Chromate Film Used for Chemical Corrosion Control on Gold-Plated PCB connectors,

Electrical intermittencies in nuclear submarine navigational computers can result from degradation of gold-coated contacts by insulating films, the latter identified as principally composed of organics and silver sulfide. The films can be removed, but without appropriate measures, they recur. To retard insulating film build-up, a protective, noninsulating chromate surface treatment was developed it has proven effective in extending service life. Because protection arises from a surface modification, investigation of the chromating process requires the use of surface-sensitive techniques. Analysis by AES, SAM, XPS, EMA and RBS reveals the presence of a thin 30 Angstroms layer, rich in Cr3 on top of the gold. This contrasts with much thicker films incorporating Cr6 species found in chromate films grown for the protection of Zn, Cd and Al. The absence of Cr6 species suggests the chromate film on gold protects via a physical rather than electrochemical mechanism. The chromate film continuity and composition can be adversely affected by mechanical contact, tarnish growth, and substrate microheterogeneity. Copper compounds are implicated in the occasional development of insulating films during field application of the chromate treatment.