Gold Wirebond on Discolored Bond Pads

Abstract

An integrated circuit wafer lot having some wafers with discolored bond pads and other wafers with normal bond pads was identified, and wafers with discolored pads were scrapped. The reason for scrap is the expectation of poor bondability or unreliable wirebonds on discolored pads. The cause of discoloration was unknown. We took the opportunity to run a bonding experiment and analysis as a student project, comparing a good wafer with one having the discolored bond pads. Unprobed die from each wafer were wirebonded for mechanical integrity analysis of the bonds. Bonding recipe designed experiments included different ultrasonic generator (USG) current settings within the bonding process window, two bonding forces, and two temperatures, all thermosonic binding 25um gold (Au) wire on AlCu pad metal of 0.7um thickness. 2% non-stick-on pads (NSOP) was found at a higher ultrasonic power settings on the discolored pads, indicating that the discolored pads can indeed be problematic in wirebonding. Analysis of wire pull test and bond shear test results indicate slightly less performance of bonds on the discolored pads. Chemical and physical analysis of the discolored pads reveals the nature of the unknown cause and leads to a hypothesis about what went wrong during the wafer processing.