Characterized by its low melting temperature of 138 °C, the eutectic Sn-58Bi solder expands the melting temperature range of interconnect joints in electronic packaging, making it widely used in multi-level packaging processes. However, its reliability at higher current densities poses a challenge. This paper employs a hybrid process combining laser soldering and hot-air reflow to fabricate Cu/Sn-58Bi/Cu solder joints in ball grid array (BGA) structures. Through mechanical testing under current loading, the effects of increasing current density (0 A/cm², 0.85 × 10³ A/cm², 1.70 × 10³ A/cm², 2.55 × 10³ A/cm², 3.40 × 10³ A/cm², 4.25 × 10³ A/cm²) were studied systematically. Results indicate that the shear strength decreases markedly with increasing current density, exhibiting a reduction of approximately 5.63% to 95.75%. This degradation is initiated by the overall temperature increase and material softening due to Joule heating. It is further exacerbated by the loss of the non-thermal electron wind’s strengthening contribution, which weakens as the dominant thermal impact escalates with current density. Fracture mode transitions from ductile failure within the solder matrix to a ductile-brittle mixture at the solder/IMC interface, with the transition initiating at 3.40 × 10³ A/cm². Finite element simulations reveal that current crowding in Sn-rich regions and at the solder/IMC interface induces localized Joule heating and thermomechanical strain, which jointly drive the degradation in shear strength and the shift in fracture path.

As the integration and power density of high-power electronic devices continue to rise, efficient thermal management has become a key challenge in ensuring their performance and reliability. In this study, liquid metal gallium was used as the matrix and copper-coated diamond (CCD) as the reinforcing phase. Using a mechanical mixing method, liquid metal/copper-coated diamond (LM-CCD) composite thermal interface materials with different filler contents (10–60 vol%) were prepared, and their microstructure, thermal conductivity, and high-temperature operational reliability were systematically investigated. The results showed that when the CCD content is 50 vol%, a complete, interconnecting thermal conduction network forms within the composite, with thermal conductivity peaking at 86.9 W/(m·K)—approximately three times that of pure gallium (~29 W/(m·K)). COMSOL Multiphysics 6.2 finite element simulations confirmed that the thermal conduction framework formed by high CCD content effectively reduces heat accumulation. However, high-temperature aging tests at 125 °C revealed a synergistic effect between the formation of the CuGa₂ intermetallic compound (IMC) via interfacial reactions and oxidation reactions, resulting in microstructural degradation and a drop in thermal conductivity to approximately 49.3% of its initial value after 240 h. After 336 h of aging, the thermal conductivity decreased further to approximately 27 W/(m·K), indicating that the thermal conductivity enhancement provided by CCD had been lost under these accelerated aging conditions. In addition, actual CPU thermal testing has shown that, compared to commercial thermal paste (~5 W/(m·K)), the LM-CCD composite material can reduce the chip’s steady-state temperature by approximately 14 °C. This represents a 4 °C reduction compared to pure gallium. Through combined experimental and theoretical analyses, this study provides a basis for understanding the degradation mechanisms of liquid-metal thermal interface materials and for designing more reliable TIMs through interface modification or oxidation-resistant encapsulation.