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42 SMT Magazine • November 2014 Because these are manual process- es, board-to-board consistency is hard to maintain. For the microblasting system, corn starch, nut shells and other specialty abrasion materials can be used to selectively remove solder mask. Care needs to be taken to make sure that excessive material is not ablated, nor are excessive ESD charges generated, which can harm components. If the bare boards will be in storage for a long period of time, then OSP or another pro - tective coating should be applied to the exposed copper in order to protect its solderability until being populated. Ultrafine patterns with accuracies to 1 mil (.0254 mm) can be ablated with the use of a galvo- controlled laser source. Solder mask applied di - rectly onto an FR-4 surface and not directly over a ground plane requires use of a depth-controlled laser in order to remove the solder mask without damaging the board. Laser mask ablation is de- signed for high-precision, high-volume require- ments as it is a highly repeatable, fast process. Physical board modifications There are a variety of methods for modifying the physical dimensions of a PCB. High-speed, high-precision numerically controlled milling machines and lasers are the most commonly-used tools in order to make these modifications on both populated and unpopulated boards. When using high-speed milling machines, it is critical to use the proper board fixturing because excessive vibration can cause solder joint cracking as well as excessive mechanical stresses on the PCB. An example of such an engineering change would be the addition of a mounting hole in a PCB. Laser cutting of the PCB brings a high-precision meth - od for shaving off a few thousands of an inch as well being able to cut difficult routing shapes and Figure 7: Manual mask ablation tool. Figure 8: Micro ablation of solder mask. figure 9: laser cutting and scribing example. (Courtesy lPKf) feaTure bare boarD ecos, ecns anD Design moDiFications continues

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