Numerical Investigation of Moisture Diffusion Effects on Underfill within Flip-Chip Packages

Abstract

In flip chip packages, the underfill between the solder and the package is generally used to improve the thermal performance and increase the thermal fatigue life. The underfill reduces the shear stresses induced in the package at reflow that is due to the thermal expansion coefficient mismatch between die and substrate. However, the underfill can be a significant source of stress when moisture is present. Prediction of the moisture concentration and resulting hygroscopic stresses are of particular importance for manufacturing processes that require high temperatures, such as lead free soldering. We numerically investigate the absorption process by solving a mathematical model for the time-dependant transport of moisture into the package through diffusion. Once the moisture concentration is known, we calculate the hygroscopic stresses due to the expansion of the underfill. The model is discretized using standard finite element analysis (FEA). We use the model to investigate the moisture sensitivity of two types of flip chip packages with underfill, the Flip Chip Ball Grid Array (FCBGA) and Flip-Chip on Board (F-COB). Also, we investigate the effects of the underfill amount on the package stresses, both for normal and shear stresses on the UBM (Underside Bump Metallurgy). Maximum stresses in the FCBGA package are determined by FEA for various amounts of underfill, for both hygroswelling loads during PCT (Pressure Cooker Test) and thermal loads that occur during reflow.
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