The Electrical Reliability of Silver Wire Bonds under High Temperature Storage

Bonds made with Pd coated pure Ag wires were studied in this work with respect to their electrical resistance under high temperature storage at 200 °C. The Pd coated 18 micron diameter Ag wires were ball bonded to the Al metallization on testchips attached to ceramic substrates. All free-air balls were made in air as Pd coated Ag wire does not need protective atmosphere such as forming gas or nitrogen. The second bonds were on the Au plated leads of the ceramic substrates. Bonded wires were grouped in pairs to form several wire daisy chains on each substrate. A setup was developed to automatically switch from daisy chain to daisy chain to perform 2-wire resistance measurements. The resistance of Ag wires bonded to Al pads was compared to that of Au wire bonds. The average resistance value of the Ag daisy chains at 50 °C was 6.28 Ω, not significantly different from that of Au bonds. These values increased ≈2 % and ≈8 % for surviving Au bonds and Ag bonds, respectively, after 1000 h at 200 °C. However, more Ag bonds survived than Au bonds. The results show that Au bonds exhibit only a low increase of resistance in the beginning which then, however, is followed by an abrupt increase to high values leading to complete failure. In contrast, the Ag bonds aging show a more gradual increase in resistance right from the beginning and are less prone to abrupt or complete failure compared to the Au bonds.