Occurrence of tin pest on the surface of tin‐rich lead‐free alloys

Abstract

PurposeThe purpose of this paper is to investigate tin pest formation in lead‐free alloys.Design/methodology/approachSamples of Sn99.5Ag3.0Cu0.5, Sn99Cu1 and Sn98Cu2 alloys were prepared in four different forms. The first group was prepared using traditional PCB technology and a hand soldering method. The next group of samples was composed of as‐received ingots of these alloys. To check the impact of mechanical treatment on the transformation process, additional cold‐worked and cold‐rolled samples were prepared (30 kN). All samples were placed initially either at −18°C or at −65°C for low temperature storage testing. Visual observations, scanning electron microscopy observations and X‐ray diffraction analysis were performed to identify the transformation process. Additional samples were prepared using a force of 75 kN and placed in a chamber at a temperature of −30°C for long‐term testing.FindingsThe detectable symptoms of tin pest in samples subjected to mechanical processing with 1 and 2 wt.% of Cu addition stored at −18°C were observed at the edges of the samples after 17 months of storage. Further aging at −18°C showed the progress of α/β transformation with time under low‐temperature stress, but only in these specimens. With the application of greater force to the pressing process (75 kN instead of 30 kN) and at a temperature of storage close to the maximal transformation rate (−30°C), there was a significant acceleration of the α/β transformation, and this dependence can be used in predicting the risk of tin pest occurrence in various lead‐free alloys.Originality/valueThe paper shows that the degree of mechanical processing had a great influence on the α/β transformation rate. Based on these observations, it is proposed that such mechanically processed samples can be used for accelerated testing of tin‐rich lead‐free alloys at low temperatures. Such tests would be appropriate for a practical estimation of the tin pest risk when the design life of some electronic equipment ranges from 15 to 25 years.