Electrodeposited cobalt-hardened gold (Au(Co)) coatings are used on electrical connectors and contacts requiring resistance to mechanical wear as well as low electrical contact resistance. However, conventionally electrodeposited Au(Co) films grown using direct current (DC) electroplating method possess complex, porous microstructures. An open question is how this porosity affects the susceptibility of these coatings to potential electrical and mechanical failure. To test the hypothesis that microstructural voids degrade the coating stability, we conducted an elevated temperature aging study to compare Au(Co) grown with a void microstructure, using conventional deposition processes, with Au(Co) grown without a void microstructure, using a new pulsed-plating process developed at Sandia national laboratories. Using characterization techniques including scanning electron microscopy, transmission electron microscopy, transmission Kikuchi diffraction, atomic force microscopy, sheet resistance measurements, nanoindentation, and atom-probe tomography, we will demonstrate how porous coatings degrade by coarsening of voids, reactions of coatings with substrates, and diffusion of the hardening agent Co from the film to the substrate/coating interface with increased annealing temperature which in turn negatively affect the coatings’ structural integrity and the stability of electrical and mechanical properties. In contrast, we will show that non-porous Au(Co) coatings display stable low resistance, surface morphology, interface with substrates, and no development of voids or diffusion of Co in the microstructure with increased annealing temperature.