Processability and reliability of epoxy adhesive used in microelectronic devices linked to effects of degree of cure and damp heat aging

摘要

In an effort to explicate the effects of degree of cure (conversion) and damp heat aging at 85℃/85% RH on physical properties of the cured epoxy system consisting of diglycidyl ether of bisphenol F (DGBF) and imidazole as a curing agent, an experimental approach was systematically performed using a dynamic mechanical analyzer (DMA) and ther-momechanical analyzer (TMA). Of interesting physical properties are the storage modulus, loss modulus, tan δ, glass transition temperature (T_g), and coefficient of thermal expansion (CTE). The die shear test was performed to evaluate changes in adhesion strength of the cured epoxy with and without an epoxy silane before and after the damp heat aging. We have found that the magnitude of storage modulus in a glass transition region and the onset temperature for degradation of storage modulus, DMA T_g, increase as the conversion of cure increases. Even though such benefits observed at ambient are lessened after extended exposure to the damp heat aging, higher conversion has less degradation than lower conversion. It is observed that T_g taken from DMA and TMA decreases approximately from 132℃ to 81℃ after 500 h of damp heat aging. Such large diminution is attributed to high moisture diffusivity of 2.0 x 10~(-8)/cm~2/s at 85℃/85% RH and can cause large dimensional instability at the critical interface supported by the epoxy adhesive. We have also found that a 3-parameter exponential function is useful to predict the degradation of T_g under the given aging condition. Although the effect of an epoxy silane is negligible at ambient, it provides better adhesion strength after the damp heat aging. Finally, we intend to give process guidelines for the usage of DGBF/imidazole epoxy system to ensure stringent reliability in electronic device applications.