| FreePCB User Guide | Version 1.2

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5. PCB Layout (continued)

5.17 Solder Mask Cutouts

Solder mask is a coating that is applied to a PCB to prevent solder from sticking to it. It is not really necessary for hand-soldered boards, but is essential for boards that are soldered automatically using solder bath or reflow techniques. In the CAM process, the solder mask is considered to be a "layer" of the PCB, and is described by a Gerber file similarly to copper or silkscreen layers. There will usually be top and bottom solder masks, with separate Gerber files for each.

Openings must be made in the solder mask for pads, since they require solder. FreePCB creates these openings automatically in the Gerber files for the solder mask layers, using the clearance specified in the CAM dialog.

Sometimes it may be necessary to create additional openings in the solder mask. For example, board edge connectors should have an opening around them to prevent "dams" of solder mask between the fingers of the connector. Or, you might want to have an area of bare copper for application of shielding or a heatsink. In FreePCB, these openings are referred to as cutouts. You can add cutouts to the solder mask layers using Add > Solder Mask Cutout. This pops up the following dialog:

You must select the top or bottom solder mask layer, and the hatch pattern. Clicking OK allows you to draw the cutout as a closed polyline, similarly to the board outline or a copper area. It will be shown in the color for the "top sm cutout" or "bot sm cutout" layer in the layer list. An example of a solder mask cutout around an edge connector is shown below. For a double-sided edge connector, cutouts should be made on both the top and bottom solder mask layers.

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