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70 PCB007 MAGAZINE I JUNE 2019 or curtain coating. When coating by screen printing, the defect does not occur. By curtain coating and spraying, two opposite edges of a panel are coated over the edge. Mask accumu- lations can also occur in the edge area, by con- tact of two panels at the edge, or by contact of the edge with a rack. In this area, the drying of the mask is insufficient and sticky. During contact exposure, this area is fully exposed and the mask remains on the panel. During direct exposure, a circulating area of about 2–5 mm is not exposed. Sticky mask accumulations might remain in this area and pollute the developer. The developer is not de- signed to remove mask accumulations. These solder mask accumulations can be transferred by the rollers on soldering surfaces. Conclusion To meet the requirements of the PCB shops, the solder mask manufacturers have optimized some solder mask types for direct exposure. With products that are more light-sensitive, the exposure time on direct exposing units is re - duced. The second advantage is that oxygen inhibition now plays a minor role and usually can be ignored thanks to the increased levels of photoinitiator, ensuring enough free radicals for a good crosslinking. The mask surface is less porous, however, so moisture absorption remains an issue. The formation of condensation during ther- mal curing is enhanced. But the service life- time of nickel baths in ENIG plating might be reduced due to the out-leaching of photoinitia- tors and degradation of products that affect the bath's service life. These decomposition prod- ucts might interfere with the nickel deposition and lead to a lower height of the deposit at the corners. It should be noted that the whole pro- cess can be improved if a UV bump is added right after developing. By adding the UV bump step, excess of photoinitiator is destroyed and blown out. Consequently, the contamination of the curing oven is lower, and the nickel-gold process is less error-prone depending on the specific ENIG and solder mask process param- eters in your facility. The results of solder mask direct imaging de- pends on the solder mask itself, the local con- ditions, and the type of direct exposing unit. Direct imaging is an elegant challenge, but the seven defects described in this article are all manageable. PCB007 Nikolaus Schubkegel retired in February 2019. For the past 12 years, Schubke- gel worked at Umicore Galvanotechnik GmbH in Germany as a technical service engineer for Taiyo products. Before that, he worked as a process engineer in the solder mask department at the former IBM-PCB plant (later STP) in Albstadt, Germany. Schubkegel obtained an M.Sc. degree in chemical engineering from the Polytech- nic Institute in Timisoara. Researchers at the University of Maryland (UMD) and US Army Research Lab (ARL) have taken a critical step on the path to better high energy batteries by improving their water-in-salt battery with a new type of chemical trans- formation of the cathode that creates a reversible solid salt layer, a phenomenon yet unknown in the field of wa- ter-based batteries. Building on their previous discoveries of the water-in- salt electrolytes, the researchers added a new cathode material which, lacking transition metal, operates at an average potential of 4.2V with excellent cycling stability, and delivers an unprecedented energy density higher than non-aqueous Li-ion batteries. Leveraging the reversible halogens intercalation in graph- ite structures, enabled by a super-concentrated aqueous electrolyte, the team generated an energy density previously thought impossible. The researchers found that the super- concentrated solution of the water-in-salt battery, combined with graphite anode's ability to automatically build and re- form a protective layer within the battery, gave a stable and long-lasting battery with high energy. The report was pub - lished in Nature. (Source: University of Maryland) Reversible Chemistry Clears Path for Safer Batteries

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