The Correlation of Microstructure, Microhardness, and Microchemistry in Resistance Welded Electronic Leads 650857
A study was made on the nature of the weld interface obtained in resistance welding of component leads used in electronic packages. To this end an investigation was made comparing the microstructure, microhardness, and microchemistry of a variety of cross wire resistance welds. Specifically, the investigation concerned itself with the variations in microhardness, and microchemistry at and near the weld interface as a function of metallographic structure, distance from the interface, and type of material being welded.
This study will help answer such questions as the amount of diffusion of one weld component into another, the effects of welding on the mechanical properties near and adjacent to the weld, and provide for a more intelligent interpretation of the variety of microstructures that have been observed.
The results of this work can be summarized as follows:
There is little or no diffusion of one material into the other. In cross wire welds that do not have a weld nugget in the microstructure (the great majority) the transition from one alloy to the other is a sharp line of demarcation. The area of indicated alloy mixing is only slightly greater than area of uncertainty of the micro-analyzer. In those welds that had Dumet (the composite material with Alloy 42 core and copper shield) for a component, the chemistry of the interface showed a sharp demarcation from the core material to the other weld component, nickel. There was only a slight trace of copper at the weld interface. No copper-nickel or copper-nickel-iron alloy formed.
In those welds that did have a weld nugget in the microstructure there was only a small amount of alloy mixing. The chemical variation, that was evident, existed in discrete pools, and as above, there was no gradual blending of one alloy into the other. For example in the weld, nickel to Alloy 42, the nickel content goes from 43% in one component to 48% in the nugget, to above 99% in the other component. The iron composition varies in a complementary manner. In all cases the transition is sharp, not gradual. The same phenomena is exhibited in welds between other materials.
There is no significant solid state diffusion. Only where a pool of molten metal is trapped is there any chemical change, and even then it is small.
Microhardness in the single phase materials, nickel, Alloy 42, and Kovar, does not vary in any systematic manner at or near the weld interface. The annealed materials show no forging action. This is confirmed by the microstructure.
The materials that were half hard prior to welding show some variations in microhardness indicating partial annealing.
Beryllium copper has the hardness of annealed material at the weld interface. At some distance back the hardness increases showing that partial precipitation has occurred. The hardness is not close to maximum hardness that could be achieved.
There is little diffusion and only minor mechanical effects due to resistance welding these materials.