Reliability of ACA Joints with Conformal Coatings in Harsh Environments

Abstract

The use of electronics is spreading to a variety of new applications. Electronics are facing different environments and reliability needs to be maintained. In harsh environments, the use of conformal coatings protects the electronics and improves their reliability. However, the performance of conformal coatings in different environments and applications needs to be ascertained. This thesis reports the use of conformal coating materials and structures to protect anisotropically conductive adhesive joined flip chips against different environments, the main focus being their use in implantable electronics. With packaging technologies, the implant may be miniaturized, and the technologies used need to be proven to be extremely reliable. The environment inside the human body is very demanding, and thus thorough studies need to be conducted to ensure the protection capabilities of the conformal coatings and the reliability of the devices. In this thesis, the reliability testing of anisotropically conductive adhesive joined flip chips on FR-4 and PI substrates was studied in constant humidity tests, temperature cycling tests, salt spray tests, and hydrolysis tests using epoxy, parylene C and epoxy-parylene C conformal coatings. The test results were analyzed mainly in terms of the selection of the failure criterion, different failure modes and mechanisms. Conformal coating materials affect the failure modes in those tests and even though the failure mechanism remained the same in them, the stresses were not generated in the same way with coating causing delaminations in different interfaces as without coating. The results confirmed the overwhelming protection capability of parylene C against moisture and impurities. To conclude, electronics miniaturization has considerable potential for medical electronics and reliable solutions can be made small and flexible if needed.