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Effectiveness of Conformal Coatings on Surface Mount Components as Tin Whisker Mitigation
Keywords: Tin whisker, Conformal coating, Coating coverage
The application of conformal coatings is a mitigation strategy to address failure risks presented by tin whiskers. With sufficient thickness and at ambient conditions, past research has shown that conformal coating can be effective at containing tin whisker growth on flat coupons; however, the effectiveness of conformal coating on actual assembly hardware has not been adequately defined. In this study, six types of conformal coating were applied by spray method or vacuum deposition on assembled Quad Flat Packages (QFPs), mounted on printed circuit boards, for evaluation of tin whisker mitigation as well as coating coverage. The test specimens were subjected to sequential rounds of temperature cycling, temperature and humidity conditions, as well as exposure to corrosive gases, and observed for whisker formation and coating penetration. In order to estimate the coating coverage, quantitative analysis is used with images taken under back scatter electron (BSE) mode on a scanning electron microscope (SEM). SEM coating coverage analysis showed that each spray-applied conformal coating had non-uniform coverage of lead edges. Estimated coating coverage from quantitative image analysis indicates that the current practices for conformal coating are insufficient for ensuring a coating's effectiveness in whisker mitigation. After the first set of accumulated loading cycles, whiskers started to grow and escape from the conformal coated specimens except for specimens coated with Parylene C. Most whiskers escaped from the coated surface at areas of thin coating. In terms of overall coating coverage and containing whiskers, Paralyne C was the most effective coating at whisker mitigation.
Curtis Johnson, Graduate Student
University of Maryland - CALCE
College Park, Maryland

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