| Abstract Scope |
This study investigates liquid/solid interfacial reactions in Sn/Ni/Cu-Be (Alloy 25) couples. Since Ni-Sn reactions are slower than Cu-Sn, the Ni layer suppresses Cu diffusion and retards intermetallic compounds (IMC) growth, enhancing reliability. Reaction couples with Ni layers (1, 3, and 5 μm) were reflowed at 240 - 270 °C for 30 - 480 s. Thinner Ni layers accelerated Ni consumption. Before complete depletion, only (Ni, Cu, Be)3Sn4 forms. Upon complete Ni consumption, it transforms into (Cu, Ni, Be)6Sn5 phase, which spalls into the Sn solder. This triggers reactions between the Sn solder and the substrate, forming three IMC layers sequentially identified as (Cu, Ni, Be)6Sn5, Sn+ (Cu, Ni)6Sn5, and (Cu, Be)6Sn5. Solder ball shear tests showed Sn ductile fractures with increasing strength over time. However, the 5 μm Ni specimen reflowed at 270 °C for 480 s exhibited mixed ductile-to-brittle fracture through the Ni/IMC layers, degrading shear strength. |