Bonding wire options and their impact on product reliability

Abstract

With the current gold price, the semiconductor industry finds itself caught in a dilemma. The industry has traditionally maintained an average 15% cost down per year per function over the range of products it offers [1]. Part of this reduction comes from packaging where manufacturing efficiencies such as cycle time reduction, supply chain management, and yield improvement allow for continued reductions in the cost per I/O. As the leaded package families are expected to increase in volume, while die sizes shrink over the same period, continued cost reduction will be difficult to achieve based on current practices and raw material prices. The need to remain competitive has compelled integrated device manufacturers and subcontractors to look at alternative material types, most notably copper wire. A recent survey of the industry has revealed that there remain very widespread concerns on migrating away from the use of gold [2]. One needs to carefully study the history of gold and copper in the manufacturing of semiconductor devices and their roles in the various interconnect methods used, both at the die level as well as the package level. Gold has been the preferred final plating layer in PCBAs for more than 65 years. Gold wire has been used for the vast majority of wire bonded interconnects for more than 50 years. Yet, we are still discovering new failure mechanism brought about, in part, by the introduction of new materials both in the wafer fabrication processes and in the package assembly area. Man has used copper for more than 10,000 years. As wiring, copper has been the preferred base material. It has also been the dominant material used in lead frames and PCBAs. For interconnections on the chip and from the chip to the lead frame, it is still in its very early stages. At the die level copper compromises dielectric properties resulting in circuit inoperability [3]. In die metallization, high concentrations of copper in the aluminum films result in localized corrosion during storage and at the wafer saw operation [4]. At the die metallization / ball bond interface, a galvanic cell is established in the presence of moisture and halides that leads to rapid degradation of the bond's electrical properties and mechanical strength [5]. In addition, research over the last two years indicates that there are reliability related issues with the copper/aluminum bond when exposed to tests that utilize high moisture levels [6,7]. The performance of such bonds is inferior to the established gold/aluminum bonds. In addition, bond pad damage can be induced in the layers beneath the pad metal also resulting in long-term reliability issues [8]. This paper will briefly review the reliability history of gold wire with respect to the above-mentioned mechanisms and elaborate on results of copper wire bonding with respect to long-term reliability and potential failure mechanisms.