The evaluation of copper migration during the die attach curing and second wire bonding process

The copper migration on the silver plated surface of the lead-frames with various heat treatments was evaluated by X-ray photoelectron spectroscopy (XPS), transmission electron microscopy (TEM) and atomic force microscopy (AFM) methodologies. The copper migration may introduce copper oxidation and result in the wedge bonding failures due to the non-stick on lead (NSOL). The experiment was performed on the two kinds of TQFP leadframes with the stamped and etched manufacturing processes. XPS results showed that the etched leadframe was the relatively better one in that less copper oxide was detected on silver surface after annealing process. However, more copper was clearly observed to diffuse onto the silver surface after annealing process in the stamped leadframe. In comparison between the stamped and etched lead-frames, the silver plated layer in latter more efficiently blocks the copper diffusion - either surface or bulk diffusion. In addition, TEM and AFM provided the additional insight of the grain structure and surface roughness measurement of silver.