With the larger size of Ball Grid Array (BGA) solder joints, the available volume for underfilling is significantly increased. Although the size of the solder joints and package dimension governs the volume of underfill material, the larger 2nd level solder interconnects are more susceptible to thermal fatigue with certain underfills and thermal profiles.
In this study, BGA packages were underfilled with two dedicated underfill materials and two soft materials used as conformal coatings and encapsulants in electronic products. Each of the selected materials was subjected to two thermal profiles, one with low mean temperature and a second with a high mean temperature. The variation in mean cyclic temperature demonstrates the influence of temperature dependent behavior of each underfill material on the loads solder joints experience in a BGA package. Material characterization was performed on the package and underfill materials and incorporated into finite element models. The influence of underfill material glass transition temperature (Tg) was found to be a critical factor on fatigue endurance of solder interconnects. Fatigue crack orientation within solder joints were found to be aligned with axial (normal) direction for BGAs with high CTE underfill materials. Simulations determined the magnitude of axial loading associated with each underfill material properties responsible for reducing fatigue life. The results developed in this paper reveal the factors associated with reduced fatigue endurance of certain underfill materials under temperature profiles with mean temperature conditions and contribute to the development of new criteria of underfill material selection for 2nd level interconnects.