Gold wire bonding performance and reliability of ENEPIG surface finishes

INTRODUCTION The expression „multifunctional PCB“, as a synonym for a PCB which is applicable with a variety of assembly techniques, is already established on the market. That means the PCB can be used for multiple reflow soldering and multiple assembly techniques like, Al-wire bonding, Auwire bonding, conductive adhesives and other interconnection methods. In the mixed COB/SMT assembly, where mainly Al-wire bonding is used, the ENIG Surface finish with Ni thickness of 5-6 μm and Au thickness of 50 – 100 nm, is a well established surface finish. But the ENIG surface finish can not be used for Au-wire bonding which is used mainly in the packaging industry. On ENIG sometimes bond Lift off ́s were reported that are often caused by an exceeding amount of Ni oxides at the Au surface because of Ni diffusion. At the other side the surface finishes electrolytic Ni/Au or electroless Nickel with autocatalytic gold are working well with Au-wire bonding but because of the high Au thickness that hinders a Ni diffusion for a longer time period. In addition the solder joint integrity thick Au of the surface is not regarded as a reliable material combination because brittle AuSn4 phases are formed.