Abstract
The growth behaviors of intermetallic compounds (IMCs) in Cu pillar solder bumps during high temperature aging were studied by microstructural observations and mathematical calculations. Different from larger solder joints, in microbumps, surface diffusion becomes much more significant when considering IMCs growth. With transmission electron microscope (TEM), lateral Ni3Sn4 (near the interface between Ni3Sn4/Ni), sidewall Ni3Sn2 (on the surface of Ni barrier) and sidewall Cu3Sn (around the surface of copper pillar) are observed for Sn atoms diffusion. In our model, we assume that the growth area of lateral Ni3Sn4 equals that of Ni3Sn4 layer, and the growth constant for lateral Ni3Sn4, which can be calculated, is about 0.0257 μm/h1/2. Based on experimental results and diffusion theory, formation mechanisms of IMCs in microbumps are proposed. The extra growth of IMCs caused by surface diffusion can be a potential failure hazard in high-density electronic packaging.
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