Impression creep of monolithic and composite lead free solders

Even though several EMS (Electronic manufacturing services) companies are currently producing “lead free” products, a general notion of apprehension still exists in the industry, primarily due to the lack of sufficient mechanical reliability data supporting the use of lead free alloys. The current study was an effort to generate an understanding of the mechanisms of creep deformation in monolithic and composite (Ag and Cu reinforced) Sn–3.5Ag and Sn–3.0Ag–0.5Cu lead free alloys in the high stress high temperature regime. Small volume solder samples were reflowed using a custom built computer controlled resistance furnace. Impression creep testing was employed to determine the activation energy and stress exponent. A careful analysis of the collected data revealed the underlying creep mechanisms and the following conclusions could be made. Both Sn–3.5Ag and Sn–3.0Ag–0.5Cu exhibited higher creep resistance as compared to the eutectic tin–lead solder under all tested conditions, with the ternary lead free alloy marginally outperforming the binary lead free alloy. Composite solders performed better as compared to monolithic solders. Furthermore, Cu reinforced solders demonstrated higher creep resistance as compared to Ag reinforced solders.