Effects of flux residue and thermomechanical stresses on delamination failure in flip chip packages

The reliability of electronic packages is strongly influenced by the thermomechanical stresses developed during the assembly processes. Understanding the mechanisms of relaxing the high stress concentrations through the selection of materials with appropriate properties, adhesive joint and bump geometry will aid in design improvements for enhanced reliability of electronic packages. Finite element analysis is conducted to evaluate the stress profiles in a flip chip assembly that contains defects and delaminations at various locations. The residual stresses evolved due to the various temperature excursions during assembly processes are taken into account. Parametric studies are made of the effects of elastic modulus and coefficient of thermal expansion of underfill resin on residual stress profiles, especially at potential failure sites amongst the various interfaces. It is revealed that the presence of flux residue at the bottom corners of solder bump does not impose a significant threat to the generation of stress concentrations, and thus the package reliability during the assembly processes and mechanical loading. On the contrary, the presence of delaminations along the solder-underfill interfaces caused more than twice the stress concentrations than those with perfect interface bonding and with flux residue at the bump corners. It is proposed that these stress concentrations can be reduced systematically by optimising the material properties, e.g. by increasing the elastic modulus of underfill resin while decreasing the modulus of solder towards those of the underfill and PCB substrate. The effects of CTEs of package materials are discussed. An experimental study is carried out to measure the interface adhesion between various package components in the flip chip package. Although the adhesive strength of the underfill was found stronger with the passivation surfaces than the soldermask surfaces, from the mechanical loading viewpoint delamination was still prone to occur near the die side. The surface energy measurements indicate that for similar solid surfaces an increase in dipole hydrogen bond resulted in stronger adhesion of the joints.