Thermal cycling of flip chips on FR-4 and PI substrates with parylene C coating

Purpose - The purpose of this paper is to study the effect of conformal coating on the thermal cycling reliability of anisotropically conductive adhesive film (ACF) joined flip chip components on FR-4 and polyimide (PI) substrates. Design/methodology/approach - Test chips were joined using flip chip technology and an anisotropically conductive adhesive. The conformal coating used was parylene C and it was applied using the vapour deposition polymerisation method. The reliability of ACF joined flip chip components on FR-4 and PI substrates was evaluated using -40/+85 degrees C thermal cycling testing. Test lots with and without parylene C coating were studied. Additionally, one test lot with initial moisture inside the coating layer and a PI substrate was subjected to the test. The reliability results were analyzed using Weibull analysis and failure analysis was performed to study the failure mechanisms using cross sectioning and optical and scanning electron microscopy. Findings - The results show a clear difference between the FR-4 and PI substrate materials. PI substrate material proved to be reliable enough to withstand the thermal cycling testing. Two different occurrences of the first failures are seen and analyzed with FR-4 substrates. The conformal coating layer did not seem to impair the reliability. Parylene C coating proved to be a reliable choice to protect, and even improve, the thermal cycling reliability of flip chip devices. Originality/value - Usually, conformal coatings are studied in humidity tests. However, it is also vital to know whether the conformal coatings affect the reliability in thermal cycling and there is a lack of reliability studies in this area. This paper gives reliability data for conformal coating users about the influence of thermal cycling.