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November 2014 • SMT Magazine 39 pad geometry. Or in some situations, the proper "fit up" for a shield or ground pattern for the component must be made. In these cases, solder mask needs to be selectively removed (Figures 4 and 5). A final grouping of this type of board modi- fications involves modifying the physical di- mensions of the PCB. Modifications which may be required include changing the physical out- line of the board, adding holes in select areas of the PCB in order to mount it, and shaving a few tenths of a millimeter from the PCB in order to fit it properly into the enclosure or housing. methods for Physical board eco/ecns and modifications For each of the different physical modifica- tions, including the addition of trace and pads, the cutting of conductors, the selective removal of solder mask and the physical modification of PCBs, numerous other methods are commonly employed to make these changes. A variety of processes can allow traces to be added while still permitting processing and as- sembly a PCB. What is critical is making sure the addition can withstand squeegee pressures and that the trace is not negatively impacted by subsequent reflow temperatures. One of the methods, if the layout of the top layer of the PCB allows, relies on a solid-state laser to selec- tively ablate the solder mask in a given area. A second method relies on either the IPC-7721 Method 4.2.1. (epoxy method) or 4.2.2. (dry film method) for the adherence of a new con- ductor by way of a replacement circuit frame. figure 3: Trace cuts on several signal lines need to be precisely cut (3 mils spaces and traces). figure 4: QfN ground plane prior to solder mask ablation. figure 5: QfN ground plane after solder mask ablation. feaTure bare boarD ecos, ecns anD Design moDiFications continues