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76 The PCB Magazine • May 2016 el/palladium/gold coating where both the palla- dium and gold have been deposited though an immersion process from DESs. This coating is termed electroless nickel/immersion palladium/ immersion gold (ENIPIG). Because of their inherent thermodynamic properties, electroless processes are fundamen- tally unstable. This can lead to spontaneous bath breakdown or extraneous plating which maCfest: manufaCturinG advanCed CoatinGs for future eleCtroniC systems requires either bath replacement or scrapping of the Pd-plated PCB, both of which can be very costly. However, immersion plating baths are fundamentally stable and not prone to ei- ther of these issues. Nevertheless, it is still es- sential that a uniform adherent coating is pro- duced to prevent the underlying nickel sub- strate from oxidation. Using the DES Ethaline 200, a uniform Pd coating of ~100 nm thick can be achieved in 20 minutes using palladium chloride as the Pd source. An example SEM im- age is shown in Figure 1 where the character- istic nodular structure of the electroless nickel can be observed. Few other features are appar- ent, despite the presence of ~100 nm palladium on the surface, because of the uniformity of the coating. Currently, immersion gold plating process- es from aqueous chemistries for ENIG and EN- EPIG coatings utilise potassium gold cyanide as the Au source. However, by using DESs, gold(I) chloride can be used as the gold source. In addi- tion, acid is required in aqueous media to help prevent passivation of the nickel surface; this is linked to the black pad phenomenon where hyper-corrosion occurs near the grain bound- aries resulting in a poorly solderable coating. Because of the unique behaviour of metal salts Figure 1: SEm of an electroless nickel - immersion palladium coating. Figure 2: PCB with an ENIPIG surface finish, pad sizes are 2.0 x 1.5 mm.