TEM study on the Cu wire stitch bonding interface

Copper wire is the potential alternative material of the traditional gold wire in the current IC package wire bonding process, because it has several advantages such as better electrical conductivity and thermal conductivity, higher mechanical strength and lower production cost. However bondability is also reduced by copper wire´s greater hardness and that copper wire can be easily oxidized. The interface structure characteristic in bonding process is crucial and will affect the yield and reliability of bonding. This paper starts with an introduction on the formation of the stitch bonding and factors of performance. Then the microstructure information of stitch bonding interface between 4N Cu wire and silver plated lead-frame or between palladium plated Cu wire and silver plated lead-frame will be reported. The micro-topography, microstructure and element interdiffusion information of Cu/Ag and Cu/Pd/Ag interfaces will be presented respectively. The similarities and differences of two interface microstructure have been compared, and the discussion is mainly focused on oxygen behaviors in Cu/Ag interface and role of palladium in Cu/Pd/Ag interface. The research results which included the stitch bonding interface microstructure could offer the essential information to improve the quality of the Cu wire stitch interface and the bonding process.