The thermal fatigue failure of SnPb solder joints of flip chip on board with and without underfill for two types of flip-chip packages was investigated by conducting thermal cycling test, scanning acoustic microscopy observation, and cross section inspection. The corresponding 3-D finite element simulation was performed to analyze the effects of underfill on thermomechanical behavior. The viscoelasticity of underfill and the viscoplasticity of solder were considered in the 3-D simulations. The Coffin-Manson equation with material constants was fitted from the combination of the lifetime measured and the shear plastic strain range simulated by 3-D model. In the case with underfill, the plastic strain of every solder joint becomes very similar and little dependent on the position of solder joints. The modeled axial strain distribution coincided well with the distribution of microstructure coarsening visible in cross sections. The mismatch of thermal expansion resulted in an overall warpage of the assembly for the case with underfill, which decreased the shear deformation of the solder joints and increased the interface stress on the chip. The interface stress distribution from the 3-D simulation agreed very well with the experimental observations.
Quantitative Mechanism of Significant Benefits of Underfill in Flip-Chip Assemblies
Contributed by the Electronic and Photonic Packaging Division for publication in the JOURNAL OF ELECTRONIC PACKAGING. Manuscript received by the EPPD Division, January 25, 2002; revised manuscript received March 15, 2002. Associate Editor: D. T. Read.
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Cai , X., Chen , L., Zhang , Q., Xu , B., Huang , W., Xie , X., and Cheng , Z. (March 14, 2003). "Quantitative Mechanism of Significant Benefits of Underfill in Flip-Chip Assemblies ." ASME. J. Electron. Packag. March 2003; 125(1): 84–92. https://doi.org/10.1115/1.1533802
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