On this page, we will see how to apply the solder mask to the Printed Circuit Board that we made in the previous post. First of all, you will need a transparent film (A4 size), a permanent marker (optional), UV solder mask that you can order here.

UV solder mask is a protection for your PCB from oxidizing. You can also use tin plating for this purpose. However, the UV solder mask gives the circuit a better and professional look. However, precaution and practice are needed to have a better circuit.

1. Cut the transparent film to a size a little larger than the size of the circuit. Do not cut it too large as we will need more transparent later. So save it for later. And also, do not cut it the exact size of the circuit, as we will tape it to a wooden board and we need some space to add the tape.

2. Place the transparent film on the circuit board and tape it. You can use masking tape as we will need to remove it again. You must stretch the film well to make the gap between the film and the circuit negligible. This is very important for what we are going to do.

3.  So we will need to mark the place where we do not need the solder mask. These are typically those places where we need to solder the components. Note that I am applying a black permanent marker. This is very important. The solder mask is sensitive to UV light. UV light makes the solder paint to harden. For places that we need to apply solder, we want it not to harden to be able to remove it easily. Note that this is not a good way to do it as the permanent marker does not block the UV light completely. You can repeat this process 2 or 3 times, and double or triple this layer to block it completely.

A better way would be to print the solder paste marks from our circuit we designed here and print it on the transparent film in an inkJet printer. A laser printer would not work as it will melt the plastic. print 2 copies of it and double the later.

4. After you have marked all the places to add solder, remove the film and cut another film, roughly the same size we did before.

5. This is the solder paste I am using. It's dark blue. You can find a link to buy it from the part list I gave on our very first post of this project.

Apply a little solder paste to the board. You can use a stick or scissor to push the paste out.

Do not apply too thick solder. Now place the transparent film we cut just before and tape it on the board. Stretch it well while doing this. We will spread it all over the circuit.
If the paste is too thick, we won't see the track and where to place the marked film we did before. You can use a card or ruler to spread the mask evenly all over.

So this is how it should look like. You can is how the paste is almost consistent all over and we can clearly see the tracks.

7. Now place the marked film on the top and align all points exactly to that of the circuit.

8. Now we have to expose it to UV light. You can buy UV light lamp or a torch for this. However, I am using sunlight for the source of UV light.

Expose it for about 4-5 minutes depending on the thickness of paste you applied. The thicker, the longer you need to expose it.

9. Now it is the moment. Remove the film very gently. Do not be too quick and this will damage it.

10. Now take a tissue paper and alcohol and clean the surfaces.

You will see in the images below that our permanent marker did now work well. All the solder paste was hardened. But that doesn't mean we need to make another one, don't worry :P.

11. We have to take a knife and scratch those places gently.

Refer to images below to see what we should get after scratching them all. You can work with this circuit. However, I like to apply the solder to the pads to make it easier to solder components later.

12. Just apply some flux all over the pad just to clean the surface. This is optional as soldering wires already contains flux.

13. Now you can take your hot soldering iron and take some solder on the tip of the soldering iron and then apply it on all the pads.

14. I also like to rub it with very fine sandpaper to level the solder and also to give the solder mask a matt look.

So this is our final PCB for our flight Controller. Time to solder all the components now.

You can watch the youtube video for this tutorial here for more understanding.

Thank you guys for watching :D