Influence of flux selection and underfill selection on the reliability of flip-chips on FR-4

Flip chip assembly is an attractive approach to further reduction of size and weight in electronic packaging. For cost-effective products, organic FR-4 substrates are used which require the application of underfill to ensure a sufficient long-term reliability. Especially, the combination of underfill and flux, which is necessary for soldering, can be regarded as main factor that determines the lifetime of flip chip modules. This paper focusses on methodologies to evaluate and to select fluxes and underfills for flip chip assembly. Three fluxes were selected from more than 15 materials with respect to robust wetting properties and inert behavior during subsequent underfilling. Seven underfills were selected from more than 20 materials with respect to fast flow speed and uniform flow profile. A large number of flip chip modules was completely assembled to evaluate the reliability of 21 flux-underfill combinations. Results of thermal cycle tests and humidity storage showed that the reliability dramatically reduced by inappropriate flux-underfill combinations. Moreover, the influence of two different solder mask designs was compared