The Reliability Impact of Highly Temperature-Dependent Underfill Material to the Lead-Free Flip Chip Package

Since today´s trend is toward `green´ products, manufacturers are moving toward lead-free schemes. The flip chip package, which has been widely utilized in high-performance products, also faces this challenge. In addition, the underfill material in the flip chip package has a specific characteristic, glass transition (T_g) temperature, and the properties of the underfill material changes drastically when it experiences temperature difference containing its T_g. With a highly temperature-dependent feature, the underfill may have an impact on the reliability of the packaging. A methodology combining the verified finite element method and factorial analysis is established to estimate this effect in this study. In factorial analysis, the CTE and Young´s modulus of the underfill at temperatures above and below its T _g are selected. In addition to the four above mentioned factors, the T_g temperature of the underfill is also chosen as a design factor. As for the response, die cracking is a concerned failure mode to the lead-free flip chip package, and hence the first principal stress is chosen as the response in factorial analysis. Aside from these, the solder fatigue life is also discussed. This study reveals that the T_g temperature is the most sensitive factor to the reliability of both die cracking and solder fatigue life. However, the lead-free flip chip package using the underfill material with a higher T_g temperature has a better solder fatigue life but has the larger stress which may likely lead to the die crack