Microalloying provides a means of tailoring solder performance by modifying solute partitioning, precipitate formation, and interfacial reactions. This dissertation investigates these effects in Sn–Bi-based low-temperature solders and the high-temperature alloys SAC305, Innolot, Alloy 10, and Alloy 39. Thermodynamic calculations, thermal analysis, electron microscopy, and mechanical testing were combined to relate alloying-induced microstructure changes to deformation and failure. This thesis presentation will focus on the studies of the high-temperature alloys designed for demanding environments.

 

Microalloying with Ag, Bi, Cu, Sb, and Ni contributes through different but interacting mechanisms. Ag forms Ag3Sn particles, Cu forms Cu6Sn5 particles in the matrix and at the interface with substrate, while Bi and Sb participate in both matrix solid solution and precipitation. Ni is incorporated into (Cu,Ni)6Sn5 precipitates, primarily at the Cu-alloy interface, modifying the IMC structure and interfacial evolution. Compared with SAC305, the multi-component high-temperature alloys generally exhibit thinner interfacial IMC layers and reduce thickening during aging. TEM–EDS identifies supersaturated Sb in the as-reflowed β-Sn matrix of Alloy 10, followed by reduced matrix Sb content after aging, consistent with SnSb precipitation. The mechanical consequences are alloy dependent. Aging reduces the peak shear stress of SAC305 but increases that of Innolot and Alloy 10, while Alloy 39 retains a comparable peak stress. Alloy 10 also exhibits improved creep resistance (tested at 120 N) after aging. These responses reflect the evolving balance between solid-solution strengthening, particle strengthening, interfacial intermetallic layer growth and matrix crystallography. Bulk strengthening can additionally shift failure toward the interfacial layer or substrate metallization. The findings demonstrate that effective microalloying requires control of both the initial microstructure and its evolution during aging to improve joint performance.