisker growth, and the later factor is confirmed by the FIB (focused ion beam) result. Besides, the statistics result shows that the longer Pb whisker corresponds to the thicker Au coating, which could be explained by the effect of Au: forming AuSn intermetallic compound and Pb rich area. This work enriches our understandings on the metal whisker growth and consequently potential failure mode of SnPb solder.