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Published in:
30-06-2018
The mechanism of Pd distribution in the process of FAB formation during Pd-coated Cu wire bonding
Published in: Journal of Materials Science: Materials in Electronics | Issue 16/2018
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Abstract
The palladium (Pd) distribution over the free air ball (FAB) and its effect on the ball’s shear strength were investigated in the Pd-coated copper (Cu) wire bonding process. It was found that for the same FAB/wire diameter ratio of 1.5, the bigger the electrical flame-off (EFO) current was, the larger would be the exposed Cu regions over FAB without Pd distribution. Combining experimental observations, a model of Pd distribution over FAB was first proposed by changing the EFO current and firing time. As the firing time increased, the coated Pd element could be dissolved into the Cu base to form a PdCu alloy at the FAB surface, to protect the bonded ball from corrosion. The Pd-coated Cu wire has higher bonding shear strength than the bare Cu wire. However, the FAB with the largest Pd coverage had the smallest ball shear strength, which revealed that the control of Pd distribution over FAB is very critical for a high-quality bonding interface.