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48 SMT Magazine • July 2014 12in. The temperature and humidity was con- tinually monitored by an electronic recording device. The date of the initial insertion of the coupons into the environmental chamber was June 15, 2004. On one occasion during the last quarter of each year, a sample of test coupons was removed from the environmental chamber and inspect- ed. The inspection consisted first of optical mi- croscopy, using the indirect light procedures de- scribed on the 'NASA Tin Whisker Homepage' website [19] . Anomalies in the integrity of the conformal coating were noted, and any areas with suspect tin whisker growth were noted and inspected further using scanning electron mi- croscopy. SEM was also used on the areas of the test coupons without conformal coating to de- termine the length and density of tin whiskers. Examination of Tin Whisker Growth Risk for Tin-Coated Braiding, Stranded Wire and Solid Single Strand Wire Three samples each of braiding, stranded wire and solid single strand wire were removed directly from stock reels. The braiding select- ed had a braid length of 4in, an average braid width of 0.172in, an average braid height of 0.029in and a strand quantity of 30. The strand- ed wire selected conformed to the M22759/34- 24-9 specification. The insulation was stripped on each sample to expose approximately 3in of wire. The solid single strand wire selected con- formed to the A-A-59551, Type S, 20AWG tin- coated specification. All of the samples exam- ined were manufactured in 2008. Prior to environmental exposure, the sam- ples were inspected by optical microscopy and scanning electron microscopy for the presence of tin whiskers. The samples were placed in an environmental chamber set to 50° ± 10°C and a relative humidity of 50% ± 15% in Septem- ber 2008. In September 2013, all of the samples were removed from the environmental cham- ber and again inspected by optical microscopy and scanning electron microscopy for the pres- ence of tin whiskers. Results and Discussion Examination of Conformal Coating as a Mitigating Material for Tin Whisker Growth A detailed review of the annual inspection results for the initial 5.5 years of environmental exposure was reported previously [20] . Original inspection of the test coupons fol- lowing application of the tin plating indicated no observable anomalies in plating integrity (Figure 3). Cross-sections of ten samples con- firmed the plating thickness to be between 215 and 225µin. The thickness of the applied con- formal coating was confirmed from several pre- vious processing tests run to evaluate process- ing parameters for required coating thickness. Inspection of the conformal coating integrity indicated no significant anomalies. The coat- fEATURE TIN WHISKER RISK MANAGEMENT By CoNFoRMAl CoATING continues figure 3: Scanning electron micrographs (800X magnification) demonstrating the condition of the sur- face of the tin plating prior to conformal coating for both the Copper C110 base metal (left) and Alloy 42 base metal (right).