The Effects of Aging on the Microstructure and Mechanical Properties of Bi-Containing Sn-Rich Alloys
Authors: André M. Delhaise, Doug Perovic, Polina Snugovsky Company: University of Toronto and Celestica Date Published: 4/1/2017
Abstract: A significant reliability concern with lead-free solder alloys such as SAC 305 is the degradation of mechanical properties after aging at both elevated and room temperature, due to the coarsening and recrystallization of the microstructure. It has been shown in earlier studies that the inclusion of bismuth (Bi) in these alloys leads to a stabilization of the microstructure and subsequent maintenance of as-cast properties after aging. The goal of this paper and ongoing research is twofold – to ascertain whether these trends hold at a large range of aging temperatures and times, and to understand the underlying metallurgical mechanisms that lead to these effects. This paper examines the effects of aging on the microstructure and hardness of Bi-containing Sn-rich alloys. The alloys studied were Sn- 1Bi, Sn-5Bi, Sn-0.7Cu-1Bi, and Sn-0.7Cu-5Bi, as well as a baseline alloy, SAC 305. These alloys were aged both at room temperature (between 10 and 365 days) and at elevated temperature (100°C and 125°C, between 1 and 14 days). As-cast microstructure and properties were also included. Cooling after elevated temperature aging was performed in air. The microstructure was evaluated using Scanning Electron Microscopy, and hardness was measured using a Rockwell hardness tester HR15X with a ¼” carbide ball indenter. For Bi-containing alloys, bismuth precipitates became more uniformly distributed as aging proceeded, and hardness did not undergo any appreciable changes. SAC 305, on the other hand, showed a predictable decay in hardness after all aging treatments.