Enhancing Ultrasonic Bond Development

To enhance ultrasonic bond development, an improved understanding is needed of the processes taking place at the bond interface. This work is designed to determine the role that bonding parameters play in contributing to bond development. Of particular concern are the effects created by the machine variables, i.e., load, power, and time. Silicon was selected as a bond surface for aluminum wire bonds. The brittle nature of silicon provides a permanent record of the bonding history. With both the crystal orientation and defects of the silicon well characterized prior to bonding, features such as the location of residual bonding strains in the silicon were determined. The pattern of partially bonded material exposed by peeling underdeveloped bonds simulates a torus (or doughnut) with an unbonded central region. Features of aluminum wire bonds to aluminum, glass, beryllium, and silicon were compared to show that a common mechanism exists independent of the bond surface material. The ability to bond to silicon varies with wire composition. For example, both Al-0.5% Mg and pure Al wires bond readily, while Al-l% Si wire does not. Two modes of material flow characterize interfacial behavior. Ultrasonic energy promotes a material softening which, in conjunction with the applied load, results in a gross flow to expose fresh material for bonding. In the second stage of material flow, a wave form is propagated through the wire to the periphery of the wire-silicon interface. This wave form is observed as a periodic cutting action into the silicon perpendicular to the pulsing direction. A fine ball-like formation in the grooves of the wave region at the bond zone was a feature common to the different bonding surfaces. The wavelength and wave amplitude vary linearly with the applied power as does the tip-to-tip displacement of the wedge. The groove spacings are of the same magnitude as the wedge displacement. For constant power and time, increased load increases the size of the central bond region that does not experience the wave action. For constant load and power, the width of the wave affected periphery increases toward the center of the bond with time. The method of thermally induced stacking faults by steam oxidation was used to characterize the residual bond strains in the silicon. Faulting was found in the peripheral region of the bond where the wave action was operable. This faulting correlated to stresses generated in the pulsing direction and not to the directions of gross material flow. Reliable bonding depends upon a proper control of the gross and wave flow processes by optimizing material properties as well as machine variables. A model has been developed to qualitatively relate the influence of these variables and the manner in which a change in one parameter affects the response of the remaining variables.