SMTA International Conference Proceedings


Authors: Kiyoshi Hasegawa, Yoshinori Ejiri, Takehisa Sakurai, and Yosiaki Tsubomatsu
Company: Hitachi Chemical Co., Ltd.
Date Published: 10/4/2009   Conference: SMTA International

Abstract: This study examines the wire bonding reliability of electroless gold plating after heat treatment. The results indicate that in the electroless Ni/immersion Au/electroless Au (ENIGEG) plating process, the electroless Ni (EN) under the immersion Au/electroless Au (IGEG) diffused onto the surface of the gold plating by heat treatment and changed to nickel oxide, resulting in the prevention of wire bonding between the gold wire and the gold plating and a decrease in the bonding strength. For restraining the diffusion of underplated nickel onto the surface of the gold plating, it was found that the application of electroless Pd (EP) between the underplated nickel and the gold plating is effective. It was concluded that the wire bonding reliability of ENEPIGEG after heat treatment was equivalent to that of conventional electrolytic Au plating on electrolytic Ni plating.

In order to investigate other causes for low wire bonding reliability in ENIGEG, bonding strengths resulting from combinations of electrolytic and electroless processes were evaluated. A noteworthy result was that the bonding strength of electrolytic Ni (SB-Watt)/IGEG was remarkably better than that of EN/Electrolytic Au after heat treatment. The wire bonding reliability of electroless Au deposit (IGEG) was excellent on electrolytic Ni (SB-Watt); however, it was poor on electroless Ni. It was found that the grain size of the electroless gold deposit coordinated with the grain size of the underplated Ni deposit with the growth of the epitaxial crystal. The large grain size of the gold deposit decreased the grain boundary of the gold deposit and reduced the grain boundary diffusion from the underplated metals onto the gold plating. In the gold wire bonding, the large gold grain improved the bonding strength by reducing the contaminations accumulating at the gold grain boundary of the interface between the gold plating and the gold wire. It was concluded that the wire bonding reliability after heat treatment depended on the diffusion behavior of the underplated metals and the grain size of the gold deposit.

Key words: ENEPIGEG, wire bonding reliability, grain size, boundary diffusion, electroless plating process, electrolytic plating process

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