Wire bond loop profile development for fine pitch-long wire assembly

Die size reductions can be achieved through "optical shrinks," compaction of existing layouts, or redesigns to finer fab geometries. For some die the limiting factors for die size reduction are bond pad pitch and bond pad size. In these "pad limited" designs, the circuitry is concentrated in the center of the die. Precious empty space exists between the bond pads in the periphery of the die and the circuitry in the die core. The only hope for die size reductions in these designs lies in advances in assembly technology that allow for reductions in bond pad pitches and bond pad size. Fine pitch assembly poses a number of challenges for conventional wire bond technology. Reducing bond pad pitch increases the probability of ball shorting, bond wire shorting, and bond wire damage. On the other hand, decreasing the die size by reducing the bond pad pitch results in longer wire lengths thus limiting some assembly options such as moving to smaller diameter bonding wires. Wire loop profile becomes a critical factor for control in fine pitch assembly. In this paper a statistical design of experiment is used in developing a wire bond loop profile control. The effect of major bonding parameters, such as kink-height, reverse loop, loop factor, wire tension, and their impact on loop profile are analyzed. The results obtained define the bond parameter requirements that must be met in order to control the wire loop profile to optimize fine pitch wire bond assembly yields.<
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