Verification of Thermal Rework Degradation Limits for Multi-Layer Board Assemblies
Verification of thermal rework degradation limits relies on coupon continuous resistance tracking and microsectioning to prevent latent inner-layer via cracking.
SAC305 Liquidus Dwell
Profile Duration
Time spent above solder melting point governs wetting and joint formation during lead-free surface mount reflow assembly. Assembly line operators calibrate SAC305 liquidus dwell between forty-five and ninety seconds to allow full solder flux activation and surface tinning. SAC305 solder alloy, composed predominantly of tin alloyed with silver and copper, reaches liquidus state at two hundred seventeen degrees Celsius.
Excessively brief dwell times prevent solder from spreading completely across copper land patterns, causing tombstoning defects or cold joints. Continuous profiling ensures thermal equilibrium across mixed-technology circuit assemblies.
Intermetallic Growth
Metallurgical reactions between molten solder and copper land metallization depend directly on thermal exposure time. Extending SAC305 liquidus dwell beyond one hundred twenty seconds promotes growth of brittle copper-tin intermetallic layers such as Cu6Sn5 and Cu3Sn. Thick intermetallic interfaces reduce solder joint resistance to drop testing and vibration stress.
Substrate overheating during prolonged dwell times increases dielectric warpage and component package stress. IPC-AJ-820 recommendations specify optimal dwell limits to balance intermetallic development against joint strength requirement parameters.
Thermal Window
Process windows must accommodate thermal mass variations across heavy copper planes and small passive components. Maintaining a uniform SAC305 liquidus dwell prevents thermal damage on lightweight components while securing complete reflow on large heat sinks. Profiling thermocouples verify dwell duration across critical board locations.