Further Observations on Tin Pest Formation in Solder Alloys

The most recent observations of the response of bulk samples of several commercial solder alloys, exposed to temperatures below the allotropic transition for tin (13A degrees C) for extended periods, are reported. Damage associated with tin pest development has been arbitrarily graded into six levels, and the formation of visible alpha-phase warts used for comparative purposes. Since the previous examination, some 2 years ago, tin pest has been observed for the first time in the traditional Sn-37Pb solder alloy after exposure at -18A degrees C and -40A degrees C, and actual warts were apparent in as-cast Sn-0.5Cu stored at -40A degrees C and in as-cast Sn-3.5Ag after exposure at -18A degrees C. No tin pest was detected in Sn-Zn-3Bi after exposure for periods up to 6 years. Tin pest continued to develop in those lead-free alloys in which it had previously been observed, indicating the probability that all would eventually disintegrate in time. In general, prior thermal or mechanical treatment accentuated tin pest formation. The influence of exposure temperature was unclear, since some alloys (Sn-0.5Cu and Sn-3.8Ag-0.7Cu) experienced more damage at -18A degrees C, but others (Sn-37Pb and Sn-3.5Ag) were more susceptible at -40A degrees C. From a consideration of the findings and other published information, it is contended that impurity levels below 0.1 mass% (1000 ppm) play a vital role in determining whether tin pest develops in realistic timescales. A major factor in the absence of tin pest, to date, on actual joints may be simply the mismatch between the timescales experienced in service and those in long-duration laboratory tests.