Once you fabricate a PCB, it pretty much instantly begins to oxidize. PCBs created in industry are coated with a couple of things to protect them from this oxidation and short circuits. The first is called a soldermask, which is a type of epoxy that literally coats the entire circuit board. If you’ve ever seen a circuit board, you’ve seen the soldermask. It is typically GREEN but can be different colors. For example, official arduinos typically have a Teal BLUE soldermask. Sparkfun uses RED. OSHPark uses Purple.
You can see below just how badly the copper oxidizes after being touched an exposed over time.
There are multiple ways to add a soldermask to a PCB. My new favorite method is using Kapton tape (explained at the end of this page), but I have tried and compared a lot of different solutions below.
Epoxy-based:
In industry, they use a specially designed paint or epoxy that is cured with ultraviolet light. This allows them to cover all the traces (the wires) but leave the pads visible so you can solder components on the board. Some folks have tutorials out there showing how to do this, but it is messy and uses nasty chemicals.
Dry-Film Soldermask:
You can also buy sheets of “dry film soldermask” which has the epoxy deposited as a flexible sheet that you adhere to the PCB, then use a photolithography method to harden it with UV light. This allows you to remove the softer material on the pads you will solder the components to. This material is not readily available, but you can find it from electronics suppliers online. Here’s an excellent tutorial on how to do this process at home.
Tinning Traces:
Another option to protect the traces from oxidizing is to tin them. Tin doesn’t oxidize as badly as copper. Essentially you can deposit tin on all the copper surfaces using a chemical deposition (electroless). This is actually done to the solder pads on commercial PCBs, but it can be done to the entire PCB. The biggest issue with this method is that it doesn’t prevent short circuits because it doesn’t add a layer of insulation to the traces. Again, it uses nasty chemicals.
Conformal Coating:
There is a conformal coating that can be painted or sprayed on a PCB after soldering the components. It coats everything. While it has been formulated for electrical characteristics, etc. I personally don’t like this option. There are Acrylic, polyurethane, and silicone based products, which you can solder through, but it only comes in clear (you though you can see it in UV light).
DIY – Nail Polish:
When I did FabAcademy in 2014, I milled a ton of PCBs. They always oxidized really badly. Some would be useless within a month. I began painting finished boards with fingernail polish. I only painted the traces in case I needed to resolder the components. (The soldered areas do not oxidize like the copper traces). This option isn’t great because fingernail polish isn’t designed for electronics, or being touched with a soldering iron, but it works and I have boards that are almost 10 years old that look brand new. This is probably one of the easiest solutions due to availability and color selection.
Lacquer:
Another thing I tried more recently was to spray the PCB with colored lacquer, then using either a laser to etch off the lacquer on the solder pads with a laser, or to just solder it directly (the lacquer melts only when touched with a soldering iron). I don’t really know the chemistry here so when you laser it or solder it, I don’t know how safe it is. I don’t see how much different it can be from the conformal coating you can buy. A bonus with Lacquer is that you can get lots of colors, though I recommend avoiding anything with glitter, pearl, or metal flakes in it.
Both nail polish and lacquer do allow multiple colors, but neither are designed for electronics. Here you can see the left board is almost 8 years old but has had its traces painted with clear nail polish for protection. The red board is from my previous article in 2021.
The best solution I’ve come up with is to mill or etch a circuit board, then export the pads layer of the design to an SVG. From here it can be cut by a laser or a vinyl cutting machine into Kapton tape. Once cut, the tape can be applied to the PCB and pressed down hard. Since kapton tape is heat resistant, it can withhold under a bit of soldering. It also has excellent electrical properties (resistance, capacitance, and inductance). It is actually used for a substrate material for flexible electrical circuits.
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Ok, so Kapton tape wins. How do you cut and apply the kapton? We tried a couple of things and both worked.
Firstly, I told Garrett (who is taking FabAcademy in our lab this semester) about my idea and asked if he’d play with the kapton tape and the laser to find out what settings to use. He set about finding the best settings. He first used it to make a solderpaste stencil for his own project. Apparently on a 120 watt epilog, for the size holes we needed, about 6-7% power worked well.
We tried a couple of methods. First we placed the tape on cardboard, cut it, then peeled and stuck it to the PCB. This worked fine, but was a little tough to unstick and weed. This is likely the method I’ll use in the future though.
The second attempt we got cocky and just stuck the tape on the PCB and lasered it directly.
It is easier to line up with the cameras on the laser, but even when we placed the PCB directly under the camera (to avoid aberration of the fisheye lens) we still didn’t get the best alignment. It was good enough to solder though. You can see the finished product at the top of this page.
