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PCB-Oct2017

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58 The PCB Magazine • October 2017 Introduction In Part 1 of this series on the importance of rinsing, the author presented an overview of the critical aspects of rinsing as it applies to the overall quality of a printed circuit board, with considerable space devoted to water conserva- tion. Thus, we now turn to how one can im- prove rinsing effectiveness without increasing water consumption and, by default, significant waste treatment costs. Rinsing Protocols The preferred rinsing method of design en- gineers is counterflow, or cascade rinsing. Still another theory widely held throughout our industry reasons that, if work is left in a rinse tank for longer periods of time, better rinsing will be the result. First, let's explore the idea of leaving the circuit boards in the rinse for a lon - ger period. 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 typ- ical rinse tank—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. This demonstrates that rinsing time in a rinse sta- tion is a non-linear relationship with respect to removing contamination from the surface of the parts. Yes, this is counter intuitive. We often think that if two minutes in the rinse removes 50% of the residues, then doubling the time in the rinse will remove 100%! This is not the case. Basically, the law of diminishing returns applies. There are many factors in play here in- The Critical Importance of Rinsing, Part 2 TROUBLE IN YOUR TANK by Michael Carano RBP CHEMICAL TECHNOLOGY

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