Wire pull on fine pitch pads: an obsolete test for first bond integrity

Abstract

This work focuses on the validity and applicability of the wire pull test being used as a measure of first bond integrity of wire bonds on fine pitch pads used in advanced silicon technology devices. The effect of geometry of the wire bond is studied by considering variations in the angle of force application during the wire pull test and the presence of nonintermetallic region (de-bonds) created by a probe mark under the ball. This paper does not address the effects of the silicon-level interconnect structure underneath the bond-pads, i.e., it is assumed that the underlying structure is defect-free, and the layers of interconnect metal and interlayer dielectric (ILD) are modeled along with the silicon as a smeared homogeneous material. Experimental data that corroborate the findings from the finite element analyses are also presented. Results from this work indicate that if the wire pull test is applied for smaller ball diameters, some incidence of unconventional failure mechanisms, such as cohesive failure of the ILD, is to be expected and allowed. A wire pull should only be considered a failure if the ball lifts and shows poor intermetallic formation or if the wire pull fails at a pull-force substantially below the "norm" for that ball diameter.