Fracture mechanics approaches have been used to study reliability problems in electronic packages, in particular, adhesion related failure in flip chip assembly. It was verified in this work that the J-integral with a special flat rectangular contour near the crack tip can be used as energy release rate at the interface between chip and underfill. Meanwhile, the delamination propagation rates at the interface was measured by using C-mode scanning acoustic microscope (C-SAM) inspection for two types of flip chip packages under thermal cycle loading. Finally, the half-empirical Paris equation, which can be used as a design base of delamination reliability in flip chip package, has been determined from the crack propagation rates measured and the energy release rates simulated.

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Plastic packaging is the most widely used package for nonmilitary applications where hermeticity is not required because plastic materials are cheaper than ceramic materials. Some surface-mount plastic packages may be moisture sensitive. For example, if they contain moisture above a threshold limit and are subjected to reflow soldering temperatures, crack and even popcorning may happen because of the vapor pressure and lead to the failure of plasticpackaged components.Absorbed moisture affects the thermomechanical properties of epoxy materials. It reduces the glass transition temperature (T g ), the modulus, and the strength, especially at high temperature. 1 Moisture diffusion through epoxy is the cause of corrosion of metals in electronic packaging. It has been reported that the dominant reason of flip-chip on-board electronic failure was corrosion of the Al pad in a moist environment. 2 The moisture inside the package also can change the electric properties of materials. Tong monitored moisture diffusion in epoxy based on the principle that the absorbed moisture increased the dielectric constant of epoxy materials. 3

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 C=5.54,β=−1.38 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.