Control of Heat Input and Deformation on Resistance Micro-Welding of Fine Nickel Wires

In the production of small electronic and medical devices, it is often necessary to produce crossed wire joints in fine nickel wires. Therefore, commercial purity nickel fine wire was welded by the resistance microwelding. A study has been performed to improve the welding process considering the welding mechanism. A welding mechanism consists of cold wire collapse, surface melting, liquid phase squeezed out and solid state bonding. Set down which indicates the depth of penetration for wires was not adequate to evaluate the joint quality. Sufficient local heat generation as well as set down is the key to high quality welds, first to generate enough transient surface melting, second to facilitate plastic deformation in order to squeezed out liquid and expand the bonded area. This requires a proper balance of high initial contact resistance and sufficient high welding force, which could be obtained by setting a low firing force.