高温高湿条件下におけるはんだウィスカの成長機構

Abstract

Corrosion is recognized as one of the potential drivers of whisker growth in regards to high-tin alloys. In this study, the growth of whiskers was observed for a period of 5000 hours on Sn3.0Ag0.5Cu (SAC305) as a function of soldering method under the environmental conditions of high temperature and humidity of 85°C/85%RH, 60°C/90%RH, and 40°C/95%RH. The soldering methods used were dip soldering, hand soldering with flux-cored wire, and reflow soldering with solder paste with all of the methods using commercially available flux formulations. The test pieces were examined at regular intervals for the onset of whisker growth, whisker density and maximum whisker length. In addition, samples were taken periodically for the measurement with the extent of corrosion and correlations between corrosion and whisker growth being explored. Findings presented in this paper can be summarized as follows: (1) Whiskers occurs at 85°C/85%RH and the growth rate, is two to four times greater than that at 60°C/90%RH and approximately ten times greater than that at 40°C/95%RH; (2) There is a positive correlation between whisker density and maximum whisker length; (3) For the dip soldering, a positive correlation between the average percentage of the corroded cross−sectioned region and the maximum whisker length suggests that corrosion is a driving force of whisker growth; (4) Different soldering methods resulted in varying degrees of copper erosion along with different distribution of flux residues, and subsequently different rates and locations of corrosion. It is concluded from these observations that the soldering method as well as the type of flux gives significant influence on the subsequent corrosion when exposed to high temperature and humidity conditions. Consequently, this also gives extensive effects on the onset time of whisker appearance and maximum whisker length.