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Effect of Soldering and Flux on Whisker Growth
Analysis Lab |
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Authored By:Keith Sweatman, Junya Masuda, Takashi Nozu, Masuo Koshi, Tetsuro Nishimura Nihon Superior Co., Ltd, Osaka, Japan TranscriptSince the electronics industry moved to lead-free solders that typically have a tin content of more than 95% there has been concern about the possibility of circuit malfunctions due to whisker growth. It is now generally accepted that whisker growth is a response to compressive stress within the tin crystal and the challenge is to identify and eliminate, or at least minimize, the processes that can generate such stress. Corrosion has been identified as one source of that stress 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 flux formulations typical of current commercial practice. These coupons, soldered but without components, were exposed to three environments for up to 3000 hours. As well as recording the location of whiskers, their density, and length as a function of time, the extent of corrosion of the solder after 1000, 2000 and 3000 hours was measured by cross-sectioning. The ultimate determinant of whether or not whiskers appeared was the environment to which the test pieces were exposed. The highest incidence, fastest growth rate, and greatest length occurred on test pieces exposed to 85 degrees C and 85% relative humidity. Whiskers occurred later, at a lower incidence, and grew more slowly at 60 degrees C and 95% relative humidity, but even after 3000 hours no whiskers were detected on test pieces exposed to 40 degrees C and 95% relative humidity. The incidence and growth rate of whiskers was found to vary with the soldering method and the type of flux. Whisker growth occurred earliest on the test pieces that had been wave soldered. The authors relate these trends in whisker growth to observations of the concurrent corrosion of the solder which in turn is related to the type of flux used. A preliminary conclusion is that the likelihood of whisker growth occurring on lead-free assemblies soldered using no-clean technologies can be significantly reduced by using a flux which does not promote the sort of corrosion that can generate compressive stress in the solder. SummarySince the electronics industry moved to lead-free solders that typically have a tin content of more than 95% there has been concern about the possibility of circuit malfunctions due to whisker growth. It is now generally accepted that whisker growth is a response to compressive stress within the tin crystal and the challenge is to identify and eliminate or at least minimize the processes that can generate such stress. Corrosion has been identified as one source of that stress 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 flux formulations typical of current commercial practice. These coupons, soldered but without components, were exposed to three environments for up to 3000 hours: 40°C/95%RH, 60°C/90%RH and 85°C/85%RH. As well as recording the location of whiskers, their density, and length as a function of time, the extent of corrosion of the solder after 1000, 2000 and 3000 hours was measured by cross-sectioning. The ultimate determinant of whether or not whiskers appeared was the environment to which the test pieces were exposed. The highest incidence (whiskers per unit area), fastest growth rate, and greatest length occurred on test pieces exposed to 85°C/85% RH. Whiskers occurred later, at a lower incidence, and grew more slowly at 60°C/95% RH but even after 3000 hours no whiskers were detected on test pieces exposed to 40°C/95% RH. The incidence and growth rate of whiskers was found to vary with the soldering method and the type of flux. Whisker growth occurred earliest on the test pieces that had been wave soldered. Geometry was found to have an effect with the concavity created on the edges of traces by the etching process apparently acting to focus the compressive stress and accelerate whisker growth in that area. The authors relate these trends in whisker growth to observations of the concurrent corrosion of the solder which in turn is related to the type of flux used. A preliminary conclusion is that the likelihood of whisker growth occurring on lead-free assemblies soldered using no-clean technologies can be significantly reduced by using a flux which does not promote the sort of corrosion that can generate compressive stress in the solder. ConclusionsUnder 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 IPC Proceedings |
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