Process Design of Fine Pitch Ball Bonding by Volume Conservation Modelling

The effect of capillary-to-capillary variation on the robustness of a fine pitch wire bonding process is studied using an analytical model developed with the goal to make predictions with respect to the shape of the bonded ball and the capillary tip geometry and tolerances for fine pitch ball bonding. The model includes stepwise deformation of the ball which is calculated based on concepts such as volume conservation. An example case is presented to demonstrate the usefulness of this volume conservation model. For the diameter of the example deformed ball, the target value is 35 μm. The wire diameter is 20 μm, the capillary tip dimensions include hole diameter 24.13 μm, chamfer diameter 29.21 μm, chamfer angle 70°, and face angle 8°. It was found the higher the ball height, the smaller the ball height variation in percent. A height to diameter ratio of 1/3 is recommended over 1/4 or 1/5 ratios. The model predicts that the smaller ratios can result in relatively large ball height variations. This leads to variations of ultrasonic energy transmitted to the bond and therefore can impact bond strength and reliability. For the example target diameter of 35 μm and a ball height of 11.8 μm, the tolerance analysis with the volume conservation model predicts a free-air ball diameter of 31.9 μm and an acceptably small ball height variation of ±8 %. The design of fine pitch or ultra-fine pitch bonding processes with modern wire materials such as Cu or Ag alloy can benefit from the reported model.