Effective Method for Wire Bonds Rework Using Conductive Epoxy

Abstract

The development of wire bonded chips requires us to go through multiple designs to achieve the best packaging performances. Nevertheless, some of the designs may not be tested due to defects preventing the wire bonds to be effectively completed. Unless an effective way to rework such failed assemblies is feasible, the invested resources in the design and manufacture are lost. The MiQro Innovation Collaborative Center, Bromont, QC, Canada, has developed a method to rework such assemblies. In this paper, we present a method where two silver-filled conductive epoxies are used to repair failed gold wire bonds. The mechanical adhesion and electrical properties of such rework assemblies were compared to reference gold wire bonds. The rework method consists of reattaching the sheared end of the wire to the landing pad on which a drop of conductive epoxy is deposited. Each group is at least composed of 40 samples for statistical analysis of the method reproducibility. Experiments were conducted on two identical test chips with gold pads on each substrate. A matrix of 1-mil-diameter gold wire bonds was realized. Then, on each substrate, the first group of wire bonds was used as a reference and the others were sheared at the stitch bond and reworked with two different silver-filled conductive epoxies. The pull tests revealed that all the wires (reference and reworked with epoxy 1 and 2) rupture at the neck of the first ball bond at a similar break load (~12 g). Visual inspection using the cross section of the reworked area showed no visual defect. We could also see that the deposited drop from epoxy 1 is much more likely to make a better electrical connection due to the presence of smaller silver flakes and a better capability to wet both the wire and the pad. The electrical resistance of reference and reworked wire bonds were measured on a two-probe station. On both samples, reworked wire bonds with epoxy 1 have similar resistance (94 $\text{m}\Omega$ ) than the reference (91 $\text{m}\Omega$ ) ones. The ones reworked with epoxy 2 have an electrical resistance (214 $\text{m}\Omega$ ) more than twice than the reference ones. Reliability testing showed increasing electrical resistance throughout 1000 deep thermal cycling cycles for epoxy 2, while epoxy 1 remained stable until 500 cycles. We demonstrated that failed wire bonds could be effectively reattached to the substrate using the appropriate conductive epoxy. This method enables us to reduce development costs by reworking failed considered assemblies to avoid the waste of resources and delays in the run to the market for the new product.