A Study on Die Stresses in Flip Chip Package Subjected to Various Hygrothermal Exposures

Moisture has been one of the major concerns for package designers and reliability researchers. It is well-known that high humidity combined with high temperature can cause a number of failure modes to electronic devices, such as popcorn cracking, delamination, or electrochemical migration. While the fundamental knowledge of moisture effects on electronic packages has been extensively explored, there have been no published studies on the effects of underfill and substrate moisture absorption on the die stresses development and delamination growth in flip chip assemblies. In this study, on-chip piezoresistive sensors were used to measure moisture-induced device side die stresses in flip chip on laminate packages under three different moisture conditions. The die stresses were also monitored during the subsequent drying to evaluate the reversibility of the moisture effects. After the initial 10 days of moisture exposure, moisture was found to have significant effects on the package, generating tensile die normal stresses of up to 37 MPa under the harshest testing environment (95% RH, 95 °C). Also, it can be observed that more harsh condition produced larger stress level, but the stresses variation under all conditions showed the same behavior. Shear stresses however were found to be quite small relative to normal stresses. Upon the subsequent drying, it was seen that the moisture-induced stress changes were almost fully recordable. In addition to the measurements of the moisture-induced die stresses, a study on hygrothermal properties of polymer materials was conducted where diffusivity, saturated concentration and coefficient of moisture expansion (CME) of underfill and BT (Bismaleimide-Triazine) substrate were characterized under the moisture conditions used in the experiments. The obtained properties were later used for the finite element simulation of the moisture diffusion. The numerical predictions were finally correlated with the experimental results. They were found to be in good agreements.