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FAB Characterization for Copper Fine Pitch Bonding
Keywords: copper wire, EFO current, FAB repeatability
With the increasing demand of copper wire as a replacement of gold wire in electronic packaging, copper wire has also started to penetrate into fine pitch processes. Cu wire down to 0.8mil is in mass production and 0.7mil and beyond are under qualification, though there are still many challenges to be overcome. One of these challenges is free air ball (FAB) size repeatability, which is especially critical in finer pitch processes. A FAB is formed by the ball bonder's electrical-flame-off (EFO) subsystem, which employs a highly controlled electric spark to melt the wire tail. Under comparable test conditions, such as wire diameter, EFO current setting, and target FAB size, copper wire FAB size variation is significantly higher than that of gold wire. In this study, we focused on FAB repeatability evaluation with 0.7mil diameter wire as a 1st phase of a broader effort to explore pitch capability below 50um. Evaluations were carried out on a K&S automatic ball bonder, equipped with a copper wire bonding kit carefully setup to provide cover gas for ball formation. EFO current and fire time were varied in order to meet a certain ball size ratio under several different forming gas flow rates. At each condition, the FAB diameter repeatability was measured. In addition, ball size variation vs EFO current was analyzed with the empirical model originally derived from the EFO-FAB energy input/output system. By considering the relationship between EFO fire time and current for a given virgin wire diameter and target ball size, the model was amended to quantitatively characterize energy distribution and understand the relationship between ball consistency and the input energy. FAB size variation as a fucntion of forming gas flow rate was also investigated and the results are discussed. This FAB size repeatabilty study demonstrated our capablity of forming repeatable FABs and our strive to provide a copper bonding solution for fine pitch bonding, at 45um pitch with 0.7mil wire diameter.
Liming Shen, Staff Engineer
Kulicke & Soffa Pte. Ltd.
Singapore 554910,

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