Effect of isothermal ageing process on microstructural changes in Bi-Ag lead-free solder alloys

Despite numerous studies on lead-free solder in recent years, only a limited number of publication is available on high temperature lead-free alternative solders to replace high lead solders. Therefore, this research focused on several candidate alloys (Bi-1.5-2.5-3.5 wt.% Ag) as alternative solders for high temperature application. Candidates of Bi-Ag alloys were aged at 160 degrees C for 0, 100, 200, 350 and 500 hours. Morphological characteristics such as grain growth, wetting angle and Cu rich particles size distribution in solder bulk were investigated. The results show that physical and mechanical adhesion between Cu substrate and solder bulk occurred in all three percentages of Ag during isothermal ageing process. In the case of Cu-rich particle size analyses, by increasing the percentages of Ag in the solder alloys, the average size of Cu- rich phase increased. The average measurement of mechanical grain boundary grooving thickness and cross-sectional wetting angle also increased with addition of Ag percentage. However, the value for cross-sectional wetting angle analysis reduce with increasing ageing time. From this work, Bi-2.5 wt% Ag have been identified as the best candidate between the three solders to be explored for future analysis such as mechanical strength data collection and electrical conductivity test.