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60 The PCB Magazine • October 2017 cluding the type of contaminants to be rinsed, affinity of those contaminants to adhere to the printed circuit board, etc. It is more desirable to have brief expo- sure 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 how to im- prove rinsing efficiency. Most of the published studies related to rinsing are based upon mul- titudes of calculations of volume of water flow relative to number of rinse tanks in use [1,2] . Discussion Although an absolute rinsing standard for all chemical processes in the PCB industry is unattainable, studies have shown that rinsing can be optimized using many rinse tanks, and the appropriate hang times. While the man- ageable levels of contaminants vary for differ- ent processes, we know that a good dilution rate equates to good rinsing across the board. De- termining the appropriate hang time of a pan- el above the process solution helps to improve the dilution rate, without incurring more wa- ter consumption. Research showed that increas- ing the hang time can decrease rinse water usage, but also that one can determine an opti- mum hang time. Beyond the optimum hang time, allowing the panel to drain longer fur- ther benefits diminish. The re- sults of this optimization will give increased quality of final product with fewer rejects, de- creased chemical costs, and lowered burden on waste treat- ment systems. Now, there are a few oth- er factors in play here. As we know, circuit boards are flat pieces with many vias in them. Most plating operations (not all) are designed to process the panels in a vertical mode. Higher aspect ratio vias will trap more solution than larger diameter vias. Regardless of the panel thickness and via size, one must contend with drag-out. One can decrease the amount of drag-out by: • Increasing hang time of the panels to insure more solution falling back into working tank • Tilting the panels at a slight angle to in sure better drainage (especially helpful for small diameter vias and thicker boards) • Withdrawing panels slowly from the working tank Several additional techniques can be em- ployed to not only improve the quality of the rinsing, but also reduce water consumption. These are described below. Counterflow or Cascade Rinsing Basically, a single water source flows through multiple rinses in a direction opposite or coun- ter to the flow of the work being processed (Fig- ure 1). Counterflow rinses provide the benefit of multiple rinse stations, but use the water flow of a single rinse. Therefore, rinsing can be greatly improved without using additional water. Both immersion rinse tanks and spray rinse systems can be counterflowed. THE CRITICAL IMPORTANCE OF RINSING, PART 2 Figure 1: Counterflow rinse diagram. (Source: IPC 740)

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