Advanced wire bonding technology for Ag wire

Abstract

With the introduction and proliferation of Cu wire bonding, the cost of wire bonding packages is greatly reduced compared to traditional Au wire bonding. Wire bonding is still the most popular interconnect technology and the work horse of the industry. Technology development and innovation in wire bonding has provided new packaging solutions that improve performance and reduce the cost. This paper reviews the pros and cons of each bonding wire type that is being used for ball bonding including Au, Cu, and Ag wire. Although Cu wire is by far the cheapest bonding wire, in certain cases Cu wire is not a viable solution due to either the lack of advanced Cu wire equipment and process or the special requirements of the package. For example, Cu wire bonding has challenges with memory devices that require low loops and thin overhang die. In this case for memory devices, Ag wire is a good alternative that can provide significant cost reduction. This paper further examines the challenges and solutions for Ag wire bonding process including Free Air Ball (FAB) formation, the 1st bond and the 2nd bond. The key developments in ball bonding equipment, process and material to overcome these challenges are discussed. We studied the FAB formation process and studied the effect of cover gas system design, EFO (Electronic Flame Off) current, EFO gap, and other factors that affect the ball formation process. We demonstrate fine pitch capability of Ag wire bonding including 1st bond and 2nd bond using 0.6 mil Ag wire. Both forward bonding processes as well as SSB (Standoff Stitch Bond) are studied using new response based processes. The different wire alloys from 87% Ag to 97% Ag are also studied and compared in order to understand the differences in wire performance.