Abstract
Flip chip technology with Au bumps on a substrate has been widely applied to electronic equipment such as smartphone. The purposes of this study are to examine the effect of Al pad thickness on the bondability of flip chip using an ultrasonic and to clarify interfacial structures between Au alloy bump and Al pad by ultrasonic bonding compared before to after thermal cycle test. Suitable Al thickness for the excellent initial Au/Al bonding without chip cracking are 0.8μm to 1.2μm. Chip cracks were occurred in thickness condition under 0.8μm because a thin Al layer could not reduce stress to a chip under an Al pad during ultrasonic bonding process. Intermetallic compounds between Au alloy bump and chip after reflows consisted of five Au-Al layers, and pure Al layer was remained. On the other hand, after temperature cycle test at 218/423K, intermetallic compounds between Au alloy bump and chip were changed into two kinds of Au-Al layers, so pure Al layer didn't existed. In addition, if thick intermetallic compound layers were existed around bonding region, bondability was easy to deteriorate by thermal stress due to a thermal cycle test, therefore open failure rate was rising when an Al pad thickness was 1.2μm.
