Many engineers have traditionally designed bonding applications to withstand extreme temperatures using the idea that substrate materials and adhesives with similar coefficients of thermal expansion (CTE) will, at a given temperature, change size at the same rate. However, the difficulty of finding substrates and adhesives that have compatible CTEs and meet all application requirements can limit design freedom and in some cases, prove impossible. In addition, the extreme temperature stress of thermal cycling can cause bond lines to weaken or fail.
A new emerging theory for designing bonding applications is to forgo designing solely around CTE and instead choose an adhesive with the ability to withstand thermal cycling stresses, such as an epoxy, silicone or ultraviolet cure adhesive. Epoxies exhibit a high glass transition temperature (Tg), an indication that the adhesive will perform well as temperatures rise. Epoxies, while normally considered brittle, can be formulated with enough flexibility to withstand temperature fluctuations. Silicones exhibit flexibility and elasticity suited for resisting thermal shocks or rapid temperature changes but offer a weaker bond strength and more limited substrate compatibility. Ultraviolet cure adhesives, though limited in temperature resistance and substrate compatibility, can be utilized as a single-component, fast-curing adhesive.
