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The impact of underfill properties on the thermomechanical reliability of flip chip on board (FCOB) assembly is addressed in this paper. FCOB assemblies using three underfill encapsulants were subjected to a thermal cycling test. The performance of the underfill encapsulants was assessed by a statistical analysis of the failure distribution of the FCOB assemblies. The failure modes in the thermal cycling test were found to be solder joint cracks, delamination at underfill/chip passivation interface, and underfill internal cracks. An attempt was made to correlate these failures with underfill properties such as the coefficient of thermal expansion (CTE), modulus, glass transition temperature (Tg), and adhesive strength to the chip. Additionally, nonlinear finite element analysis (FEA) was conducted to verify the experimental results.