Characterization of palladium coated copper wire prior physical bonding assessment

While the well known challenge for conventional bare Cu wire is oxidation and the maintenance of available techniques and tools to control the consistency of oxidation free remain an extra precaution step, the introduction of Pd coated Cu wire has therefore been welcomed by the package assembly manufacturers. An understanding on the behavior of this wire type is essential to enable better engineering judgment in wire bonding process. Characterization was conducted to check the Pd coating uniformity using conventional metallographic technique and thin film measurement software feature that used Energy Dispersive X-ray Spectrometry (EDX). Micro thickness variation was observed on the Pd coating layer of the wire in both inspection techniques. Grain textures of the free air ball (FAB) that formed under different sparking parameters were examined with the attempt to correlate the texture appearance to its hardness. Distribution of Pd during FAB formation was studied by analyzing the presence of Pd on both surface and inner Cu core layer. Planar cut via Focus Ion Beam (FIB) was conducted for this purpose with the detection of Pd done using Scanning Electron Microscopy (SEM)-EDX. Pd was found slow in diffusion with majority still remained at the outer surface of the FAB. Optimization of sparking condition and other analytical means would still be needed to further investigate the whole distribution flow and understand its impact to bondability and reliability.