Gold-to-Aluminum Bonding for TAB Applications

Abstract

The gold-to-aluminum bond is a commonly used interconnection in VLSI packaging. An example is gold wire bonding to aluminum pads on silicon devices. It is well known that the reliability of the bond is strongly influenced by the formation of gold-aluminum intermetallics during bonding or in use. In wire bonding, ultrasonic or thermosonic techniques enable low temperature bonding. This limits intermetallic formation, thus providing more reliable bonds. In thermocompression bonding used in tape automated bonding (TAB) technology, the gold-to aluminum interface is one of the most difficult bonds that can be formed, because a thin aluminum oxide layer inhibits bonding at low temperatures. The usual bonding temperature is, therefore, expected to be higher than 500°C. This results in extensive growth of intermetallics. In an effort to reduce the bonding temperature of a TAB application, a laser-processed gold-plated balltape was used for thermocompression bonding to aluminum. The effects of hardness, thickness, and morphology of the gold layer on thermocompression bonding have been investigated. As a result, an improved gold layer surface morphology has been developed by controlling the plating rate. The new bumped tape structure was bonded at temperatures from 500°C down to as low as 350°C. © 1992 IEEE