Hydrolysis testing of ACF joined flip chip components with conformal coating

The reliability of electronics is very important, especially in implantable applications. Electronics need to be protected against moisture and body fluids. Various coating solutions are used for this. Polymer coatings offer protection against harsh environment, and even if metallic or other coatings are used, polymer conformal coatings are used underneath. Thus, their reliability issues need to be considered. In vitro testing of implantable devices is needed prior to in vivo testing and implantation. In this study, altogether three test lots were prepared for in vitro tests. FR-4 substrate was used and an 8 × 8 mm chip with daisy chain test structure was attached using anisotropically conductive adhesive. Epoxy and parylene C conformal coatings were studied. The test lots had epoxy, parylene C, and epoxy and parylene C composite structure as conformal coatings. All the test lots were medically sterilized prior to testing using gamma rays. The hydrolysis testing was conducted using sodium phosphate buffer solution. The duration of the test was ten months. The test samples were measured once a week for their functionality and the pH value of the solution was controlled to ensure that the samples were not dissolving in the solution. The aim of the study was to compare the different conformal coatings in the hydrolysis test and to determine the failure mechanisms occurring in the ACF joints.