Swelling and time-dependent subcritical debonding of underfill during temperature-humidity aging of flip chip packages

Stress corrosion cracking and the associated time-dependent subcritical debonding of the underfill-die interface is believed to be responsible for a large disparity between the reliabilities of flip chip-underfill packages subjected to two different aging conditions: 1) 85°C/85% RH (>3500 h) and 2) 121°C/100% RH (∼100 h). The ratio of the transverse stress at the bump (Sy) to the shear stress at the die edge (Sxy) has been found to correlate well with the two characteristic autoclave failure signatures: random local failure and die edge delamination. The first failure mode is promoted by a high Sy-Sxy ratio. A practical finding from this study is that raising the curing temperature of the underfill offers a simple and effective means of enhancing the autoclave performance of the flip chip-underfill package.