Ageing characteristics of Cu Wire Bonds on Palladium Surface Finishes

Abstract

During the past two years, fine pitch copper wire bonding has finally entered high volume production. It is estimated that nearly 15% of all wire bonders used in production are now equipped for copper wire bonding. Most of these are used exclusively for copper wire bonding. In terms of pitch, copper wire is only barely lagging behind the most advanced gold applications. The most commonly used copper wire is 20um in diameter and 18um copper wire is already being used in mass production. Evaluations with even finer wire are underway. Although some technical challenges remain, many years of research have now resolved most of the problems associated with copper wire bonding and attention is beginning to shift from merely ensuring reliable manufacturing processes to optimizing processes for efficiency and throughput. The most advanced wire bonders now have pre-configured processes specifically designed for copper. In addition to throughput optimization, further cost reductions are being sought. Among these is the desire to eliminate the high-cost gold not just from the wire, but also from the substrate. On the substrate side the electronics packaging industry still works with electrolytic nickel / electrolytic (soft) gold (Ni/Au) for copper wire bond applications. This surface finish works with copper wire bonding but includes some disadvantages, such as:- Thick expensive Au layers of 0.1 to 0.4μm- Electrically connected pads (bussing for the plating) which requires added space on the substrate.- Pd-coated copper wire often delivers better results on gold covered finishes, but is two to three times more expensive as pure copper wire Furthermore electrolytic Ni/Au was not chosen for Cu wire bonding as a result of in-depth investigations for the most effective surface finish. The selection was made because it was the surface finish with the highest distribution in the market for wire bond packages. This paper is offering the results of a two company joint work regarding alternative copper wire bondable surface finishes. The result of the project is separated in 2 papers/publications. The first publication [1] presents the investigations with Cu wire bond pull forces and process windows of 23 different surface finish variations. The main aim was to identify alternative surface finishes for copper wire bonding. Within this study the surface finishes ENEP (Electroless Nickel/Electroless Palladium) and “Direct Electroless pure Pd on copper” (pure EP finish) was identified as copper wire bondable finishes with the pure Cu wire. The second part of the evaluation summarizes an in-depth study of copper wire bonding tests after thermal aging with ENEP and the pure EP surface finish using the pure copper wire. The results of this investigation does include results with pull forces after thermal aging and an IMC Investigation with FIB Pictures of the copper wire/surface finish connection in order to evaluate the reliability of such an interconnection.