Palladium surface finishes for copper wire bonding (Part I: The selection of 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 entering final qualification. 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 require 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 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 an alternative copper wire bondable surface finish for substrates mainly with palladium as the final copper wire bondable layer. This offers further cost reduction possibilities. Furthermore, copper palladium intermetallics are regarded as very reliable.