Microstructural evolution of joints with and without Sb, Ni in Sn58Bi solder under electro-thermal-force coupling

Severe atomic migration occurs in the solder under the effect of multi-physics field coupling, leading to serious reliability problems. Under the influence of multi-physics field coupling, severe atomic migration occurs in solder, resulting in significant reliability issues. In Cu/Sn58Bi/Cu joints subjected to 0.5 × 10⁴ A/cm² current density and temperature cycling from −45 °C to 60 °C, the primary diffusing atom was Bi. However, when the temperature range was extended to −45 °C–90 °C, Cu atoms became the primary diffusing species. This led to rapid thickening of the interfacial intermetallic compound (IMC) and failure due to penetration cracks and excessive consumption of the Cu interconnect layer. The addition of Sb to Sn58Bi to form (Cu, Sb)₆Sn₅ IMC inhibited Cu atom migration and mitigated rapid dissolution of the Cu interconnect layer and overgrowth of the interfacial IMC. The formation of a biphasic structure adjacent to the interfacial IMC improved joint deformability and prevented premature joint failure. COMSOL analysis revealed that the highest current densities in the Cu/Sn58Bi/Cu interconnect layer and solder joint were 6.041 × 104 A/cm² and 1.368 × 10⁴ A/cm², respectively, under the influence of 0.5 × 104 A/cm² current. The current density in the Cu interconnect layer was 4–5 times higher than that inside the solder joint. However, the addition of Ni formed (Cu, Ni)₆Sn₅ IMC, promoting Cu atom diffusion and accelerating joint failure under multi-physics field coupling. This study provides key insights into how alloying elements modulate atomic migration in Sn–Bi-based solder joints.