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Electroless nickel immersion gold (ENIG) has been around the printed circuit industry for more than 25 years. The first version of the IPC-4552 ENIG specification was issued in 2002. Initially, the specification only addressed tin/lead solder; now, lead-free solder, like SAC 305 and its variants, dominate soldering in electronics. Although the occurrence of corrosion was recognized, a better understanding of the defect has led to a series of improvements over time.
Today, it is well established that Ni corrosion occurs in the immersion gold step, and the most important method for eliminating the defect is through process control. ENIG is a complex chemical process with multiple process steps, and each step must be completed successfully before proceeding forward. ENIG remains a very popular surface finish and offers a series of benefits at assembly: it is easy to inspect, has an extended shelf life, and is suitable for a wide range of assembly applications.
The IPC-4552 Rev A, issued in 2017, specifies the deposit thickness: nickel from 3–6 µm (120–240 µnis) and gold from 0.04–0.1 µm (1.6–4.0 µins). The upper limit for gold at 0.1 µm (4.0 µins) would require an extended dwell time in the immersion gold bath. The extended dwell time makes the deposit susceptible to nickel corrosion. The recommended immersion gold deposit thickness is 0.04–0.07 µm (1.6–2.8 µins). If a higher gold thickness is a design requirement, an alternative to immersion gold should be used for deposition. Two available alternatives are reduction-assisted immersion (RAI) gold and electroless gold.
Controlling the outcome of ENIG plating starts with the parts coming to the line; parts must be free of tin and organic residues. Tin is used as an etch resist during the circuitization of the board. Tin must be completely stripped to allow for uniform catalyzation of the copper surface. Residual tin interferes with the deposition of the palladium catalyst or activator.
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 photoimageable mask, tack drying, imaging, developing, and curing. Attention to the details of processing solder mask is paramount to achieve the desired ENIG deposit. The proper 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. Organic residues, like tin residues, contribute to the non-uniformity of the Ni deposit. In addition, monomers from partially cured solder mask leach out in the electroless nickel bath and contribute to instability, reduced deposition rate, and shorter life of the bath.
The objective of pretreatment is to ensure a pristine copper surface to ensure an even/uniform 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 effectively reduce the profile of the copper surface that was previously roughened to ensure proper 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 sulfuric 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 bath lays down the foundation on which the nickel, and eventually the gold, will deposit. The bath is, in most cases, composed 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 copper 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 significant part of the ENIG finish.
The Electroless Nickel Bath
Electroless nickel (EN) baths are formulated to give a specific range of incorporated phosphorus, expressed as a percentage range, in the deposit. A low-phos deposit would range from 4–6%, a mid-phos deposit would range from 6–8%, and a high-phos deposit would range from 8–10%. Mid-phos nickel is widely used for ENIG application, where soldering and corrosion control are required. With the emphasis on eliminating nickel corrosion, newer formulations now favor a higher phos content (>8%) to further expand the corrosion control window.
EN is a well understood multi-component bath. The primary constituents are nickel sulfate as the source of nickel and sodium hypophosphite (hypo) as the reducing agent. The latter supplies the electrons needed to reduce the nickel ion to the nickel metal. The hypo reaction also produces phosphorus and the byproduct orthophosphate. The phosphorus is incorporated in the electroless nickel deposit. The buildup of the orthophosphate byproduct in the bath determines the life of the bath.
The nickel deposition reaction requires a specified temperature (ranging from 175–185°F) and a weak acidic pH medium. In addition, there are a series of other important proprietary constituents that have a direct impact on the quality of the nickel deposit, including stabilizers, surfactants, complexing agents, accelerators, and buffers. Different vendors or suppliers may use various ingredients to achieve the goal of a uniform deposit with an optimized deposition rate and achievable/reproducible operating conditions.
The nickel bath requires good management as it is a dynamic, ever-changing bath. The nickel and the hypo are consumed during the deposition; in addition, there is a buildup of the byproduct orthophosphate. The pH of the bath must be controlled within a narrow range. It is necessary to replenish those ingredients that are being used up. This is best achieved by a controller that will automatically replenish the desired components as well as maintain the pH. To compensate for byproduct buildup, newer, more sophisticated controllers increase the nickel concentration and the pH operating ranges, as the bath ages to maintain a consistent rate of deposition.
The Immersion Gold Bath
To minimize corrosion, new developments include:
- Neutral pH formulations for reduced corrosion
- Reduced gold concentration for cost control
- RAI gold for corrosion elimination
RAI gold is a mixed reaction bath that initiates as an immersion bath and continues as an electroless bath. It is capable of depositing 4–6 µins of gold with no corrosion. RAI gold is ideally suited to deposit thicker gold if it is a design requirement.
The ENIG line is one of the most complex chemical lines in a board shop. It requires a good understanding of how the process works and the critical parameters that must be maintained. The ENIG line has little tolerance to deviations, particularly to extending the bath life of any of the process steps. Shops that have good engineering and documentation of the manufacturing process, coupled with a dedicated, experienced ENIG operator and backed by a capable analytical laboratory, run defect-free ENIG day in and day out, producing a consistent product that meets customer requirements.
This column originally appeared in the December 2019 issue of PCB007 Magazine, click here.