MICROSTRUCTURE AND PROPERTY ASSESSMENTS OF UV-CURABLE CONFORMAL COATINGS FOR PB-FREE ELECTRONICS

摘要

UV-curable polyurethane acrylate (PUA) coatings are expected to dramatically reduce process time and cost for conformal coating applications for electronics while having significant environmental benefits for curing processes with low or no volatile organic compounds (VOC's). Optimally processed coatings can also effectively mitigate tin whisker growth from tin-rich surfaces inside Pb-free electronics. It is therefore crucial to have accurate property assessments of the PUA coatings to evaluate their potential in such applications. In this study, key microstructural and chemical bonding features were examined through low-kV SEM, Raman, and FT-IR spectroscopy to correlate them with the resultant mechanical properties. Microscale and macroscale mechanical properties were characterized via nano-indentation and tensile testing, respectively. In addition, coating adhesion to the tin surface was evaluated for several conformal coatings by measuring their debond energy from tin surface. Conformal coverage of the coatings on actual electronic devices was examined in this work via cross-sectional microtome analysis of polyurethane (PU) coatings on component leads. Thin coating sections were identified across the component lead geometry through these results. In particular, to assess the effectiveness of the coatings on tin whisker mitigation, tin-plated slotted leadframe coupons were coated with PUA coatings and exposed under high-temperature, high-humidity environment (85°C/85RH).