Metal Finishing Guide Book

2013

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words, the concentration of grain refiner that gives a smooth, satin finish at 100 A/dm2 will produce full bright deposits under similar conditions at 200 A/dm2. Pre- and Posttreatments Since silver is being plated selectively the electrolyte is exposed to cleaned and activated substrate surfaces (typically copper-rich alloys or nickel-iron alloys) that are prone to forming immersion silver deposits. Any silver detected outside the package profile (i.e., on the outer leads) is cause for rejection of the leadframe due to fears that silver outside the package will migrate across the dielectric and cause short circuits. Hence, all traces of silver must be eliminated outside of the selectively plated spot itself. A typical process flow is degrease and rinse, electroclean and rinse, acid dip and rinse, anti-immersion predip, selective silver plate, silver drag-out rinse, silver back-stripper, multiple rinses, dry. The "anti-immersion predip" consists of a dilute solution of a mercaptan or similar compound, which will attach itself to active surface sites and minimize immersion deposition of silver without inhibiting adhesion of the desired electrodeposit. Since there is no rinse between the predip and the plating solution it is essential that the predip agent should not adversely affect the electrolyte performance. Most predip agents are actually added to the electrolyte during solution preparation so as to enhance the immersion deposition inhibiting effect. Some also act as an additional grain refiner. Even with these precautions some silver can be detected outside the spot area and must be removed. This is usually achieved using a "back-stripper," which removes a small layer of silver from the entire surface. The most popular solutions are succinimide-based and many apply a reverse current in order to better control the amount of material stripped in this cell. This plating technology has been fully developed and is projected to continue until the phasing-out of the lead frame itself. SUMMARY After more than 150 years, rack and barrel silver plating is still performed using a cyanide electrolyte that closely resembles the electrolyte in the original 1840 patent. After many years of R&D effort directed at finding an alternative to cyanide in silver plating, two processes are being offered commercially. Additional work is continuing with the objective of producing full-bright silver deposits from an electrolyte as robust as the traditional cyanide process. The use of electroplated silver on electronic components is well established and continues to expand into new applications such as on waveguides for cellular telecommunications systems. 316

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