Cohesive-Zone Explicit Sub-Modeling for Shock Life-Prediction in Electronics

Modeling transient-dynamics of electronic assemblies is a multi-scale problem requiring methodologies which allow the capture of layer dimensions of solder interconnects, pads, and chip-level interconnects simultaneously with assembly architecture and rigid-body motion. Computational effort needed to attain fine mesh to model chip interconnects while capturing the system-level dynamic behavior is challenging. Product-level testing depends heavily on experimental methods and is influenced by various factors such as the drop height, orientation of drop, and variations in product design. Modeling and simulation of IC packages are very efficient tools for design analysis and optimization. Previously, various modeling approaches have been pursued to predict the transient dynamics of electronics assemblies assuming symmetry of the electronic assemblies. In this paper, modeling approaches to predict the solder joint reliability in electronic assemblies subjected to high mechanical shocks have been developed. Two modeling approaches are proposed in this paper to enable life prediction under both symmetric and anti-symmetric transient-deformation. In the first approach, drop simulations of printed circuit board assemblies in various orientations have been carried out using beam-shell modeling methodologies without any assumptions of symmetry. This approach enables the prediction of full-field stress-strain distribution in the system over the entire drop event. Transient dynamic behavior of the board assemblies in free and JEDEC drop has been measured using high-speed strain and displacement measurements. Relative displacement and strain histories predicted by modeling have been correlated with experimental data. Failure data obtained by solder joint array tensile tests on ball grid array packages is used as a failure proxy to predict the failure in solder interconnections modeled using Timoshenko beam elements in the global model. In the second approach, cohesive elements have been incorporated in the local model at the solder joint-copper pad interface at both the PCB and package side. The constitutive response of the cohesive elements was based on a traction-separation behavior derived from fracture mechanics. Damage initiation and evolution criteria are specified to ensure progressive degradation of the material stiffness leading to cohesive element failure. Use of cohesive zone modeling enabled the detection of dynamic crack initiation and propagation leading to IMC brittle failure in PCB assemblies subject to drop impact. Data on solder interconnect failure has been obtained under free-drop and JEDEC-drop test.