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68 SMT Magazine • December 2015 require a large mechanical force to scrape it off. With either the use of a knife, scraper, pick or eraser, thin layers of mask can be removed by a skilled and certified repair technician. A mul- titude of equipment rework stations contain- ing drills, grinders, rotating brushes, and the like are commercially available that the rework technician can use (Figure 2). There are several drawbacks to this process, namely operator fatigue on large projects, and the lack of repeatability when ultra fine dimen- sional requirements need to be adhered to. Even with steady hands and fabricated masks the tol- erances that can be held by skilled personnel is to ±10 mils. In addition, the depth control using this technique is very poor and underly- ing laminate or copper layers may be damaged as there is a lack of depth control. The positive aspects of this method are that no special set-up is required and it can be done by almost anyone for prototype or low-volume rework. Another method that can control the depth of the cut is through the use of a milling ma- chine to remove the solder mask. It is a more precise technique for removing solder mask than using a manually-controlled depth cut of a hand grinding tool. Since high-speed cutters are used when milling, precision depth control is essential since the cutters will tend to pull into the coating and may penetrate into the board surface. Not damaging the underlying laminate is the key risk in using this approach. Turning the milling cutter in the opposite direction can be an effective technique for controlling the depth. Operator skill and experience as well as board fixturing are paramount to making this technique work. chemical means Another method for solder mask removal relies on a chemical stripping agent and is most effective when removing solder mask from cop- per or soldered surfaces. Tapes and other masks or other protective materials are placed on the PCB surface to isolate the area before the chem- no TIme For A boArD SPIn? SeLecTIVe SoLDer mASK remoVAL Figure 2: Microabrasion on a PCB. knoCking Down THe bone Pile

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