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

Flip-chip packages can achieve high interconnect speeds, high-density and b e made in thinner dimensions. Despite these, the demand for flip-chip packa ges is still relatively low when compared to packages using wire bonding. T his is due to the requirement of 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 em erge. 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 an y other material within the flip chip package is the underfill. The interfa ces that the underfill attaches to include the die surface, solder bump and the soldermask. This paper looks at the change of contact angle over time for three underfills on the die and the soldermask surfaces. This change is also measured for varying temperatures. This is then correlated with the v iscosity of the underfill at those temperatures. Certain time and temperatu re based relationships are derived. The properties of the three underfills are also discussed and their performance under varying duration in humidity and thermal shock cycling are analyzed.