Effectiveness of Conformal Coatings on Surface Mount Components as Tin Whisker Mitigation
Tuesday, August 28, 2012 | Sungwon Han, Curtis Johnson, Michael Osterman, and Michael Pecht, CALCE, University of Maryland
Conformal coating is considered to be a mitigation strategy to address the failure risk presented by tin whiskers. While various conformal coatings have been shown to be effective on whiskering tin plated coupons, the effectiveness of conformal coating on actual assembly hardware has not been adequately examined. In this study, sets of assembled quad flat packages were conformally coated and evaluated for tin whisker mitigation as well as the coating coverage. Six coating materials were examined. Whisker growth was examined after specimens were subjected sequential to temperature cyclings, corrosive gases and temperature and humidity conditions. Whiskers were observed on coated surfaces that had relative thin layer of coating. Paralyne C coating was found to be the most effective coat-ing at whisker mitigation in terms of overall coating coverage and containing whiskers.
Conformal coatings are thin polymeric layers, designed to protect printed circuit boards (PCBs) from the harsh environments, such as moisture and chemicals . Tin whiskers are electrically conductive crystalline structures that can grow long enough to bridge adjacent conductive surfaces. The application of conformal coating on exposed electrical conductive surfaces is considered to be one of the most effective mitigation techniques for preventing electrical short failures caused by tin whisker [2, 3]. In terms of tin whisker mitigation, a conformal coating may prevent whiskers from contacting a coated surface or contain whiskers under the coated surface.
Previous studies have been found that conformal coating served well in whisker containing . However, Woodrow et al. [5, 6] reported on the effectiveness of several differences conformal coating materials to contain tin whiskers and found that tin whiskers penetrated all test coatings regardless of mechanical properties such as modulus, tensile strength and hardness of conformal coating. Most studies of conformal coating as a tin whisker mitigation strategy have used coupons with flat surfaces with uniform coating thickness.
This study evaluated the effectiveness of six conformal coating materials for tin whisker mitigation on assembled quad flat packages (QFPs). Coating coverage was examined and the whisker growth after assemblies were subjected to environmental loading sequences with temperature cycling (TC), elevated temperature humidity (TH) and mixed flow-ing gases (MFG) was documented.
A 184 IO QFP with Sn plated alloy 42 leads was used for this assessment. Parts were assembled with Sn-Pb solder paste to a custom designed printed wiring board that allowed individual part assemblies to be separated after assembly. Sets of part assemblies were coated by one of six conformal coating materials. Table 1 shows six conformal coating materials and their coating method. ALD-Cap O5TA200 coating is atomic layer deposition (ALD) with targeted thickness of 200 nm. All the conformal coating materials were applied by qualified commercial conformal coating services. One set of part assemblies were left un-coated to compare the whisker growth on the conformal coated and non-coated specimen.