Effects of au plating on small-scale resistance spot welding of thin-sheet nickel
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I. INTRODUCTION
THE application of resistance spot welding in the fabrication of electronic devices and components (e.g., batteries, capacitors, and sensors) is generally termed as micro-, fine-, or small-scale resistance spot welding (SSRSW), since the metal sheets to be welded are relatively thin or small in diameter (⬍0.2 to 0.4 mm).[1,2] There are many differences between SSRSW and large-scale resistance spot welding (LSRSW), which is used to join sheet metals of thickness larger than 0.5 mm, mainly in the automotive and appliance industries.[3–6] These differences are not only due to the difference in the scale of the joints, but also due to the fundamental difference in the electrode forces (pressures) used.[3,4] Simply downsizing from LSRSW to SSRSW may lead to problems such as electrode sticking, metal expulsion, and nonrepeatable welding.[2] Furthermore, metals to be welded in SSRSW are mostly nonferrous, while the workpieces in LSRSW are mainly steels and, to a much smaller extent, aluminum alloys.[2,3,7,8] The base metals (such as Ni, Kovar, and Cu) of many electronic devices and components are frequently plated with materials (such as Au, Ag, and Ni) to improve corrosion resistance or to obtain a unique combination of mechanical, thermal, and electrical characteristics.[1,9] However, these plating materials may dramatically influence joining parameters used during the subsequent assembly processes and affect joint formation and quality, compared to the joining of the same base metals without plating materials. For example, Biro et al.[10] have found that Au/Ni and Ni plating reduced the power density required to form a joint during laser welding of very thin aluminum sheets. A joint formed by a combination of fusion welding and brazing was observed in laser welding of Au/Ni-plated Kovar and Ni; the braze layer caused a shift of the location of tensile-shear failure away from the fusion boundary and into the heat-affected zone or base metals. However, there is little work published on how plating materials affect joint formation and joint quality in SSRSW. In the automotive industry, Zn-based coatings are often W. TAN, Graduate Student, Y. ZHOU, Assistant Professor, and H.W. KERR, Professor Emeritus, are with the Department of Mechanical Engineering, University of Waterloo, Waterloo, ON, Canada N2L 3G1. Contact e-mail: [email protected] Manuscript submitted September 6, 2001. METALLURGICAL AND MATERIALS TRANSACTIONS A
used to improve the corrosion resistance of the sheet steels.[5,11] The use of these coatings has significantly affected the weldability of sheet steels in two ways. First, these low-melting-point coatings require a higher welding current or longer weld time in LSRSW. For example, in a study on LSRSW of sheet Zn-coated steel, Gedeon and Eagar[12] have observed that the Zn coating melts at the sheet/sheet faying interface to form a molten Zn disk at a very early stage of welding. This molten coating is then pushed away from the central area under the electrode pressure to form a molten Zn
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