Correlation of underfill viscosity and contact angle on surfaces in a flip chip package

Flip-chip packages can achieve high interconnect speeds, high-density and be made in thinner dimensions. Despite these advantages, the demand for flip-chip packages is still relatively low when compared to packages using wire bonding. This is due to the requirement for bumped dice for these packages as well as capital expenditure for new equipment dedicated to flip chip packaging. In any case, the positive benefits are expected to drive the industry towards flip chip technology and are expected to take off and grow in the years to come. As this happens, problems unique to flip chip packaging are set to emerge. Unlike traditional packaging, flip chip packaging has more interfaces that are interacting with one another. The ability of these interfaces to stay reliable under various stress conditions is critical to the success of the package. One material that has more interfaces to contend with then any other material within the flip chip package is the underfill. The interfaces that the underfill attaches to include the die surface, solder bump and the solder mask. This paper looks at the change of contact angle over time for three underfills on the die and the solder mask surfaces. This change is also measured for varying temperatures. This is then correlated with the underfill viscosity at those temperatures. Certain time and temperature based relationships are derived. The properties of the three underfills are also discussed and their performance for varying durations in humidity and thermal shock cycling are analyzed.