Ball Bond Process Optimization with Cu and Pd-Coated Cu Wire

Abstract

Cu and Pd-coated Cu (PdCu) wire have been replacing Au wire in electronic packaging. The biggest reason for this transition is the high cost of gold. Methods for process characterization and optimization in Cu and Pd-coated Cu wire differ substantially from those typically used for Au wire. This paper will examine how modifications and newly developed metrology tools are used in the characterization of PdCu and Cu wire bonding. Shear strength, traditionally the most important measure of bond quality in Au wire bonding, is of only limited use in Cu wire bonding. Ball pull has become a much more useful measure of bond quality since it is a more direct measure of too little bonding (ball lifts) and too much bonding (pad peeling). Other important measurements for Cu wire bonding include measurement of Al pad splash and bonded ball cross-sectioning to inspect the ball pad interface and the remaining Al thickness. For Au bonding, the process recipe can be fairly easily optimized and transferred to another bonder. Process windows are much narrower in the Cu and PdCu 1st bond process. It is very critical to develop a robust process with an acceptable process window for Cu wire bonding and to have good control for equipment and material during production. Due to the high demand of process optimization, equipment repeatability, material and production control, collaborations between companies in the electronic packaging supply chain is important to ensure advances in Cu wire bonding technology.