Micro-Mechanisms of Solderless Wrapped Joint Formation

Abstract

The manner in which the various surfaces interact during wire wrapping, and the structure at the contact sites in the completed joint, have received only cursory attention. This paper describes an investigation designed to elucidate these phenomena using metallographic, scanning electron microscopic and electron microprobe analysis techniques. Series of joints made under known sets of parameters are examined to provide data on post-into-wire penetration, metal displacement and contact area :for different wrapping loads. It is shown that at high wrapping loads, the wrapping system tends towards an unstable state analogous to the onset of necking in tensile strain. Scanning electron microscopy and microprobe analysis of the surfaces of dismantled joints are used to show irrefutably that a substantial amount of cold welding occurs during the wrapping sequence. The transfer of metal from wire to post, and vice versa, by a welding-followed-by-fracture mechanism is clearly demonstrated for a variety of materials. This contradicts currently held views on the subject and indicates additional guide-lines for wrapped joint design. Above all, it is evident that the presence, and subsequent behaviour during wrapping, of thin plated layers on wire and terminal post cannot be ignored, and indeed plays a significant role in joint formation. A theoretical model is used to derive variation in contact area with wire diameter for typical joints. A change in wire diameter from 0.050 cm to a micro-wrap dimension of 0.017 cm is seen to reduce the area of individual contact sites by an order of magnitude, i.e. to approximately l0-4 cm2. It is argued that as the contact areas of wrapped joints tend towards this microscopic level, the phenomena discussed under the heading of "micro--mechanisms of wrapped joint formation" assume great significance.