Wettability test method for surface mount technology assessment

A study was performed to develop a different experimental methodology to as sess wettabilities of solders on various printed wiring board (PWB) finishe s, based on a modified spreading test in which solder pastes were heated fo llowing temperature reflow profiles representative of those used in surface mount technology (SMT) instead of using a fixed rate temperature ramp. Thr ee solder alloys (Sn63-Pb37, Sn96.5-Ag3.5, and CASTIN(TM): Sn96.2-Ag2.5-Cu0 .8-Sb0.5), two fluxes (rosin, mildly activated, RMA, and no-clean, NC), and seven PWB finishes (Pd, Au/Ni, Ni. Ag. Sn, and organic solderability prese rvatives: OSP), and bare copper were involved in the study. setter wettabil ities were observed in the current study than the results reported in the l iterature for conventional tests an the same combination of solder alloy, f lux, and substrate. The different results in measurement of wettabilities o btained in the current study were attributed to the more adequate heating p rocess allowing flux activation, which reduced reoxidatian of solder powder s and substrates during the reflow process and thus improved wettabilities of solders. Compared to the results obtained from the popular wetting balan ce test, the current study demonstrated a more realistic simulation of, and approach to, assessing the wettability of solder far SMT.