Detecting the Internal Distribution and Structure of Pd-Doped Ag Wire Bonds Using an RF Technique

Abstract

A vector network analyzer was used to obtain the S parameters of Pd-doped Ag-alloy wire bonds up to 67 GHz. The alternating-current (AC) resistance was extracted at around 3 GHz, 8 GHz, 16 GHz, and 20 GHz before the resonance; it was found that Ag alloy wire bonds with 3.5 wt.% Pd had the lowest AC resistance, with consistent trends at these frequencies. Because Pd has higher resistance than silver, and at high frequencies the current tends to be distributed at the periphery (according to the skin effect), the inward movement of Pd can account for this lowest AC resistance. Analytical methods including electron probe microanalysis (EPMA), energy-dispersive x-ray spectroscopy (EDS), and focused ion beam (FIB) were employed to determine whether Pd had higher concentration in the central region for Ag wire with 3.5 wt.% Pd. EPMA and EDS showed evidence of higher Pd concentration in the central area for Ag wires with 3.5 wt.% Pd. Also, from FIB micrographs, the central area of the Ag wires with 3.5 wt.% Pd had regions of high-angle grain structure, which may explain why the Ag wires with 3.5 wt.% Pd performed better in reliability testing. It is hypothesized that regions of high-angle grain structure are associated with higher Pd concentration in the core region, but verification of this hypothesis lies beyond the scope of this work.