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94 SMT007 MAGAZINE I SEPTEMBER 2019 fold increase in oxygen depth post dipping ENIG at 80°C versus room temperature measurements was recorded [4] . Palladium's inherent resistance to oxidation provides a robust, oxida- tion-free protective layer for solder applications. Also, palladium dis- solves in the molten solder allowing the formation of a copper-tin inter- metallic directly with the underlying copper surface. Another contributing reason for the soldering performance differ- ence between ENIG and EPIG is the tin intermetallic formed. ENIG forms a nickel-tin intermetallic whereas EPIG forms a copper-tin intermetal- lic. The gold and palladium layers readily dissolve into solder, so the bonded interface is directly between copper and tin. Hence, any thin oxi- dation or other environmental con- tamination of the palladium layer is essentially dissolved and thus incon- sequential to the copper-tin bonding interface. A follow-up verification test was made by coating ENIG solder cou- pons with an organic anti-tarnish topcoat. The topcoat is a solderable organic anti-tarnish coating that is intended to be a barrier to oxygen penetration. The coating is applied on top of the immersion gold layer, essentially sealing the thin porous gold. Solder wetting tests using the organic topcoat on ENIG substan- tially eliminate the adverse effects of steam conditioning. Starting at one-hour steam exposure through to eight hours limited the degra- dation in the ENIG's solderabil- ity, which indicates that the nickel layer remained relatively oxide- free with solder wetting occurring after eight hours of steam stressing (Figure 12). Figure 10: EPIG normal wetting after one-hour steam exposure. Figure 11: EPIG wetting after eight hours of steam exposure. Figure 12: Organic anti-tarnish protected ENIG wetting balance after eight hours of steam stressing.