Characterization of Moisture Induced Die Stresses in Flip Chip Packaging

Moisture is a significant contributing factor to the failure of microelectronic packaging including phenomena such as popcorn cracking, delaminations, and interfacial fracture. While the effects of moisture have been examined extensively in plastic encapsulated packages (e.g. DIP and PBGA), there have been no published studies on the effects of underfill and substrate moisture absorption on the die stress evolution and delamination growth in flip chip assemblies. In this study, on-chip piezoresistive sensors were used to perform a variety of measurements of moisture-induced device side die stresses in flip chip packaging. Both flip chip on laminate and flip chip ceramic ball grid array (CBGA) packaging configurations have been studied. The two types of assembled flip chip packages were exposed to MSL-1 conditions (85/85) in an environmental chamber for various durations from 0 to 240 hours. Both the sample weight gain and transient die stresses were monitored as a function of the exposure time in the high humidity environment. In addition, the moisture-exposed samples were subsequently baked in a dry atmosphere to drive the moisture back out of the samples and to see whether the effects of moisture absorption were reversible. After the initial 10-day moisture exposure and subsequent redrying, selected samples were then subjected to moisture cycling to characterize the evolution of the die stresses from cycle to cycle. The 85/85 hygrothermal exposures were found to generate tensile die normal stress changes of up to 30 MPa in the flip chip on laminate assemblies, where both the underfill and BT substrate absorbed significant water. Relatively small die stress changes were found in the flip chip CBGA, where the ceramic substrate was hermetic and only the underfill absorbed moisture. Upon redrying, it was observed that the moisture-induced stress changes were almost fully recoverable. The hygrothermal properties of the underfill and substrate were characterized, and used within finite element numerical simulations of the absorption process in the flip chip on laminate assemblies. Good correlations were obtained between the sample weight and die stress predictions of the simulations and the analogous measurements made in experimental testing.