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Thermal Cycle Consideration in Applying Lead-Free TFBGA Simulation to a Design
Keywords: thermal cycle, fatigue life, strain energy density
Three-dimensional (3D) stacked die packaging has become the trend to improve efficiency for transmitting signals, reduce the volume of the package, and help with functions integration. It is more and more popular in telecommunications when applying it to DRAM and Flash dice in electronic products. Solder joints, where there are the interconnections in the packaging structure, are more easily influenced and damaged by critical stress from thermal cycle loading when the component is operating, because the coefficient of the thermal expansion of various materials mismatch. People have studied the common single die during the thermal cycle loading for a long time, but the research for the stacked die ball grid array are not as complete. This is the reason that we focused on investigating the stacked die package. In addition, to protect the environment and comply with legislation, the semiconductor industry wants to find a lead-free material to replace the lead solder joints. SnAgCu solder is accepted as the best lead-free material so far. Therefore, most extant or new types of packaging started using lead-free material to do thermal cycle testing. There are many studies focusing on the effects of the design variations in package dimension [1-8]. The critical solder ball location is not the same as in a single die package where at the die corner, due to it is influenced by stacked die with different geometry, the difference in dice length helps to redistribute the die stress. Darveaux’s approach is a common methodology used to calculate the fatigue life based on energy and damage accumulation theories [9-12]. Focusing on this, we would follow the literature to investigate deeply and explore the area which is not as well-studied. By using the finite element analysis method to analyze the critical solder ball and integrate this with Darveaux’s approach; we understand the life cycle of solder joints during thermal cycle. In this paper, we used FEA software, ANSYS, to build up a 3D thin-profile, fine-pitch, ball grid array (TFBGA) model. The modeling investigated the effect on solder joints under thermal cycle and included deformation, von-Mises stress and strain energy density (SED). Then we utilized Darveaux’s approach to predict the fatigue life of the critical solder joint and modify our model with the theory. We changed various kinds of SAC solder joints with different ratios of the components and Anand’s constants, which were used to calculate strain energy density, analyze the divergence, what they have in common, and compared them with the lead solder material. We carried out the metallographic experiment with actual package structure, observing the failure phenomenon on solder joints. This was done in order to ensure our simulations are credible and as close to the real thing as possible. We could develop a model to forecast the failure phenomenon and fatigue damage on an electronic package. This would help engineers and designers raise the reliability of the package. In addition, we hope to find a lead-free material that can be substituted for the lead solder material. If this is the case, based on simulating results, we hope to improve it and have much a better reliability performance than the original lead solder.
Yi-Chuan Tsai,
National Sun Yat-Sen University
Kaohsiung, Taiwan, R.O.C.

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