Issue link: https://iconnect007.uberflip.com/i/339176
July 2014 • SMT Magazine 55 polyurethane conformal coating. The regions in tension similarly showed no evidence of tenting or tin whisker penetration for any of the coated areas of the Copper C110 base metal samples. For the bent Alloy 42 base metal samples, there was tin whisker protrusion of both the 1.0 mil thick acrylic and polyurethane conformal coatings in both the regions of compression and tension as reported previously. The Alloy 42 base metal samples with acrylic conformal coat- ing of 2.0 mil thickness exhibited tin whisker protrusion in the compression region (Figure 8), and the Alloy 42 base metal samples with poly- urethane coating of 2.0 mil thickness exhibited tenting of the coating in the compression re- gion (Figure 9). The 3.0 mil thick acrylic and polyurethane coatings showed no indication of tenting or tin whisker protrusion in either the tension or compression regions. Examination of Tin Whisker Growth Risk for Tin-Coated Braiding, Stranded Wire and Solid Single Strand Wire Following exposure of the samples to a temperature of 50°C and a relative humidity of 50% for five years, the tin coated braiding and the stranded wire exhibited no observable tin whisker growth. For the solid single strand wire, there was a significant concentration of tin whiskers along the surface of all three sam- ples (Figure 10). The longest whiskers present on the surface of the wire measured up to 40µm in length (Figure 11). Conclusions The Alloy 42 base metal test coupons exhib- ited higher tin whisker densities in uncoated re- gions than that of the Copper C110 base metal test coupons. The stressing of the test coupons by applying a 45° bend in two locations caused a significant increase in tin whisker density for figure 8: Scanning electron micrographs detailing the appearance of tin whiskers protruding through 2.0 mil thick acrylic coating in the compression region of a bent Alloy 42 base metal coupon. figure 9: Scanning electron micrograph characteristic of tenting of the 2.0 mil thick polyurethane coating in a region of compression on a bent Alloy 42 base metal coupon. TIN WHISKER RISK MANAGEMENT By CoNFoRMAl CoATING continues fEATURE