Effect of grain size evolution in an Au-wire ball on debonding failure with the Al-pad in semiconductor devices.

This article details how a 99.99% Au-wire ball with an appropriate grain size prevents void formation at the interface region with its underlying Al-pad, which is responsible for wire debonding failure in lead-frame packages during thermal-cycling. Metallurgical examinations showed that an Au-wire ball with a grain size smaller than 3 μm in diameter allows the faster diffusion of Al atoms into it, leaving troublesome voids at the interface region with the Al-pad region. Meanwhile, the investigation revealed that an Au-wire ball with a grain size larger than 8 μm effectively suppresses the void formation due to the slower diffusion of Al atoms from the Al-pad to it. However, an Au-wire ball with such a large grain size (bigger than 8 μm) did not allow the formation of a tough intermetallic compound such as AuAl 2 , which is favorable for the reliable adhesion with the underlying Al-pad. Instead, the Au-balls with such a larger grain size were more favorable for the formation of a brittle intermetallic compound such as Au 4 Al at the interface region due to the lower density of active Al atoms, resulting in bonding reliability degradation. Consequently, in order to ensure the reliability of Au-wire bonded on an Al-pad in lead-frame packages during thermal-cycling, the wire ball should have a grain size of between 3 μm and 8 μm in diameter. [ABSTRACT FROM AUTHOR]