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52 SMT Magazine • July 2014 metal coupons (128 whiskers/mm 2 in regions of compression and 59 whiskers/mm 2 in regions of tension). The length of tin whiskers growing along the surface of the samples from both base metals approached 8.0 mils (Figure 6). The observations of the conformal coated test coupons as of the fourth quarter of 2013 are summarized in Table 4. Inspection Results for Straight, Scratched Coupons with Conformal Coating There were no observed tin whisker protru- sions through any of the conformal coating As of the fourth quarter 2013 sampling, the whisker densities in the stressed regions were considerably higher than that for the control samples (Figure 5). The regions in compression exhibited greater whisker densities than the regions in tension for both the Copper C110 and Alloy 42 base metal coupons. The whis- kers tended to grow along stress cracks in the tin plating caused by the bending process. The Alloy 42 coupons had significantly greater tin whisker density in both regions of compression (367 whiskers/mm 2 ) and tension (213 whiskers/ mm 2 ) than that of the bent Copper C110 base fEATURE TIN WHISKER RISK MANAGEMENT By CoNFoRMAl CoATING continues figure 5: Scanning electron micrographs (100X magnification) demonstrating the density of tin whis- kers on the uncoated areas in the compression regions the bent coupons (top two micrographs) and the regions of tension for the bent coupons (bottom two micrographs).