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42 SMT Magazine • July 2014 fEATURE there is a resulting risk of current leakage and shorting due to bridging of adjacent conduc- tors. There have been multiple studies into the mechanisms for whisker growth and both en- vironmental and mechanical factors that may promote whisker growth [1-5] . Similarly there have been multiple stud- ies on methods for mitigating tin whisker growth [6,7] . Mathew et al. reviewed research into mitigation strategies such as conformal coating, electroplating techniques, surface treatments, alloying tin, use of various under-plates and an- nealing of tin [6] . The conformal coating miti- gation strategy has shown multiple results using various coating materials and envi- ronmental storage conditions. NASA studies [8-10] indicated that bright tin plated brass coupons, conformal coated with a uralane-based material, was able to prevent tin whis- ker protrusion following nine years of ambient storage when the coating thickness was at least 2.0 mils. If the coating was thinner, there were obser- vations of tin whisker protru- sions. Woodrow and Ledbury re- leased two papers [11,12] examining tin whisker growth through mul- tiple conformal coating materials. Both studies used bright tin plated brass test coupons. For the first study, when the test coupons were sub- jected to an environmental chamber set to 50°C with 50% relative humidity (RH), tin whisker penetration was noted after approximately one year for coatings of 1.5 mils and less, but not for coatings of at least 3.9 mils. For the second study, the conformal coatings examined were urethane-acrylic hybrid, silicone, acrylic and parylene. The test coupons were subjected to an environmental chamber set to 25°C with 97% RH. All of the test coupons exhibited tin whis- ker penetration of the conformal coatings, even on samples with up to 6.0 mils of coating. The University of Maryland's Center for Advanced Life Cycle Engineering (CALCE) has studied the interfacial strength of conformal coatings in comparison to the whisker buck- ling force [13] initially presented by Kadesch and Leidecker [8] . Preliminary testing indicated that conformal coatings of 25 microns (approximate- ly 1.0 mil) or less with a modulus of 100MPa or less are at risk of tin whisker penetration. Nak- agawa et al. similarly identified harder coatings with high adhesion strengths as the most likely materials to prevent tin whisker protrusion [14] . Han et al. review the effectiveness of confor- mal coatings as a tin whisker mitigator on ac- tual circuit cards and determined that coating coverage is an essential factor [15] . As the coating thinned along the edges of component leads, the potential for tin whiskers to protrude through the coating, regardless of the coating type increased dramat- ically. This was confirmed by the NPL's Hunt and Wickham who designed a test vehicle to determine the propensity of whiskers to grow through coatings and contact a neigh- boring plate [16] . Reviewing the multiple studies on the use of confor- mal coatings as a mitigation technique for tin whiskers indi- cates that tin whiskers can grow through a coating. One of the lead- ing factors for the risk of a protrusion through a coating is the thickness. Coating thicknesses below 2.0 mils appear to present a greater risk of tin whisker penetration, although extreme en- vironmental conditions coupled with the type of tin plating could promote tin whisker growth through any type of coating at relatively large coating thicknesses. This company conformal coats 99% of all circuit boards using one of three different con- formal coating materials: acrylic, polyurethane and parylene. This study examined the effects of tin whisker growth on the three coatings ap- plied to test coupons at varying thicknesses. Acrylic conformal coatings are perhaps the most popular of all conformal coating materi- als due to their ease of application, removal and TIN WHISKER RISK MANAGEMENT By CoNFoRMAl CoATING continues Reviewing the multiple studies on the use of conformal coatings as a mitigation technique for tin whiskers indicates that tin whiskers can grow through a coating. One of the leading factors for the risk of a protrusion through a coating is the thickness. " "