Material Selection for Low-Temperature Soldering of FPC Boards

　　Low-temperature soldering is essential for assembling flexible printed circuit (FPC) boards, as their heat-sensitive substrates—often polyimide or polyester—can degrade at temperatures exceeding 200°C. Selecting appropriate soldering materials is critical to maintaining the FPC’s mechanical flexibility and electrical conductivity while ensuring strong, reliable joints.

　　Lead-free solder alloys are preferred for environmental compliance, with tin-bismuth (Sn-Bi) alloys emerging as a top choice. Sn-Bi alloys with 58% bismuth content melt at approximately 138°C, well below the threshold for substrate damage. They offer good wetting properties, forming strong bonds with copper conductors, and their low viscosity during reflow allows for precise application in fine-pitch FPC designs. However, Sn-Bi alloys are relatively brittle, so adding small amounts of silver (Ag) or indium (In) can improve ductility without significantly increasing the melting point.

　　For applications requiring even lower temperatures, indium-based alloys (e.g., In-Sn) melt as low as 117°C, providing excellent flexibility in the solder joint. These alloys are ideal for FPCs used in wearable devices or medical sensors, where substrate integrity is paramount. However, their higher cost and lower shear strength make them less suitable for high-stress industrial applications.

　　Flux selection is equally important: no-clean fluxes with low activation temperatures (100–150°C) prevent residue buildup that could compromise flexibility. Water-soluble fluxes are an alternative but require thorough cleaning to avoid corrosion. Ultimately, material choice must balance temperature sensitivity, mechanical stress, and cost, ensuring the soldered FPC meets both performance and reliability requirements in its intended application.