Resistometric Characterization of the Interface between Au Wire Bonds to AlCuW Bond Pads

The contact resistance (RC) of Au wires bonded to AlCuW bond pads (98.5% Al, 0.5% Cu, 1.0% W) and pure Al bond pads was measured continuously using the high-resolution resistometric method during multiple high-temperature storage (HTS) tests. Oven storage temperatures from 200°C to 250°C were utilized to accelerate the intermetallic formation and resistance degradation at the bond interface. This study used ceramic side-brazed packages (CERDIPs) with 1 mil Au bond wires ball bonded to the AlCuW bond pads. All devices under test (DUTs) were measured using Kelvin (4-wire) contacts. The mean contact resistance (RC) of an unstressed Au wire ball bond to AlCuW bond pad at room-temperature was 0.186 ohm. The failure time criterion was defined as when the resistance at stress temperature (RT) exceeded 2x (100%) of the initial resistance (R0) but all tests were monitored to a 500% resistance increase. The AlCuW failure distributions show a clear acceleration of the intermetallic degradation mechanism due to increasing temperature. A thermal activation energy was determined using Arrhenius analysis of the median time to failure (t50%) from four different temperatures. A comparison of the contact resistance (RC) intermetallic degradation behavior between the AlCuW and Al bond pad metallurgies is discussed. Scanning Electron Microscope (SEM) images of Focused Ion Bean (FIB) cross-sections confirm that the failure mechanism is due to the formation of intermetallics and subsequently Kirkendall voids. Several Au-Al intermetallic phases were also evident along the Au wire ball bond to Al bond pad interface.