High-temperature lead-free soldering is essential for rigid PCBs in harsh environments (automotive, aerospace), where operating temperatures exceed 125°C. This process uses lead-free alloys like SAC305 (Sn-3Ag-0.5Cu) or SAC-Q (Sn-3.8Ag-0.7Cu with additives), which withstand higher temperatures than traditional tin-lead solders but require precise process control.

The process starts with solder paste application, using stencils with 50-150 μm thickness to deposit paste on pads. The paste contains solder powder (20-45 μm particle size) and flux (10-15%) to remove oxides. For high-temperature alloys, no-clean fluxes with rosin or organic activators are preferred, ensuring residue compatibility with rigid PCB materials like FR-4 or polyimide.

Reflow soldering follows, with a temperature profile tailored to the alloy: preheat (150-200°C for 60-120 seconds) to activate flux and prevent thermal shock; soak (200-220°C) to melt flux; peak temperature (245-260°C for 30-60 seconds) to ensure complete solder melting and wetting. Cooling rates (3-5°C/second) are controlled to avoid micro-cracks in solder joints.

Challenges include thermal stress—rigid PCBs with large copper areas or thick cores (≥2.4 mm) may warp, so support fixtures or stepped heating profiles are used. Solder joint integrity is verified via X-ray inspection (for BGA/CSP) and shear tests (>50 N for 0.8 mm pitch components). Post-soldering, cleaning (with aqueous or semi-aqueous solutions) removes flux residues if required, preventing ionic contamination. This process meets IPC-A-610 and J-STD-001 standards, ensuring reliable joints with tensile strength >20 MPa, suitable for high-vibration, high-temperature applications.