SMT Solder Cracking Due to No-Clean Flux and Outgassing Materials Reacting Under Bottom-Terminated Components (BTC)

ABSTRACT Bottom-terminated component (BTC) fractures originating from vertical stress are a significant reliability concern, often indicating the presence of trapped volatiles beneath low-standoff parts. Inadequate venting prevents complete outgassing or burns-off of solvents and activators in no-clean flux, leaving residues that may react with airborne materials or moisture. This can result in an expanding reaction product that exerts upward force, stress and potentially cracking solder joints. The issue is exacerbated under shields, FETs, QFNs, and other ultra-low standoff components that further restrict volatile escape. This report presents field and laboratory findings on the interaction between no-clean flux residues and thermal pad silicone, demonstrating its role in open solder joints.