If you’ve ever seen a PCB you are no doubt familiar with their oddly green coloring. This thin green protective covering on the top and bottom of a PCB is called the solder mask and it helps protect traces from shorting when debris is present on your PCB. Not every board includes a a solder mask and there may be circumstances where you want a solder mask on the top layer, but not the bottom or vice versa (at high speeds there are some performance issues to consider with solder mask). If you do go with a solder mask, you must provide files for all layers where this covering is desired (top and bottom are the only options). One final detail, solder masks are specified as negatives. In other words, areas that aren’t to be covered, will be filled in on your solder mask layer. This is apparent when examining the example solder mask image above. The only purple spots on the top layer are for component pads. Of course, we can’t have solder mask covering where our components need to make electrical contact with the board! Solder paste is a material used to assist in the soldering of surface mount components on a PCB. The pads on the surface mount components you use in your design will specify solder paste within the exposed copper region (not covered by solder mask). When the board is populated with surface mount components, solder paste helps the solder flow better as it bonds the component pin to the copper on your PCB. Solder paste files are necessary for the top and bottom side of your design when using surface mount components on those layers. There are a handful of surface mount components on the top side of our example board so in the image above you’ll notice the gray coloring for solder paste on the pads for those components.
