Power Semiconductor Module Busbar Welding Using a Single-Mode Fiber Laser (Part 2): Effects of Busbar Thickness and Heat Input on Weldability during High-Speed Multi-Pass Welding

This study investigates the effects of busbar thickness and heat input on the weldability of large-area Cu-Cu busbar joints fabricated by single-mode fiber laser multi-pass welding for power semiconductor module applications. Based on the optimized welding conditions obtained in Part 1 for the 0.8-0.8 mm busbar combination, thicker Cu busbar combinations of 0.8-1.2 mm and 0.8-1.5 mm were evaluated under increased heat input conditions. The effects of overlap condition and heat input on solidification cracking behavior, heat-affected zone (HAZ) formation, mechanical performance, and thermal history were systematically investigated. Under optimized overlap conditions, defect-free multi-pass welds without observable HAZ were successfully achieved for both thickness combinations, whereas non-optimized overlap conditions resulted in severe solidification cracking. Compared with the 0.8-0.8 mm busbar combination in Part 1, increased heat inputs of 5.7 and 13.3 J/mm were required to achieve stable weld penetration for the thicker busbars. Nevertheless, the multi-pass welds maintained high mechanical reliability with maximum shear tensile loads exceeding 3.40 kN, which were comparable to those obtained for the thinner busbar joints. Thermal history analysis using the Thermo-Calc Additive Manufacturing module revealed rapid cooling behavior with an average cooling rate of approximately 4 × 10<sup>4</sup> K/s, contributing to the suppression of heat accumulation and HAZ formation even under increased heat input conditions. These results demonstrate that single-mode fiber laser multi-pass welding can provide reliable HAZ-free joining for various Cu busbar thickness combinations in high-power semiconductor module applications.