Copper Wirebond Compatibility with Organic and Inorganic Ions Present in Mold Compounds

Copper wirebond (CuWB) packaging is emerging as a replacement to gold wire technology because of the increases in the price of gold. However, copper is more susceptible to corrosion than inert gold so there are several reliability concerns regarding the integrity of the Cu-Al interface. Copper wirebond compatibility with the surrounding encapsulating mold compound is critical, as this material combination can produce a corrosive environment in the presence of moisture and ionic impurities. Mold compounds consist of different chemical components that can yield several ionic organic and inorganic species. The effect of chloride on CuWB reliability has been published in the literature but the impact of other ions typically present in the mold compound matrix is not widely discussed. It is important to understand the role of these ions on the CuWB reliability to design appropriate mold compounds for product encapsulation as some of the ions are corrosive while others are benign or beneficial in nature. This paper examines the impact of organic anions such as formate, acetate, and oxalate, and inorganic anions phosphate and nitrites, on CuWB reliability with regard to the integrity of the vulnerable Cu-Al interface. Corrosion depends on the pH conditions of the mold compound matrix in the presence of moisture. Cu-Al integrity under various ionic concentrations and pH conditions will be presented and the effect of these ions on the corrosive behavior of chloride will be examined. The combined influences of different anions on the Cu-Al integrity will be discussed. Finally, a relatively simple method to examine the integrity of the Cu-Al interface will also be described.