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

Electronic package reliability is influenced by thermomechanical stresses developed during assembly. Finite element analysis is conducted to evaluate stress profiles in a flip chip assembly containing defects and delaminations. Residual stresses evolved due to temperature excursions during assembly are taken into account. Parametric studies are made of the effects of the elastic modulus and CTE of underfill resin on residual stress profiles, especially at potential failure sites on the interfaces. Flux residue at the bottom corners of solder bumps does not pose a significant threat to generation of stress concentrations, and thus to package reliability during assembly and mechanical loading. However, solder underfill interface delaminations 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 material properties, e.g. by increasing underfill resin elastic modulus while decreasing the solder modulus towards those of the underfill and PCB substrate. The effects of package material CTEs are discussed. Experiments are carried out to measure interface adhesion between package components in the flip chip package. Although underfill adhesive strength was stronger with passivation surfaces than solder mask surfaces, delamination was still prone to occur near the die side under mechanical loading. Surface energy measurements indicate that for similar solid surfaces, an increase in dipole hydrogen bonds led to stronger joint adhesion