Power Semiconductor Module Busbar Welding Using a Single-Mode Fiber Laser (Part 1): Heat-Affected Zone Formation and Solidification Cracking Behavior during High-Speed Multi-Pass Welding

This study investigates the heat-affected zone (HAZ) formation behavior and solidification cracking characteristics in large-area Cu-Cu busbar joints fabricated by single-mode fiber laser multi-pass welding for power semiconductor module applications. The effects of welding condition and overlap ratio on weld integrity and mechanical performance were systematically evaluated. Under the conditions of heat input 3.5 and 4.5 J/mm, solidification cracking was effectively suppressed when the overlap ratio was controlled below 10 %, whereas higher overlap ratios resulted in severe solidification cracking. Under optimized multi-pass welding conditions, defect-free welds with HAZ-free characteristics were successfully achieved. The shear tensile strength of the multi-pass welds reached approximately 3.60 kN, corresponding to approximately a 920 % increase compared with that of the single-pass condition. Thermal history analysis using the Thermo-Calc Additive Manufacturing module revealed rapid heating and cooling behavior with an average cooling rate of approximately 7 × 10⁴ K/s, which contributed to the suppression of heat accumulation and the prevention of HAZ formation during multi-pass welding. These results demonstrate that precise control of overlap ratio and heat input is essential for achieving reliable large-area Cu busbar joints in power semiconductor modules.