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56 PCB007 MAGAZINE I JUNE 2021 DIG stands for "Direct Immersion Gold." e acronym is used to specify direct deposi- tion of gold on copper as a surface finish. It is a metallic solderable finish. At assembly, DIG forms a Cu/Sn intermetallic with the gold layer dissipating into the bulk solder. DIG has been around for at least 15 years. Gold will readily immerse on copper based on their respective positions in the EMF se- ries. e reaction is driven by +1.22 volts. As in all immersion reactions, the reaction will con- tinue as long as the substrate is available to the displacement reaction. As the substrate is cov- ered by the depositing species, it becomes less available, rendering the reaction self-limiting. e original formulation of DIG produced a relatively thin gold layer that had a reddish hue to it from the partial diffusion of copper. Copper diffusion to the surface continued with time; aer a year of storage at ambient condi- tions the deposit color became increasingly reddish, almost brown. Checking the solder- ability of the discolored surface using wetting balance methods showed excellent wetting and no signs of soldering degradation. At that time, DIG did not seem to offer any breakthroughs as other established finishes like OSP, immersion silver, and immersion tin. ese surface finishes were well-established, their limitations were well understood, and they were deployed extensively in PCB man- ufacturing. DIG was a more costly finish and created apprehension as it was clear that the copper would diffuse through the immersion grain boundaries into the gold, altering the as- received surface. A new generation of DIG was developed to meet the needs for wire bonding and high frequency signal propagation. High frequency RF signal loss is associated with thicker nickel deposits. e new DIG process uses a reduc- tion-assisted immersion gold (RAIG). e use of an RAIG gold allowed for the deposition of a thicker gold layer up to 0.3 µm (12 µins) that prevented the diffusion of copper to the surface. DIG: The Next Generation The Plating Forum by George Milad, UYEMURA

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