Study of conductor and substrate lossy characteristics of Ag-based wire-bonds

In this paper, high frequency characteristics of Ag alloys, for example, Ag-Au-Pd and Ag-Pd, were studied. At 20°C, the electrical resistivity of pure Ag, Au, and Pd is 1.587 μΩ·cm, 2.214 μΩ·cm, and 10.54 μΩ·cm, respectively. Depending on the metallurgical distribution of Pd in the AgAu alloy or Ag alloy wire bonds, the high frequency behaviors of the wire bond would vary significantly. For example, if Pd is distributed mainly on the surface, the surface impedance of the Ag-alloy wire bond could be drastically increased as the frequency is increased. Agilent's Vector Network Analyzer VNA was used to obtain the S-parameters up to 67 GHz, well beyond the first resonance frequency. R, L, and C were extracted by converting the measured S-parameters. Indeed, it was found that Ag alloy wire bonds with 3.5 %wt Pd had the least AC resistance. The AC resistance was obtained at 3 GHz and 20 GHz. The trends were consistent at both frequencies. It indicates the Ag alloy wire bonds with 3.5 %wt had a different Pd distribution. We also studied AC resistances at 2 GHz, 4 GHz (the first stage), 8, GHz (the second stage), and 16 GHz (the third stage), and found wirebond had exhibited losses in addition to the conductor's skin effect related loss. To our knowledge, this paper could well be the first one to report the effects of Pd distribution on the RF characteristics due to the skin effect of the Pd coated/doped Ag alloy wire bonds. Analyzing the high frequency characteristics along with reliability assessment of Pd coated/doped Ag-alloys such as cross-sectional micrographs, an optimized Ag-based wirebond can be picked.