Effect of Reflow Temperature on Interfacial IMC Growth in Pb-Free/Pb Mixed-Assembly Micro-Solder Joints

This study examines interfacial intermetallic compound (IMC) growth in Pb-free/Pb mixed assemblies comprising 450 µm SAC305 solder balls and Cu pads printed with a 0.12 mm layer of Sn63Pb37 solder paste. Samples were reflowed at peak temperatures of 200, 220, 240, and 260 °C, with the time above 217 °C fixed at 40 s. Scanning electron microscopy, energy-dispersive X-ray spectroscopy, and electron backscatter diffraction were used to characterize the interfacial composition, morphology, layer thickness, grain size, and kernel average misorientation (KAM). The IMC layer thickened from 1.5 µm at 200 °C to 6.0 µm at 260 °C, while KAM_ave increased from 0.36° to 0.59°. At 220 °C, the IMC thickness changed only slightly to 1.6 µm, accompanied by local Pb and Ag enrichment. At 240 °C, the IMC thickness increased sharply to 4.7 µm and KAM_ave increased to 0.55°, together with larger Cu₆Sn₅ grains and localized coarsening. At 260 °C, the IMC thickness reached 6.0 µm and KAM_ave reached 0.59°, while Pb-rich regions became more pronounced and the Ag content decreased markedly, indicating substantial interfacial solute redistribution and increasingly heterogeneous IMC growth. The results link the reflow temperature to changes in interfacial chemistry, and microstructure and can help define a suitable process window for Pb-free/Pb mixed assembly.