SMT007 Magazine

88 SMT007 MAGAZINE I SEPTEMBER 2019 Article by Jon Bengston and Richard DePoto UYEMURA INTERNATIONAL, USA SOUTHINGTON, CONNECTICUT Abstract Electroless nickel immersion gold (ENIG) is now a well-regarded finish used to enhance and preserve the solderability of copper circuits. Electroless palladium immersion gold (EPIG) is a new surface finish also for enhancing and preserving solderability but with the advan- tage of eliminating electroless nickel from the deposit layer. This feature has become increas- ingly important with the increasing use of high-frequency PCB designs whereby nickel's magnetic properties are detrimental. We exam- ine these two finishes and their respective sol- dering characteristics as plated and after steam aging and offer an explanation for the perfor- mance deviation. Comparing the results of steam-age test data shows a clear benefit of EPIG over ENIG. After even a short duration exposure to steam, ENIG finishes failed to solder. Much longer steam exposures produced little to no effect on EPIG plated samples. Rapid oxidation of the electro- less nickel phosphorous layer when stressed with heat and moisture explains the superior EPIG result. The ability to demonstrate excellent solder- ability following steam exposure represents increased fabrication reliability under non- ideal storage conditions. Introduction Assessing the solderability of a given final finish can range from a simple solder dip as coated or plated to quite complicated proce- dures involving the final finish type, solder alloy, flux characteristics, test preconditioning, soldering apparatus (wetting balance), and part geometries. Focusing on two types of sol- derable finishes and one preconditioning pro- cedure allows the clear reporting of results and expectations that those finishes afford. Morris, Lukaszewski, and Genthe [1] describe a need for verification methods for acceler- ated testing of electronics. Specific to soldering Comparing Soldering Results of ENIG and EPIG Post-steam Exposure

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