A corrosion study of Ag–Al intermetallic compounds in chlorine-containing epoxy molding compounds

Abstract

Typical epoxy molding compounds (EMC) contain chlorine ions that cause silver wire-bond failures under highly accelerated temperature/humidity stress test (HAST). To understand the corrosion mechanisms, experiments were designed to emulate the HAST environment and conditions and performed. Specifically, Ag–Al joints, Ag, Al, Ag3Al, and Ag2Al samples were encapsulated in chlorine-containing EMC, respectively, and went through HAST. EMC with high chlorine content (1000 ppm) was chosen to accelerate the corrosion rate and shorten experiment time. The experimental results show that chlorine-induced corrosion occurs only on Ag3Al and Ag2Al compounds. It does not occur on Ag or Al samples. The corrosion rates of bulk Ag3Al and Ag2Al disks were measured under HAST up to 125 h. The corrosion rate of Ag3Al is twice of that of Ag2Al, meaning that Ag2Al is more resistant to corrosion. Careful evaluations reveal that the corrosion is caused by both moisture and Cl− ions. Microstructures, compositions, and phases of corroded regions were examined in details. In the corroded regions, Ag3Al and Ag2Al compounds disintegrate into aluminum oxide pieces and dispersed Ag precipitates with voids and cracks. The chlorines detected near voids and cracks suggest formation of aluminum chloride as an intermediate product that is hydrolyzed into aluminum oxide. A corrosion chemical model is proposed to establish the reactions of Ag3Al and Ag2Al compounds with chlorine-containing EMC. New information obtained in this research can help the search of new processes and techniques to counter or reduce the chlorine-induced corrosion of devices and packages. The corrosion test method presented may possibly become an acceptable test standard to consider.