STRATEGIC ENVIRONMENTAL RESEARCH AND DEVELOPMENT PROGRAM (SERDP)NANOPARTICLE ENHANCED CONFORMAL COATING PROJECT: COATING MODELING FOR TIN WHISKER MITIGATION

摘要

In 2006, the European Union RoHS legislation prohibited the use of lead in many electronic systems. This has resulted in increased use of pure tin finishes and tin rich lead-free alloys in aerospace and defense electronic systems. While aerospace and defense are currently exempt from the RoHS legislation, these industries adapt or directly use many consumer parts and assemblies. Unfortunately, the tin rich materials in these consumer items have a higher short circuit risk due to tin whiskers than heritage tin-lead materials. The whisker shorting potential can be mitigated by applying conformal coatings traditionally used for moisture protection to electronic assemblies. However, tin nodules and odd-shaped eruptions can also form, which can rupture the coating and reduce moisture protection and whisker mitigation effectiveness. The application of conformal coating to the original "tin free" surface alters the surface, changing the whisker nucleation and growth characteristics. A combination of finite element, classical plate deflection and adhesion models have been developed to examine the critical pressure that a tin nodule/whisker can exert on a coating before rupture or delamination occur. Supporting experimental results reveal that a high strength high modulus polyurethane conformal coating can inhibit nodule/whisker formation provided the coating is sufficiently thick and well adhered to the tin.