Lifetime of solder joint and delamination in flip chip assemblies

The effects of underfill on thermomechanical behaviors of two types (B and D) of flip chip packages, with different bumping size and stand-off height were investigated under thermal cycling by both experiments and finite element simulation. The results show that the use of underfill encapsulant increases tremendously (∼20 times) the thermal fatigue lifetime of SnPb solder joint, and weakens the effects of stand-off height on the reliability, and changes the deformation mode of the package. It was found that the thermal fatigue crack occurs in the region with a maximum plastic strain range, and the Coffin-Manson type equation could then be used for both packages with and without underfill. The effects of material models of underfill, i.e. constant elasticity (EC) and temperature dependent elasticity (ET) as well as the viscoelasticity (VE), on the thermomechanical behaviors of flip chip package were also studied in the simulation. The VE model gives comparatively large plastic strain range, and big displacements in shear direction, as well as sequentially low lifetime of solder joints. The ET model gives the close results to VE model and could be used instead of VE in simulations for the purpose of simplicity. Underfill delamination analysis of flip chip on low-cost board is also presented. The delamination propagation rates at the interface between chip and underfill have been measured by using C-SAM inspection of flip chip assemblies under thermal cycle loading. The experimental measurement was done in four cases, that were type B with fine solder joint and fractured solder joint, and type D with fine and fractured solder joint. In the finite element simulations the strain energy release rates G and the phase angles φ near the delamination crack tip were calculated for four measurement cases by employing the fracture mechanical method. The Paris half-empirical equations were determined from the delamination propagation rates measured and the energy release rates simulated. Meanwhile, the energy release rates G with a different delamination crack length were also simulated. The G ∼ a curve representing a convex shape when the crack propagates and indicates that the delamination crack may be stable. After propagating for a certain lengt- h the crack will be arrested in the flip chip assembly.