Shear Strength and Failure Modes of As-Bonded Gold and Copper Ball Bonds on Aluminum Metallization
The shear test failure modes of “as-bonded” gold and copper ball bonds are fundamentally different. Soft gold balls are typically sheared by the tool, leaving a lower section of the ball bonded to the aluminum metallization. In contrast, copper balls do not undergo appreciable plastic deformation and are sheared completely away from the bond pad. Evidence is presented to show that the different failure modes of gold are copper are due to the relative strength of the balls. Copper balls are generally harder than gold balls, and the failure mode changes to the weakest interface, which is at the ball– aluminum bond pad. Gold ball bonds fail due to plastic deformation of the ball, while copper ball bonds fail by a process of plastic deformation in the aluminum bond pad.