Intermetallic Compound Formation and Mechanical Property of Inner Lead Bonding Joints

A Sn-plated Cu lead is bonded with an Au bump to form an inner lead bonding (ILB) joint, and the adhesion strength of the joint is examined using peeling test. Effects of bonding temperature (force) on the joint's adhesion are investigated. The 200-nm-thick Sn layer at the bottom of the Cu lead is consumed, and two reaction product layers are formed at the Cu/Au interface after bonding. One is the ζ-(Au,Cu)5 Sn phase next to Cu, and the other is the α phase, that is, the (Au) or (Cu) solid solution dissolved with minor Sn, next to Au. Nanocracks are easily formed in the α phase layer after peeling test, indicating that the α phase was the mechanically weakest point in the ILB joint. In the ILB joints fabricated at lower bonding temperatures (forces), nanocracks are initiated by peeling test and propagate along the bonding interface, resulting in complete detachment of the Cu lead from the Au bump. Raising the bonding temperatures (forces) contributed to a better coverage of Sn on both sides of the Cu lead and deeper embedment of the Cu lead in the Au bump. The adhesion strength of the ILB joint is enhanced, and accordingly the joint withstands the peeling test without delamination.