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

Purpose - The purpose of this paper is to investigate tin pest formation in lead-free alloys. Design/methodology/approach - Samples of Sn99.5Ag3.0Cu0.5, 5n99Cu1 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 degrees C or at -65 degrees 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 degrees C for long-term testing. Findings - The detectable symptoms of tin pest in samples subjected to mechanical processing with 1 and 2 wt.% of Cu addition stored at -18 degrees C were observed at the edges of the samples after 17 months of storage. Further aging at -18 degrees C showed the progress of alpha/beta 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 degrees C), there was a significant acceleration of the alpha/beta transformation, and this dependence can be used in predicting the risk of tin pest occurrence in various lead-free alloys. Originality/value - The paper shows that the degree of mechanical processing had a great influence on the alpha/beta 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.