The effect of high temperature intermetallic growth on ball shear induced cratering

A mechanism that explains the high incidence of silicon cratering when thermosonic gold ball bonds are sheared from aluminum metallized pads over silicon and SiO₂ is proposed. The mechanism is based on the transmission of energy from the ball shear ram through the rigid intermetallic weld to the underlying dielectric. The concentration of this transmitted energy in the form of stress concentrators was estimated, using finite-element modeling, to be highly dependent on ball and weld geometry. This failure mode does not require that the substrate be cracked initially through the use of improper bonding conditions, as cratering was observed to occur even though the range of bonding parameters used in this study produced no visible damage to the underlying dielectric material. The above mechanism may be partly responsible for shear-induced cratering in plastic-encapsulated packages. Recommendations are made for reducing this effect by altering bonding parameters, pad metal thickness, and cure schedule