Further Observations on Tin Pest Formation in Solder Alloys

Abstract

The most recent observations of the response of bulk samples of several commercial solder alloys, exposed to temperatures below the allotropic transition for tin (13°C) for extended periods, are reported. Damage associated with tin pest development has been arbitrarily graded into six levels, and the formation of visible a-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 À18°C and À40°C, and actual warts were apparent in as-cast Sn-0.5Cu stored at À40°C and in as-cast Sn-3.5Ag after exposure at À18°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 À18°C, but others (Sn-37Pb and Sn-3.5Ag) were more susceptible at À40°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.

A method for the tin pest presence testing in SnCu solder alloys

“…Sn0.5Cu) should be more prone to tin pest at Ϫ18°C than Ag-containing alloys (e.g. Sn3.5Ag), which appear more vulnerable at Ϫ40°C (Plumbridge, 2010). The results obtained in this experiment showed slightly different results, but they were also influenced by the thermodynamics and kinetics of the transformation.…”

“…As can be noticed, in the first stage, the transformation rate was higher at the temperature of Ϫ30°C for some time, but after about 75 days of exposure to sub-zero temperatures, the rate at Ϫ30°C became lower than the rate at Ϫ18°C. As described by Plumbridge (2010), Cu-containing alloys (e.g. Sn0.5Cu) should be more prone to tin pest at Ϫ18°C than Ag-containing alloys (e.g.…”

“…After much Ͻ 14 days (within days for storage at Ϫ30°C, as can be seen from Figure 6), it was possible to observe the first visible symptoms of the transformation, while detection usually takes months or many years (Plumbridge, 2010). This means that the proposed new approach leads to a diagnostic test which produces results many times more quickly.…”

“…The transformation consists of an incubation stage, needed for ␣-tin nucleation, and a growth stage (Gialanella et al, 2009). The incubation stage leads to the nucleation of ␣-tin, the formation of a diamond cubic unit cell with dimensions of just below 10 Ϫ9 m (Plumbridge, 2010). Once nucleation has started, the transformation propagates through the material with coherent, diffusionless displacements of a large number of atoms, following a typical martensitic pattern (Gialanella et al, 2009).…”

“…Tin pest (gray ␣-tin) is the allotropic transformation of white ␤-tin (metallic bonding; s 2 p 2 electronic configuration; d ϭ 7.28 g/cm 3 ; and tetragonal structure) into gray ␣-tin (semiconducting properties; covalent bonding; sp 3 configuration; d ϭ 5.75 g/cm 3 ; and diamond cubic structure) at temperatures Ͻ 13.2°C which additionally causes a volume increase of about 27 per cent (Styrkas, 2003;Plumbridge, 2010). This transformation also changes the chemical bonding and reduces the coordination number from six to four (Styrkas, 2003).…”