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Thermally enhanced ball grid arrays (BGAs) are designed to have reduced thermal resistance through features such as heat slug, heat spreader, and thermal solder joints. This paper studies the design comparison of five types of thermally enhanced BGAs, i.e., conduction cooled BGA (C2BGA), metal-core BGA, exposed-die land grid array (LGA), slug LGA, and spreader LGA. The solder joint reliability performance of thermally enhanced BGAs is benchmarked with conventional thin-profile fine-pitch ball grid array (TFBGA). Both global and local three-dimensional finite-element analysis (FEA) models are established to predict the fatigue life of solder joints during thermal cycle testing. Detailed pad design with realistic geometry of solder balls and nonlinear material properties are considered in the model. The fatigue model is based on a modified Darveaux's approach with nonlinear viscoplastic analysis of solder joints. For the test vehicles studied, the critical solder joints are located near the package corner. Design variations investigated include the effects of key package dimensions and material properties. Design variations are mainly reported using C2BGA package as the trend for the other four thermally enhanced BGAs was similar. The choice of mold compound (MC) material is critical, and a material with higher coefficient of thermal expansion (CTE1) and lower modulus is preferred. Die size, die attach, and slug-attach material have little effect on solder joint reliability. It is observed that there is good correlation of fatigue life between modeling prediction and thermal cycle testing for C2BGA. Reliability of C2BGA thermal solder joints is proven to be excellent, and heat can be effectively conducted away from the die to the PCB. This is crucial to the design of C2BGA. In addition, solder joint fatigue life is found to be related to package warpage induced during thermal cycling test. A design with less package warpage usually has a longer fatigue life.
Date of Publication: May 2006