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90 The PCB Magazine • July 2017 the situation, and rejects resulted. Thankfully the industry, and its suppliers, have grown up since that time, and silicates are no longer pres- ent in most of the cleaners used by the PCB in- dustry. Regardless, do not assume only alkaline cleaners are more difficult to rinse. Some acid- based cleaners, due to the nature of the surfac- tants and wetting agents contained in the for- mulation, also pose rinsing challenges. Wetting agents and surfactants can cause havoc with rinsing and subsequent cleaner drag-in to downstream operating process steps (Figure 1). When one sees excessive amounts of foam in subsequent processing cells, the red flag should be raised. Still another theory, widely held through- out our industry, reasons that if work is left in a rinse tank for a longer period, better rinsing will be the result. Let us apply the dilution model we discussed earlier, as a review of this informa- tion would prove this theory to be false. When a rack of PCBs is immersed in a rinse tank, the residual surface contamination is re- duced to a practical minimum within 30 sec- onds, as the solution carried in on the surface of the work disperses into the rinse waters. A typical rinse tank of 100 gallons, with a water flow of five gallons per minute would decrease the concentration of the solution contaminants at a rate of only 5% for each minute that it re- mains in the rinse tank. Leaving the work in any longer would have virtually no effect. It would be much preferable to have brief exposure times in many (presumably cascade) rinse tanks, which will result in a better dilution rate of the contaminants, than a long exposure time in just a few rinse tanks. There have been countless studies per- formed on rinsing mechanisms, and the litera- ture is rife with information on achieving high- quality rinsing. Most studies are based upon multitudes of calculations of volume of water flow relative to number of rinse tanks in use. In some areas, cascade rinsing is currently held in high favor even though this method can substantially increase water consumption. In other areas, PCB fabricators are constant- ly concerned with amount of water consumed. Both the price tag attached to each gallon of water consumed, and the cost of waste treating the rinse waters are major factors. We see there- fore, that the "more is better" attitude to im- proving rinse water quality cannot be accept- able across the board. This brings us to the ultimate question. Namely, how can we further improve rinse qual- ity without increasing water consumption and waste generation? This question as well as ad- ditional concerns related to rinsing will be ex- plored in a future column of "Trouble in Your Tank." PCB Michael Carano is VP of technol- ogy and business development for RBP Chemical Technology. To reach Carano, or read past columns, click here. THE CRITICAL IMPORTANCE OF RINSING, PART 1 Many birds, insects, and other animals can both walk and take flight. Robots with similar versatil- ity would open many possibilities, including machines that could fly into construction areas or disas- ter zones that aren't near roads and squeeze through tight spaces on the ground to transport. Researchers from MIT's Computer Science and Artifi- cial Intelligence Laboratory (CSAIL) are aiming to develop ro- bots that can both maneuver around on land and take to the skies. In a new paper, the team presented a system of eight quadcopter drones that can fly and drive through a city-like setting with parking spots, no-fly zones, and landing pads. Drones that Drive

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