Abstract
Au/Al wire bonding is the traditional bonding method of circuits of electronic packages, and still a very important bonding technique. Bonding strength was measured by the shear test of a gold ball on an aluminum pad. Delamination behavior was categorized into three types, cohesive fracture in the gold ball, interfacial delamination between the gold ball and the aluminum pad, and fracture from inside of the aluminum pad. The cohesive fracture in the gold ball is the most reliable in these three types of fracture. We investigated the mechanisms causing the different types of fracture from the shear test. First, we observed the surface of aluminum pads using a scanning electron microscope (SEM) and measured the orientation of crystal grains using the electron backscatter diffraction (EBSD). A porous structure was observed on the surface of the aluminum pad that caused the fracture from the inside of the pad. Almost all of the crystal grains on the aluminum pads were aligned (111). Then, we measured the material properties of aluminum pads using a nano-indenter. Aluminum pads with stiff surfaces showed interfacial fracture. Aluminum pads with soft surfaces and large absorbed plastic energy showed cohesive fracture of the gold balls.
