SMT007 Magazine

SMT-June2014

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34 SMT Magazine • June 2014 Exploring High-Temperature Reliability Limits for Silicone Adhesives Abstract The thermal stability of silicone polymers, fluids and resins has been well documented and studied extensively. The high-temperature performance of silicone adhesives and seal- ants used for electronics applications has only moderately been investigated. This report doc- uments the effects of very high-temperature exposures to electronics-grade silicone adhe- sives and sealants for such properties as tensile strength, elongation, tensile modulus, weight loss, shrinkage, durometer, and lap shear adhe- sion. The goal of the work is to determine ap- plication "life expectancies" of the products as well as an extrapolated estimate of the Under- writer's Laboratories' "continuous use" temper- ature rating— the highest temperature at which a product is expected to lose no more than 50% of its original value for whatever key property degrades the fastest. Four different formulations of silicone adhe- sives and sealants were evaluated for high-tem- perature stability. For these products, elongation was found to be the fastest degrading property among those tested. The data was found to fit a power curve of exposure temperature vs. time to reach a 50% loss of initial tensile strength and elongation to an R-squared value of 0.99 and to a linear fit in an Arrhenius plot to the same very strong fit. These plots could be used to closely estimate the effects of heat aging on the material over a wide range of temperatures. Introduction Silicone is the generic name used by many to identify a family of products based on the polydimethyl siloxane (PDMS) molecule with unique characteristics. For electronics appli- cations silicones can be used as adhesives, en- capsulants, gels, protective coatings, thermal feaTure by Carlos Montemayor dow Corning CorporaTion

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