Metal Finishing Guide Book

2011-2012 Surface Finishing Guidebook

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Page 148 of 707

electroplating solutions GOLD POST-DIP TO IMPROVE CORROSION-RESISTANCE PROPERTIES BY OLAF KURTZ, JÜRGEN BARTHELMES, FLORENCE LAGORCE-BROC, TAYBET BILKAY, MICHAEL DANKER, AND ROBERT RÜTHER, ATOTECH DEUTSCHLAND GMBH, BERLIN, GERMANY Electrolytic or electroless gold depositions are used to provide a conductive and corrosion-resistant final coating for electronic applications. Because copper or copper alloys are the predominant choice for the base material, a nickel or nick- el–phosphorus layer is often used as a diffusion barrier.1,2 Cost efficiency and reduced precious metal content are the main criteria to sat- isfy the ever-increasing technical and quality requirements for components used in the electronic, telecommunications and automotive industries. The dramat- ic increase in the price of gold within the last decade reached its highest position in March 2008, amounting to $1,000 per ounce.3 The gold coating thickness must be adequate to ensure pore freedom and pro- vide sufficient corrosion resistance. Each defect or pore formation within the gold or barrier layer can lead to corrosion, resulting in both a reduced contact area and an increase in contact resistance. In general, a pore-free deposit is normally achieved at a deposit thickness of 0.5 to 2 µm,4,5 depending on the substrate type and pretreatment process used. For cost efficiency, the combination of palladium–nickel and ultra-thin gold coatings are used as an alternative to conventional gold deposits.1,6,7 As an alternative route, special pre-treatment such as electropolishing and the use of "inexpensive" electrolytic nickel–phosphorus barrier layers increase cor- rosion resistance at the expense of a significant reduction in gold thickness.8,9 Post-treatments may also help. However, care must be taken to ensure that they do not impair the essential surface properties (i.e., contact resistance and solder or bonding functions needed for most connector applications). Lubricants are Process Parameters pH Temperature Agitation Immersion time Optimum 5.8 50o C Required Reel to Reel 5 sec Rack and Barrel 2 min Table 1: Process Parameters for Betatec Gold Post-Dip 147 Process 5.7–6.0 20–60o C Required Reel to Reel 3–10 sec Rack and Barrel 1–3 min

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