Solderability is not a major factor in the selection of termination materials. "This standard establishes the method of identifying, specifying and limiting, to a minimum, the combination of basic materials, the thickness and composition of the plating..." This statement from MIL-STD-1276, Leads for Electronic Component Parts, summarizes a basic process reality that limiting the number of variables is imperative in process control.
The existing variety of lead materials is a result of varying manufacturing and application requirements. The differences in material and internal construction of components result in different termination materials, as can be seen in the application of Kovar or alloy 42 leads used in the packaging of glass components to maintain hermeticity requirements. Barrier layers are very important for materials that are not readily solderable.
(1) Limit the number of termination materials by type, design, and end use.
(2) Barrier layers should be used with brass, BeCu alloys, phosphor bronze and with most NiFe alloys used in glass scaled components.
3.4 Future Investigations
Recommended areas for investigation include:
(1) Barrier materials that have acceptable plating characteristics, provide solderable surfaces, and minimize detrimental interfacial metal reactions.
(2) Establish requirements for microcircuit terminations similar to those defined in MIL-STD-1276.