Effect of plasma treatment of Au-Ni-Cu bond pads on process windows of Au wire bonding

The wire bondability of Au-Ni-Cu bond pads with different Au plating schemes, including electrolytic and immersion plates, are evaluated after plasma treatment. The plasma cleaning conditions, such as cleaning power and time, are optimized based on the process window and wire pull strength measurements for different bond pad temperatures. Difference in the efficiency of plasma treatment in improving the wire bondability for different Au plates is identified. The plasma-cleaned bond pads are exposed to air to evaluate the recontamination process and the corresponding degradation of wire pull strength. The changes in bond pad surface characteristics, such as surface free energy and polar functionality, with exposure time are correlated to the wire pull strength, which in turn provides practical information about the shelf life of wire bonding after plasma cleaning.

I. Introduction

Wire bonding is still the far most popular first-level interconnection technology used in electronic and photonic packaging. The conventional wire bond pads consist of a 1–1.25--thick electrolytic Au plate over a 4–5--thick electrolytic Ni layer on a FR-4 woven glass reinforced epoxy laminate printed circuit board [1]. In recent years, techniques based on immersion Au plating over electroless Ni plating have been developed to replace the conventional expensive electrolytic Au plating [2]. In our previous paper [3], process windows were successfully established and the corresponding wire bond strengths were evaluated for several different Au–Ni bond pads that were produced using different plating methods. Important properties of the bonds pads, including Au layer thickness, surface morphology, surface roughness, and elemental composition were characterized to correlate these parameters with the wire bonding quality. It was highlighted that the surface morphology of bond pads produced by different Au plating methods was different. The immersion plating method produced a fine thin Au layer. The surface roughness of bond pads created by electrolytic plating decreased as the Au layer thickness increased. The thinner the Au layer in the electrolytic plate, the higher the hardness, the surface roughness, and the risk of Ni migration onto the Au surface, both of which adversely affected the wire bondability and process window. A much wider process window was constructed for the immersion Au plate than the electrolytic plate with an equivalent Au thickness, confirming the immersion plating process is a low-cost alternative for wire bonding. Its bond strengths with respect to bond power were also comparable to those for the bond pads with a thick electrolytic Au layer.