Theoretical Studies and Root Cause Identification of the Pinhole Defects on Al Bondpads in Wafer Fabrication and Packaging Processes

In this paper, we will study a case of NSOP failure and identify the root causes. Through research, we determined and concluded that NSOP was due to the pinhole defects on Al bondpads and the pinhole defects were caused by galvanic corrosion and silicon dust corrosion. Moreover, we found that it was introduced in wafer fab process, but during wafer sawing process step, the corrosion was enhanced and got worse at assembly house. We propose and present theoretical models of galvanic corrosion (Al-Cu cell) in wafer fab and silicon dust corrosion during wafer sawing process; also discuss failure mechanisms and key characteristics of these defects chemically and physically; and how to eliminate them so as to improve yields in wafer fabrication and advanced packaging processes.

I. Introduction

It is well-know that NSOP (No-stick on pad) problem will affect the yield in wafer fabrication and advanced packaging processes. Failure analysis engineers face more challenges as they not only need to know what caused the product failure, but also want to know whether the problem comes from either wafer fab or packaging/assembly processes.