Authored By:
Keith Sweatman, Junya Masuda, Takashi Nozu
Masuo Koshi, Tetsuro Nishimura
Nihon Superior Co., Ltd
Osaka, Japan
Summary
Corrosion has been identified as one source of the compressive stress that is a driver of whisker growth in high tin lead-free solders and in this paper the authors report a study directed at identifying the relationship between the extent of corrosion and the concomitant whisker growth. Printed circuit coupons with an OSP finish were soldered with SAC305 solder using wave, reflow, and hand soldering methods with "no-clean" fluxes typical of current commercial practice.
These coupons were exposed to conditions of 40°C/95%RH, 60°C/90%RH and 85°C/85%RH for up to 5000 hours. As well as recording the location of whiskers, their density, and length as a function of time, the extent of corrosion was measured on cross-sections through the solder. The highest incidence and fastest growth rate occurred on test pieces exposed to 85°C/85% RH. The incidence and growth rate of whiskers was found to vary with the soldering method.
Conclusions
Under conditions of 60°C/90%RH and 85°C/85%RH corrosion that appears to be related to the character of the residues used in the soldering process can cause SAC305 solder to produce whiskers long enough to compromise circuit reliability. Where circuitry vulnerable to failure by shorts caused by whiskers is likely to be exposed to such conditions. Consideration should be given to effective removal of flux residues or the selection of fluxes with residues that do not support the sort of corrosion that seems to drive whisker growth.
Initially Published in the SMTA Proceedings
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