Elastic modulus variation due to moisture absorption and permanent changes upon redrying in an epoxy based underfill

In this study, the influence of moisture on the elastic modulus of a no-flow underfill is characterized over a wide range of accelerated testing environments, and the permanent changes are evaluated upon fully redrying. Moisture saturation is reached in all environments prior to testing, thus the inherent wet modulus of the underfill is identified for each respective level of moisture preconditioning. The change in the elastic modulus as a function of moisture concentration is measured, as well as the reversible and irreversible effects from moisture uptake in the underfill upon redrying. Thermal aging from the temperature component of the accelerated testing environment is found to have no effect on the elastic modulus, while the moisture uptake is found to notably decrease the modulus at higher humidity levels. Recovery experiments demonstrate that most of the loss in the elastic modulus from moisture exposure is recoverable, although more irreversible damage did occur at higher humidity levels. Hydrolysis contributed to the irreversible loss in the modulus, while the reversible component is attributed to plasticization of the underfill from moisture uptake.