The Performance of Solid Solution Strengthened No Silver Lead Free Solder
Authors: Takatoshi Nishimura, Mu Dekui, Shuhei Sawada, Keith Sweatman, Keith Howell, Guang Zeng, Kazuhiro Nogita Company: Nihon Superior Co., Ltd., The University of Queensland Date Published: 5/19/2015
ICSR (Soldering and Reliability)
Abstract: In the first stage of lead-free implementation the strengthening effect of silver on alloys based around the tin-copper eutectic was a major consideration in solder formulations. Subsequently a search for cost effectiveness as well as improved performance in drop impact has prompted a trend to lower silver levels and the increasing use of solders that contain no silver. However another factor prompting a review of the reliance on silver has been the increasing evidence that the strengthening effect of silver declines as the fine dispersion of Ag3Sn particles that provides that effect coarsens and the interparticle spacing increases as a result of Ostwald ripening during isothermal aging and thermal cycling. There is concern that this decline in the strengthening effect of the silver could affect the long term reliability of medical, infrastructure, automotive and aerospace electronics from which a long service life is expected. There is, therefore, an interest in alternative strengthening mechanisms that could complement or replace the strengthening effect of silver to provide more assured long term reliability. One such mechanism is the addition of elements that provide solid solution strengthening of the tin matrix and bismuth has been identified as a promising candidate. In this paper the results of tensile testing and high speed ball shear of a tin-copper eutectic alloy with an addition of bismuth will be reported. The distribution of the bismuth in the as-cast condition and the change that occurs during isothermal aging and thermal cycling was monitored by EPMA. The results indicate that solid solution strengthening with bismuth can provide a stable and cost effective means of increasing the reliability of tin-copper lead-free solders.