Reliability of Ag bonding wires and its coated variants from the perspective of IMC degradation and its correlation to wire and epoxy molding compound material properties under corrosive environment

Abstract

Ag wires and its coated variant, the novel Au-coated Ag (ACA) wire were investigated following the IMC degradation performance of Ag wires using fast corrosion test with Au wire as control. Higher purity 96% Ag wire showed more propensity for bond interface intermetallic compound degradation than the 89% purity Ag wire. The coated ACA wire was analyzed of its material properties and showed similar mechanical strength and hardness with that of Au wire for the same wire diameter. There are two types of ACA wires investigated, type 1, 20um and type 2, 23um, with similar core composition but difference in microstructure and Au coating thickness. In terms of its wirebond processability such as looping profile, wire pull and ball shear strength, the wirebond process response of ACA wire is comparable to Au. In the initial build using Au wirebond parameter, the FAB was concentric and can be formed consistently but the bonded ball of the ACA wire was not squashed well. The ACA wire which has >98% Ag core purity and wirebonded using Au parameter showed high sensitivity to IMC degradation after the fast corrosion test. The degraded IMC interface also showed presence of Cl and O which can be attributed to the Cl ions from the EMC and the subsequent oxidation of the aluminum bond pad. Improvements on corrosion test performance were made by optimizing the wirebonding parameters, increasing the bonding force by 25% and the ultrasonic force by ~7% from the baseline Au wire parameters. The 20um ACA wire was able to withstand corrosion test until 480 h with presence of microcracks at the interface while the 23um ACA showed worse degradation even at 240h with the A1 bond pad becoming completely porous. This indicated that the microstructure and Au coating thickness play significant role in controlling the diffusion of Au to the ball periphery to retard degradation.