Correlation Study of Pd Metallurgical Distributions and RF Characteristics of Pd Coated/Doped Ag-Alloy Wire Bonds

Agilent's Vector Network Analyzer VNA was used to obtain the S-parameters up to 67 GHz. The AC resistance was extracted at 3 GHz and 20 GHz, it was found that Ag alloy wire bonds with 3.5 %wt Pd had the least AC resistance. The trends were consistent at both frequencies. It indicates the Ag alloy wire bonds with 3.5 %wt might have a different Pd distribution from other Ag-alloy wires of other Pd percentages [4]. Here, we use several analytical methods to investigate the Pd distributions that may lead to explain why Ag alloy wire bonds with 3.5 %wt Pd had the least AC resistance. First, we employed EPMA (Electron Probe for Microanalysis) to obtain the distribution of Pd at internal wire (after cross-section the sample). It was observed that the 3.5%wt had a higher Pd concentration at the middle region compare to that of the peripheral region, which validated our AC resistance results, since Pd has the highest electrical resistivity. The opposite was observed for distributions of 2.5%wt and 4.5%wt. Second, we tried FIB (Focus Ion Beam) and SEM (Scanning Electron Microscopy) to study the metallurgical compositions. We obtained the distributions of grain structures, and compared the difference among three samples (2.5%wt, 3.5%wt, and 4.5%wt). It clearly shows that only 3.5%wt has such high angular GB (Grain Boundary) structures in the middle region, other two samples both have columnar crystal structures, instead. It again validates 3.5%wt Pd had the least AC resistance. Lastly, we employed EDS (Energy Dispersive Spectrometer) to obtain the compositions of the wire, that is, the amounts of Pd in the central and the peripheral regions. The results of 3.5%wt show central region's Pd is 0.9%~1% more than the peripheral region. And 4.5%wt shows central region's Pd is similar to peripheral region. The higher concentration of Pd toward the central region explains the lowest measured AC resistance of 3.5%wt (higher conductivity Ag move toward the peripheral, where the current density is highest due to Skin effect). To our knowledge, this paper could well be the first one to correlate the metallurgical distributions of Pd to the AC resistance (due to the skin effect) of Pd coated/doped Ag-alloy wire bonds.