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48 PCB007 MAGAZINE I DECEMBER 2019 der mask. This involves cleaning and rough- ening the copper surface, the application of a photoimageable mask, tack drying, imaging, developing, and curing. Attention to the de- tails of processing solder mask is paramount to achieve the desired ENIG deposit. The prop- er adhesion between the mask and the copper surface has to be achieved. After development, the sidewall should be straight, with no signs of negative or positive foot, and there should be no organic residues left on the copper pad surfaces. This is particularly important if the design includes solder mask-defined pads. Or- ganic residues, like tin residues, contribute to the non-uniformity of the Ni deposit. In ad- dition, monomers from partially cured solder mask leach out in the electroless nickel bath and contribute to instability, reduced deposi- tion rate, and shorter life of the bath. ENIG Pretreatment The objective of pretreatment is to ensure a pristine copper surface to ensure an even/uni- form catalyst deposit. Pretreatment involves a series of steps, namely cleaner, microetch, and acid predip. The cleaner serves a series of functions: the detergent component removes soils and organic residues (fingerprints), and the acidic component removes oxidation and the surfactant present wets the surface. The new development in cleaners is the use of low surface tension surfactants. A properly wetted surface will help dislodge any entrapped air in the narrower vias. Vibrating the parts in the cleaner bath is recommended for high aspect ratio holes and small vias. Good rinsing should follow the cleaner. The microetch removes a layer of surface copper and modifies the surface topography. The proper choice of microetch can effective- ly reduce the profile of the copper surface that was previously roughened to ensure prop- er solder mask adhesion. Peroxide/sulfuric based micro-etch is the preferred choice here. Usually, 30–50 µins of copper are removed. This should be monitored and maintained. Good rinsing should follow the micro-etch as well. Failure to rinse off all microetch residues, particularly from small holes, will interfere with the immersion (charge transfer) based palladium catalyst deposition. A heated sul- furic post-dip is recommended here. This step helps in removing any traces of oxidant trapped in small holes. Again, this followed by rinsing. If all the aforementioned items adhered to, the copper surface now has a low profile, is organic residue-free, oxidation-free, and, more importantly, charge neutral. The Catalyst/Activator The catalyst bath lays down the foundation on which the nickel, and eventually the gold, will deposit. The bath is, in most cases, com- posed of palladium sulfate in a sulfuric acid low-pH medium. Here, the Pd ion in solution will be reduced to Pd metal at the expense of copper metal (the substrate) that is oxidized to the Cu ion. This is an immersion reaction and is based on electron transfer, where a metal ion higher up in the electromotive series will displace a substrate metal lower in the series. Nickel will not plate on a non-catalyzed cop- per surface. Proper rinsing is important after the catalyst bath to ensure no drag-in of Pd into the electroless nickel bath. The catalyst is a very thin layer applied to the copper surface to initiate the Ni deposition and is not a signifi- cant part of the ENIG finish. The copper surface coming to the ENIG line, in most cases, follows the application of solder mask. This involves cleaning and roughening the copper surface, the application of a photo-image- able mask, tack drying, imaging, developing, and curing.

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