Free Air Ball Consistency of Palladium Coated Copper Wire

Abstract

The formation of good free air ball (FAB) in palladium coated copper wire depends on many factors including the type of protection gas used, flow rate, Electric Flame Off (EFO) current, firing time and gap between the tip of wire and EFO wand. These factors can individually or, in combination of which, affect how good a ball can be formed prior to bonding onto the bond pad of a device. It is therefore important to find an optimum condition in forming good FAB so as to avoid poor bonded ball quality, or worse, non-uniform ball hardness that could lead to damage of the bond pad upon impact. The microstructure of the free air ball from palladium coated copper is unlike that of pure copper wire. It consists of palladium or palladium rich phase that can either exist partially or majority on surface of the ball making the structure non-uniform and inconsistency from one condition of EFO firing to the either. The hardness of the ball is also complicated by the non-uniform microstructure in such a way that determining the microhardness of the ball is never a straight forward characterisation. This paper covers the effect of process condition on the FAB size and quality of the wire. Microstructure and hardness of the FAB processed with different conditions are also presented and discussed.