The migration of silver through and on the surface of insulating materials

Silver possesses a combination of properties which encourages its use for a variety of purposes in electrical components such as electrical contacts, printed circuits, and metal-ceramic combinations. Its electrical resistivity is 17 × 10^-6 as compared with 21.3 × 10^-6 for copper and 30.8 × 10^-6 for gold. Its oxide decomposes at temperatures slightly above 100°C where the dissociation pressure equals the partial pressure of oxygen in the atmosphere. The free energy of formation of the oxide at room temperature is -2395 calories as compared with -26,000 for copper. For these reasons silver has some of the properties of noble metals. Unfortunately, it has two very undesirable and potentially hazardous properties. It combines with sulfides under certain conditions to form surface films of sulfides which are permeable to silver and, therefore, continue to increase in thickness, resulting in high electrical contact resistance. The second undesirable property is the high solubility of the oxide in water which is probably the basis of the tendency for silver to migrate in electrical fields. The term silver migration as employed here may be defined as the electrolytic transport of silver from its initial location and its deposition as metallic silver in some other location. The deposited silver may assume several different forms-1) dendritic structures which normally develop from cathodic areas; 2) colloidally deposited silver in the form of conducting or non-conducting films in the vicinity of either the cathode or the anode; 3) dendritic or colloidal deposits detached from either the cathode or the anode. In this investigation the test specimen was clamped between two sets of silver bar electrodes one inch long and 3/16 inch wide so as to provide a leakage path 1/8 inch long running parallel to the specimen which was in the form of a strip one inch widen Migration occurs either at the surface of the sheet in the case of nonporous materials or through the material in the case of fiberous or porous materials. The specimens clamped between the two sets of electrodes were supported from the cap of a bottle maintained at constant temperature and. containing salt solutions of the desired relative humidity. A d.c. voltage of 45 or 200 volts was then applied to the test specimen and the progress of migration followed by resistance measurements and by visual observation.