Despite the ruggedness and durability of quadcopters they are full of very sensitive electronics and they are often used in less than forgiving environments. Would you take your home computer half the places you end up putting your quad and still expect it to work? One of the biggest enemies of electronics is moisture, depending on where you live it can be hard to avoid when you are crashing into damp grass or trees, especially in winter. It’s time to waterproof FPV Electronics.
The best way to protect your machine is by using silicone conformal coating, this article will step you though the basics of how to conformal coat your electronics so that you can fly with more confidence when there is a chance you might get your quad wet. Of course, it’s not going to make your quad invincible to the effects of rain or snow, but it is more likely that it won’t freak out or kill components when that stray bit of wet grass finds its way into your stack.
Tools for the Job
This one’s pretty easy, you need some conformal coating (more detail below). For the best job possible you want some kind of UV torch or one of those invisible ink kids pens with the UV LED on the back, failing that a bright blue LED will sometimes work, and as a last resort bright sunlight actually causes the coating to become more visible.
You will want a clean, well ventilated place to work and some paper towels to catch any stray drips is a smart idea.
Conformal Coating Basics
Conformal coating comes in a huge range of products from Aerosols through to 20L drums, for multirotor electronics only a very small amount is needed, and you need to be able to apply it precisely so a small bottle with a brush is the best solution. You will be able to coat a large number of quads with just a small 50ml bottle.
There is more than just one type of conformal coating, the one that you want to look for is ‘Silicone Conformal Coating’. The other commonly available types are acrylic or urethane conformal coating, while they will also work they are not as durable and can not be reworked as easily.
Silicone conformal coatings generally contain Xylene as a solvent, make sure that you use the product in a well ventilated area, it smells nasty and if you breathe in too much of the vapour it can leave you with a wicked headache.
There are many conformal coating manufacturers. I have tried two of them, Electrolube and MG Chemicals. The MG Chemicals product is MUCH nicer to work with, it has a lower viscosity allowing it to be applied more easily, and barely smells. The Electrolube product on the other hand is quite difficult to apply and has a smell that gets into everything!
What to Coat
When coating your electronics, you need to concentrate on the parts that will be most sensitive to voltage changes. The most important area is the legs of the IC’s on the PCB’s, these legs are often sensing and transmitting extremely small voltage changes and can be disrupted by current carried through water droplets from other connections.
Second most important is the motor esc connections, these deal in larger currents than the on-board IC’s but also ‘listen’ to the very small current sent back from the motor in between firing each phase. If one of the ‘firing’ phases is shorted to the ‘listening’ phase by a water droplet or wet piece of grass it can fry the ESC, or the motor, or both.
It’s counter intuitive, but the main power connections for batteries and ESC’s are not very important to coat. Yes, they deal with the most current, but because of the relatively low voltages and the lack of sensitive transmission water does not affect them as easily. Of course, if you put water between one of these main contacts and a sensitive circuit it will cause trouble so cover as many electrical connections as possible.
How to Conformal Coat your Electronics
The best time to conformal coat your electronics is on a fresh build, when everything is soldered together but the quad has not been fully assembled, it is clean and not covered yard goop. You can conformal older builds, but try and clean the parts as well as possible first, a toothbrush and some rubbing alcohol is the best way, but even window cleaner can work, just make sure to allow the parts to dry thoroughly before coating .
I plan ahead when doing a build that I know I will conformal coat and will purposefully only route wires and install parts temporarily, so they can be easily removed to be coated before final assembly.
IMPORTANT NOTE: Do not conformal push buttons or plug sockets, if you get the coating on a button/switch it can wick inside the parts and prevent it working which can ruin the part.
Start with all the peripheral components that can be disconnected with plugs. This usually includes the VTX and Receiver. Remove the heat shrink and paint the coating onto the PCB, concentrating on the legs of the IC’s and making sure to avoid switches and plugs. It is a good idea to paint the soldered connections on the back of plug sockets though.
Once you are done with the peripheral components, you can concentrate on the wired components, including the ESC(s) and FC. You should have most the components wired together on the frame, this allows you to make sure all the wire lengths are correct and tidy, remove the motors and the parts from the frame and you should end up with what I affectionately call a Quadopus.
If you are building a machine with separate ESC’s then hopefully you remembered to put the heat shrink loose on the wires before soldering them together, push this up out of the way and liberally apply the conformal coating to all the components, paying attention to get it between all the components, especially the capacitors which tend to be tightly packed. Also carefully coat the solder joints for the motors and any exposed motor wire. 4in1 ESC’s are exactly the same process, but you obviously do all of them at the same time.
Coating the FC is pretty much the same process, make sure to get all the IC’s completely covered and be sure to avoid the boot button if your board has one. If not, make sure not to cover the boot pads as conformal coating will prevent you from being able to short these to flash your board.
Once you have the parts all coated it’s time to get out the UV torch and play CSI detective, the conformal coating contains a UV tracer that fluoresces when exposed to UV light, this is very helpful as it makes it much easier to accurately see where the conformal is and isn’t applied correctly. Simply shine the torch on the parts and check that everywhere glows nice and evenly, again focusing on the IC legs etc. You might be quite amazed at how many spots you missed even when the board looked properly coated to the naked eye. Touch up any spots you have missed and leave the boards to properly cure. With the MG Chemicals product they can usually be handled and installed within 30mins or so. I leave mine on the radiator to cure them faster.
Now you are done with the conformalling, you can re-cover the peripheral parts and ESC’s with the heat shrink and do the final assembly.
Hopefully this has helped explain how to conformal coat your electronics. You can now pick up the required items and protect your machine, so that next time you try to squeeze in a flight between rain showers or go for a blast over the snow covered landscapes, you can do so with more confidence that if something happens and you go down, your machine won’t freak out or be damaged by the water.
Keep those props spinning, Happy Flying.