A hybrid fracture-damage model for computationally efficient fracture simulations in solder joints

The ability to accurately simulate the response of solder alloys during cycling tests and assess its reliability is critical to developing reliable packages. A novel hybrid approach based on damage and fracture mechanics is proposed and used to predict crack trajectory and fatigue life of a solder joint subjected to temperature cycling and power cycling conditions in this paper. A 144 I/O TI Microstar Chip Scale Package (CSP) is experimentally characterized to study the thermomechanical fatigue of the Sn-Pb solder joints under both traditional temperature cycling test and power cycling tests. Through this characterization, the developed hybrid damage-fracture model is validated. The damage criterion chosen in this study is shown to possess a form similar to other damage measures for fatigue fracture that have been recently proposed in the literature dealing with Cohesive Zone Models of fracture mechanics. A hybrid computational approach in which a first order expansion of the damage rate is used to propagate the crack is proposed. This approach is shown to be superior in its computational efficiency by two orders of magnitude compared with the either the classical fracture analysis or the classical damage models. Overall, the model is shown to predict the fatigue life of the joint to within 20% of the experimentally determined life. More importantly, the predicted crack trajectory is demonstrated to agree very well with the experimentally observed trajectory. The experimentally observed microstructural changes are also demonstrated to show an excellent agreement with the changes in the rate of the damage criterion used in the model.