Assured Reliability in Soldered Connections: Solderability as Parameter of Assurance

The reliability of any connecting process is statistically governed by the number of process parameters. Significantly, for reliable welding a multiplicity of parameters requires close control, but reliable soldering depends primarily upon one - solderability of the surfaces to be joined. The theoretical and practical implications of solderability and tests for its evaluation are discussed. An entirely new test is described which is rapid, simple, and significantly correlated with connection quality. The test is based upon an estimate of the contact angle between a molten solder preform and the metal surface. This concept is also linked theoretically with the surface tension-interfacial tension relationship controlling "wetting". This test has been used extensively in the evaluation of component lead wires and in measures to raise their solderability to the highest possible level, with remarkable effects upon connection quality. Formation of perfect fillets having mechanical and electrical redundancy becomes automatic, independent from the human element and little affected by variations of solder, flux, or procedure. Nonaggressive fluxes, and, in some cases, flux-less processes may be used. Highly reliable connections are obtainable with very low-melting and very highmelting solder alloys. Thus, controlled high solderability conveys to solder processes a reliability aspect which is unique and, in some respects, unsurpassed in other connecting processes.