The influence of aging on the creep behavior of underfill encapsulants

Microelectronic encapsulants exhibit evolving properties that change significantly with environmental exposures such as isothermal aging and thermal cycling. Such aging effects are exacerbated at higher temperatures typical of thermal cycling qualification tests for harsh environment electronic packaging. In this work, the material behavior changes occurring in flip chip underfill encapsulants (silica filled epoxies) have been characterized for isothermal aging at four different temperatures that are below, near, and above the T_g of the material. A microscale tension-torsion testing machine has been used to evaluate the creep behavior of the underfill material at several temperatures, after various durations of environmental exposure. A novel 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 isothermally aged for up to 10 months at 80, 100, 125, and 150°C. Creep tests were then performed on both non-aged and aged samples at three different elevated temperatures where creep is significant (80, 100, and 125°C). The changes in mechanical behavior were recorded for the various aging temperatures and durations of isothermal exposure. Empirical models have been developed to predict the evolution of the creep strain rate as a function of temperature, aging time, and aging temperature.