Material behavior changes in underfill encapsulants exposed to isothermal aging

Silica filled epoxy encapsulants used for microelectronic packaging are known to exhibit evolving properties that change significantly with environmental exposures such as isothermal aging and thermal cycling. Most aging effects are typically exacerbated at higher temperatures at or above the glass transition temperature (T_g) of the encapsulant. Such extremes are often used during the high temperature dwells present in thermal cycling qualification tests for harsh environment electronic packaging. In this work, the first measurements of material behavior changes occurring in flip chip underfill encapsulants exposed to isothermal aging at temperatures near the T_g of the material have been performed. A microscale tension-torsion testing machine was used to evaluate the uniaxial tensile stress-strain and creep behaviors of an underfill material at several temperatures, after various durations of isothermal aging at 125 degC. A method has been developed to fabricate underfill uniaxial test specimens so that they accurately reflect the encapsulant layer present in flip chip assemblies. Using the developed specimen preparation procedure, samples were prepared and then aged for up to 500 hours at 125 degC. Stress-strain and creep tests have been performed on both non-aged and aged samples, and the changes in mechanical behavior were recorded for various durations of isothermal exposure. The obtained results showed an obvious degradation of the underfill mechanical properties and creep behavior as a function of the duration of the prior aging/preconditioning at 125 degC. Both the elastic modulus and ultimate tensile strength decline monotonically with the amount of isothermal aging. The observed trends suggest that this underfill will eventually lose all rigidity and cohesion if aged appropriately long at 125 degC