The microstructure evolution and failure mechanism of Sn37Pb solder joints under the coupling effects of extreme temperature variation and electromigration

The effect of electromigration and thermal shock on the microstructure evolution and failure mechanism in the Sn37Pb/Cu solder joints were investigated in this study. With increasing thermal shock cycles, Pb atoms acted as the dominant effective charge carrier, which migrated with the electron flow and accumulated at the anode side. As the number of thermal shocks extended to 20 cycles, the dissolution of Cu trace was found at the corner positions where the current was crowded in the solder joints. High and low temperature thermal shock demonstrated the acceleration of solder failure that accompanies an increase in peak temperature and shock frequency during electromigration process. Combining the extreme temperature thermal shock (–196 to 120 °C) with the current density of 2.0 × 10⁴ A/cm², the cracks were prone to initiate at the corner and then propagate along the IMC layers with thermal shock cycles. The high thermal stress caused by the thermal expansion mismatch between Sn37Pb solder and Cu pad with the large temperature variation (ΔT = 316 °C) significantly aggravated the crack propagation, which induced the failure of Sn37Pb solder joints after 22 cycles.