Reliability evaluation of bonding between Cu wire and Al pad

There are several reliability problems, while Cu wire bonding has been used instead of Au wire bonding in microelectronics packaging. A previous work indicated that after autoclave test maintaining at 394 K, 100% relative humidity and 0.2 MPa for 300 h, a maximum shear strength of Cu wire bonding was decreased to about half that of initial Cu. In this work, we study the reason for decrease in a maximum shear strength. From quantification I method which is one of multivariate analyses, it was found that the value of pH in the molding compound has a significant effect on the decrease in a maximum shear strength. Furthermore, Cu wire bonding to Al pad molded by higher hydrogen index (pH=5.8) compound is higher reliability than that by lower hydrogen index (pH=5.1) compound. From SEM, TEM and EDX observation, it is found that oxides are formed in Cu-Al alloy layer between Cu wire and Al pad in specimen having lower pH. It is because that lower pH atmosphere in molding compound during autoclave test enabled Cu-Al alloy to corrode. As a result, these oxides reduced the shear strength of bonding between Cu wire and Al pad.