Abstract
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. Copper migration may introduce copper oxidation and result in the wedge bonding failures due to the nonstick on lead (NSOL) phenomenon. Experiments were performed on the two kinds of TQFP leadframes with the stamped and etched manufacturing processes subjected to various heat treatments and bonding conditions to understand the underlying physics. TEM and AFM provided the additional insight of the grain structure and surface roughness of silver. XPS results showed that the etched leadframe was relatively better one that less copper oxide was detected on the silver surface after annealing process. However, more copper was observed to diffuse onto the silver surface after annealing in the stamped leadframe. In comparison between the stamped and etched lead-frames, the silver plated layer in later one is more efficient in blocking copper diffusion-either surface or bulk. Finally a full factorial design of experiment (DOE) with wedge bond pull strength as response was performed to verify the results of XPS, TEM, and AFM. The evaluations based on XPS, TEM, and AFM analyzes can really help to improve the yield of the wedge bonding process and optimize the IC manufacturing process window.
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