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58 PCB007 MAGAZINE I JUNE 2021 During deposition the immersion gold initi- ates on the copper surface and that triggers the electroless deposition, which will continue to deposit as the copper substrate gets plated over and is no longer available to sustain the immer- sion reaction. e electroless reaction does not require substrate contribution as it is driven by a reducing agent in the electrolyte. e elec- troless deposit is non-granular and not porous. Unlike the original immersion DIG, this new mode of deposition can produce a thicker lay- er that is pore-free (no grain boundaries) thus limiting the migration of copper. e deposit has the lemon-yellow color that is expected of a gold deposit in contrast with the reddish yel- low of the original DIG. At 0.2 to 0.3 µm, the deposit shows no signs of copper diffusion to the surface. Hofstetter PCB AG in Küssnacht/Switzer- land is a supplier of various coatings in micro- electronics. Hofstetter supplies a complete spec- trum of surface finish plating to the microelec- tronics industry. ey offer some of the newer finishes like EPIG (electroless palladium im- mersion gold), ISIG (immersion silver immer- sion gold), and DIG (direct immersion gold). eir R&D department invests in studying the properties of some of the newer finishes coming to the market, exploring solutions to meet the ever-changing demands of their customers. Hofstetter has conducted a comprehensive study on the gold wire bondability of the new DIG process. ey studied the consistency and strength of the gold wire bonds under different tem- peratures, namely 90–130 o C for the wedge bond and 150 o C for the first (ball) bond. ey included room temperature bonding in the study. Room temperature bonding has a nar- row operating window compared to a heated table. Success at room temperature bonding was achieved by the optimization of the DIG surface topography and the US (ultrasound) bonding force. e study concluded that the new DIG at 8-12 µins (0.2–0.3 µm) thickness is capable of producing wire bonds consistently and reproducibly with optimized bonding pa- rameters. e DIG surface at 8–12 µins exhibits a dis- tinct fine roughness, which is very evenly formed over the entire surface. e new DIG surface requires minimum processing steps. Deposition requires a standard cleaner and mi- cro-etch followed by the gold bath. is con- trasts with ENEPIG which requires cleaner, micro-etch, catalyst, electroless nickel, elec- troless palladium, and immersion gold. e new DIG is a surface finish that offers ad- vantages over the traditional finishes like OSP, immersion silver, and immersion tin. OSP is a fragile, organic solderability preservative that cannot be used as a contacting surface. Immer- sion silver is a good contacting surface but was susceptible to creep corrosion and occasional- ly formed voids at the IMC surface. Immersion tin will form a Cu/Sn intermetallic during stor- age and may lose some of its wetting proper- ties at assembly. e new DIG, which is commercially avail- able today, is solderable and aluminum and gold wire bondable, and it is ideally suited for plating small features with limited spacing. e absence of nickel in the surface finish makes it well-suit- ed for high frequency applications. PCB007 George Milad is the national accounts manager for technology at Uyemura. To read past columns or contact Milad, click here. A new generation of DIG was developed to meet the needs for wire bonding and high frequency signal propagation.