Characterization and degradation of β-Sn particles in thermal aged Pb-rich solder joint for low-temperature co-fired ceramic (LTCC) applications

High-lead solder joints are still playing an indispensable role in military and space applications. Nevertheless, in-depth characterization of high-lead solder joints and the underlying degradation mechanisms remain unexplored. This research first performed aging tests on Sn10Pb90 solder joints, the shear strength at room and elevated temperatures gradually reduced, and the resistance increased. Here, a two-layered Ni–Sn intermetallic compound (IMC) structure was identified using transmission electron microscopy (TEM), which could be attributed to the change of Sn content in the solder. Moreover, the internal annealing twin of a Sn particle was discovered, which could be attributed to creeping induced by thermal expansion coefficient (CTE) difference between Sn and Pb. Detailed analysis of partial and whole annealing twins was conducted through high-resolution TEM (HRTEM). Finally, four degradation mechanisms were proposed. Thickening of the IMC layer would result in increased brittleness and resistivity. For particle coarsening, apart from diminishing the ductility and toughness of the solder joint, it would also accelerate the creeping rate by weakening the phase boundary strength. Regarding voids and cracks induced by phase boundary sliding, wedge-shaped cracking and pore-shaped cracking were discovered and their formation was analyzed. Most importantly, the consumption of Sn resulted in a depletion of wettable layer, leading to the formation of Pb streams and isolated IMC islands, also known as the spalling and delamination of IMCs. Pb diffusion followed a spiral path, which was mutually influenced by orientation misfit and concentration gradient. A technique to prevent cracking was proposed. This research is expected to provide significant technical references for high-lead solder joints.

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