Soldermask and solder past layer on PCB

When a PCB is soldered it is necessary to keep the areas that are not to be soldered protected by a layer of what is termed solder resist. The addition of this layer helps prevent unwanted short circuits on the PCB boards caused by the solder. The solder resist normally consists of a polymer layer and protects the board from solder and other contaminants. The color of the solder resist is normally deep green or red.

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 soldermask and it helps protect traces from shorting when debris is present on your PCB. Not every board includes a solder mask layers 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!

In order to enable the components added to the board, either leaded or SMT to solder to the board easily, exposed areas of the board are normally "tinned" or plated with solder. Occasionally boards, or areas of boards may be gold plated. This may be applicable if some copper fingers are to be used for edge connections. As the gold will not tarnish, and it offers good conductivity it provides a good connection at a low cost. If you want to save PCB cost, then HASL(Hot Air Solder Level) is a good option for you, it will provide excellent solder ability, just the PADs will be so flat as other PCB surface treatment.

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.