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80 PCB007 MAGAZINE I MARCH 2019 dip before the OSP process to remove any residues remaining. This step is not necessary if hydrogen peroxide-sulphuric acid is used. Also, it is important to note that this process does not require a pre-coat step. The OSP (if a conveyorized system is used) can be applied by spray or flood, but spray is preferred. The ideal configuration provides spray nozzles just immersed in the OSP solution (Figure 1). A second critical design feature for OSP equipment is to ensure minimal drag-out of the OSP chemistry. To minimize drag-out, PVA sponge rollers should be installed at the exit of the OSP chamber. These super-absorbent roll- ers squeegee the excess liquid from the panels, and it remains in the OSP chamber. The PVA roller covers are tubes of the sponge material with an inner diameter sized to fit the core and an outer diameter sized slightly larger than the current solid rollers. This slightly larger outer diameter ensures good contact with the boards and compression of the sponge to create the squeegee action. Compression of the rollers is aided by either the weight of the top roller core or a screw compression device that pushes down on the upper roller shaft. The compression also wrings out the rollers between panels to return the excess to the chamber (Figure 2). These rollers should also be present at the entry of the OSP chamber to prevent the potential of rinse water drag-in to the OSP. Drag in of water (potentially acidified from the microetch) will dilute the effective concentration of the azole compound and reduce the operating pH. Both situations will reduce the thickness of the OSP coating on the copper. Deleterious effects on solderability can be expected. Summary The performance of chemical processes used in the fabrication of PCBs depends on equip- ment function and design, not just chemistry. Keep that in mind when designing equipment for future purchase. PCB007 References 1. R. Schueller. October 2009. "Proper Section of Final Finishes," presentation to SMTA local branch meeting, Minneapolis, MN. 2. Prismark Partners LLC. 2013. "Final Finish Survey." Michael Carano is VP of technology and business development for RBP Chemical Technology. To read past columns or contact Carano, click here. Figure 1: OSP spray chamber configuration. Figure 2: Top view of the OSP chamber (yellow arrows point to the PVA sponge rollers).

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