THE EFFECT OF THIN-FILM STRUCTURE AND PROPERTIES ON GOLD BALL BONDING

The gold ball bonding process is widely used for making interconnectio ns between integrated circuit chips and package lead frames, yet the r elationships between the wire/substrate materials properties and the b ond formation processes are not yet well understood. While the creatio n of a metallurgical bond at the interface between the wire and substr ate is required, the deformation of the wire and substrate also play a n important role in bond formation. Bonding to thin film substrates is of particular interest, since thin films often exhibit mechanical beh avior distinctly different from bulk materials. In the present study, a systematic investigation has been conducted to understand the effect s of the structure and properties of aluminum thin films on the qualit y of gold ball bonds. A series of aluminum thin films was fabricated w ith systematic variations in hardness, roughness, thickness, and compo sition. Gold wires were ball bonded to these substrates, and the bonda bility and bond shear strengths were assessed. Metallographic sections of several of these specimens were made and examined in the scanning electron microscope. The results show that the film thickness has the most dominant effect on the bondability and bond strength; films that were 0.5 mu m thick often exhibited low strength or poor bondability. Very hard films also gave poor results. Ultimately, these results can be used to predict the wire bond reliability expected from various typ es of thin film metallization.