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44 SMT Magazine • July 2014 edges. The test coupons were placed in an en- vironmentally controlled temperature / humid- ity chamber to promote the growth of the tin whiskers. At specific time intervals, a sampling of test coupons were removed from the temperature / humidity chamber and evaluated for tin whis- ker growth on the plated and uncoated surfaces versus the plated and conformal coated surfac- es. Data samples were collected and examined under high magnification, photomicrographs, or scanning electron micrographs. Energy dis- persive spectroscopy (EDS) analysis also provid- ed metallurgy to confirm anomalies as tin whis- kers. All data collected was documented, logged and charted to show whisker growth and other variations. The test had three primary objectives: 1. Grow tin whiskers on the bright tin plated test coupons. 2. Provide positive evidence that conformal coating, over a bright tin plated coupon protects against tin whiskers through growth reduction, abatement or containment. 3. Evaluate the different conformal coating materials and thicknesses to evaluate which materials and coating thicknesses provide the best protection against tin whisker growth. Examination of Tin Whisker Growth Risk for Tin-Coated Braiding, Stranded Wire and Solid Single Strand Wire Three samples each of pure tin coated braid- ing, stranded wire and solid single strand wire were taken directly from stock reels manufac- tured in 2008. The samples were inspected for the presence of tin whiskers using optical mi- croscopy and scanning electron microscopy pri- or to exposure to an environmentally controlled temperature/humidity chamber to promote the growth of the tin whiskers. Following five years of exposure, the samples were removed from the temperature/humidity chamber and evalu- ated again for the presence of tin whiskers. The primary objective of the testing was to determine if the typical pure tin coating present on braiding and wire would grow tin whiskers of the size capable of causing electrical failure to an assembly. Procedure and Materials Examination of Conformal Coating as a Mitigating Material for Tin Whisker Growth A diagrammed outline of the test coupon preparation procedure is shown in Figure 1. The test coupons measured 1in x 4in x 0.032in. The bright tin coating was applied to the test cou- pons by electrodeposition according to ASTM B545. The thickness of the tin coating was 215- 225µin. The test coupons were supplied and tin plated by Alexandria Metal Finishers. There were a total of 99 coupons with the Copper C110 base metal and 99 coupons with the Alloy 42 base metal. fEATURE TIN WHISKER RISK MANAGEMENT By CoNFoRMAl CoATING continues figure 1: Test coupon preparation procedure.