Low-temperature instantaneous large-area laser soldering of Al/Cu flexible printed circuit board lap joints with exceptional fracture energy absorption capability: Toward carbon-neutral energy reduction assembly

To address the growing demand for carbon-neutral technologies, this study proposes a novel interconnection method for assembling aluminum flexible printed circuit boards (FPCBs) into battery management systems (BMS) with copper FPCBs using low-temperature laser soldering (melting point: 140 °C). This work focuses on mitigating embrittlement issues commonly associated with SnBi solder by employing a rapid surface scanning laser soldering technology. This method enhances energy absorption capabilities while minimizing embrittlement risks. Despite applying similar laser power levels, the intermetallic compound (IMC) layer at the SnBi solder bonding interface was approximately 62 % thinner (~0.36 μm) compared to SAC305 solder (~0.98 μm). This significant difference in IMC growth is attributed to the temperature-dependent heat capacity and solidification behaviors of the two solder types. Additionally, the distinct physical properties of SnBi and SAC305 solders profoundly influence recrystallization and grain growth in Al FPCBs, highlighting the critical role of low-melting-point solders in achieving robust joints. The trends in IMC growth, driven by excessive heat input, can explain the underlying mechanisms of severe degradation and decomposition at the electrode-polyimide (PI) interface, which ultimately result in differences in fracture energy absorption capabilities. •Low-temperature laser soldering enables carbon-neutral assembly for Al/Cu FPCBs in energy storage devices.•SnBi system low-melting point solder can achieve excellent fracture energy in laser process-based FPCB lap joints•The laser soldering process achieves reliable, large-area bonding within 2 s, ideal for flexible electronics and battery systems.