Influence of a rapid annealing on the surface oxide film of a tinned copper wire
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Lionel Boyer and Camille Bodin Laboratoire de Genie Electrique de Paris, Ecole Superieure d'Electricite, Universites Paris VI et Paris XI, U.R.A. C.N.R.S. D 0127, Plateau du Moulon, 91190 Gif/Yvette, France
Germaine Binder Alcatel-cuivre, B.P. 30, 02301 Chauny Cedex, France (Received 6 June 1990; accepted 20 November 1990)
A continuous annealing, using the Joule effect, is performed during the fabrication of a tinned copper wire. During this operation, which lasts between Mo and Moo of a second, the wire is brought in air to a temperature which exceeds the melting point of tin. The influence of the continuous annealing on the nature of the surface layer of the tin coating is studied using XPS. The thickness of the tin oxide film covering the metallic tin is shown to be reduced after the annealing. This result is confirmed by measurements of the electrical contact resistance using the crossed rods method.
I. INTRODUCTION
The tinning of copper conductors has two main functions: protection of the copper from corrosion and improvement of the electrical continuity between the conductor and the other components such as terminals, sockets, and printed circuit tracks. It is clear that the second function is closely linked to the first since, whether it concerns brazing or mechanical connection, the presence of surface oxides, sulfides, chlorides, or more complex species, often has detrimental consequences (increased contact resistance, poor solderability). When the tin coating is subject to friction, as is the case with the active elements of electrical connectors, a cleaning effect can be counted on to remove a large proportion of the surface contaminants and provide a good electrical continuity. However, when this cleaning effect is reduced, as is the case when crimped connections are used, the nature of the covering's initial surface state takes on a greater importance. The tin surface has been the subject of numerous publications. Their authors, relying mainly on electron spectroscopy, have shown that metallic tin is covered with a tin oxide layer, made up of either SnO alone2'5'6'9'10 or a mixture of SnO and SnO2.3"5'7'8 Bevolo et al. also mention the influence of temperature on the composition of the surface oxide layer.8 Industrially tinned copper wire, the subject of our study, undergoes a dynamic annealing aimed at the recrystallization of the copper, work-hardened during the wiredrawing. The dynamic annealing is a Joule effect annealing, which takes the tinned wire to temperatures reaching 350 CC for durations of Vw to Moo of a second. 492 http://journals.cambridge.org
J. Mater. Res., Vol. 6, No. 3, Mar 1991 Downloaded: 01 Apr 2015
The primary goal of the research, whose results we present here, was to study the influence of the dynamic annealing on the nature of the surface layer of the tin coating. We then try to correlate the results of the analysis of the surface layer with our measurements of the electrical contact resistance, performed using the crossed rods method. II. EXPERIMENTAL PROCEDURE
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