If you are building your own printed circuit board (PCB), you most likely encounter problems where solder goes where it’s not supposed to go. The worst scenario is when dealing with closely spaced solder tracks and pads; one wrong move and you’ll end up shorting connections with solder. The answer for this predicament is the use of a PCB solder mask.
Solder mask is actually the green (or sometimes other) color you see in mass produced PCBs. Makers often don’t consider applying solder mask for their DIY PCBs but the benefits are undeniable. In this post, I will demonstrate how to apply this to your own PCB.
Choosing the Right Solder Mask
For us makers, the best solder mask would be the UV curable one. UV curable means we can dry the mask out under UV light or under the sun. The alternative would be using the Epoxy Liquid type but we need a silkscreen for that. Using the one pictured below will be easier although limited to small-sized PCBs.
Printing the Solder Mask Pattern
The idea of the mask is to allow solder only into the PCB pads. Thus, we need to cover up the pads so that the mask will not reach it. This is done by printing a mask pattern into a transparent film.
If you are using Eagle CAD, click on the Layers tool or type Display on the command line. The layers window should show up:
The layers with highlighted numbers are the ones currently visible. What we need is to show only the pads and vias. So click on the other visible layers to hide them. Your layout should now look like this:
You then print out the pattern into a transparent film. I suggest printing out two copies and placing these two on top of the other. These will make the pattern dark enough for UV not to penetrate.
Applying the Solder Mask
Once you have your mask pattern., it’s time to apply the solder mask. It’s good practice to clean the PCB first with alcohol for better results. Apply the solution and cover it with transparent film. Then use a plastic card to even it out on the board.
If all parts of the PCB is now covered with solder mask, put the mask pattern on top. Make sure the mask aligns with the pads on the board.
Next, expose the board to sunlight or a UV lamp. If you’re using the latter, place the lamp about 3 inches away from the board and wait for around 15 seconds. If you’re using sunlight, wait at least a minute.
After the curing process, remove the films and wash the board with alcohol. Then, exposed the board again to UV light for about a minute. This will make sure the mask has hardened.
You should now have a professional looking PCB ready for drilling and soldering.