The impact of underfill properties on the thermomechanical reliability of FCOB assembly

The impact of underfill properties on the thermomechanical reliability of f lip chip on board (FCOB) assembly is addressed in this paper. FCOB assembli es using three underfill encapsulants were subjected to a thermal cycling t est. The performance of the underfill encapsulants was assessed by a statis tical analysis of the failure distribution of the FCOB assemblies. The fail ure modes in the thermal cycling test were round to be solder joint cracks, delamination at underfill/chip passivation interface, and underfill intern al cracks. An attempt was made to correlate these failures with underfill p roperties such as the coefficient of thermal expansion (CTE), modulus, glas s transition temperature (Tg), and adhesive strength to the chip. Additiona lly, nonlinear finite element analysis (FEA) was conducted to verify the ex perimental results.