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I have recently begun etching my own circuit boards for miscellaneous Arduino projects and I need to apply a solder mask so that my copper traces don't oxidize.

I bought this acrylic latex spray paint for a different project, but I am wondering if it could be used as a solder mask.

My thought is that I would solder my components on first and then spray the board. Possibly, I would just cover any sensitive components with a small piece of masking tape first, but largely I wouldn't even think this would be necessary(?).

Any thoughts or reasons why this would mean certain doom for the board or, more importantly, why this may be unsafe or a fire hazard?

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    \$\begingroup\$ What you're describing is actually called conformal coating. Soldermask is applied before soldering components. \$\endgroup\$ – AndrejaKo Jul 7 '12 at 6:25
  • \$\begingroup\$ What a FINE group of contributors this community has! All three answers were correct but Oli's answer was the most direct in regards to my question so I picked for any other layman who happens upon this post. \$\endgroup\$ – Matt Cashatt Jul 7 '12 at 11:59
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It looks like it might work "okay", certainly unlikely to damage your circuit in any way (unless sprayed in pots/switches/etc) Check if it says flammable in the datasheet to ascertain fire safety.

However, if you are just looking for a "sealant" to add after populating the PCB (as you mention in your question), then there are plenty of coatings around specifically for this purpose. They are usually know as Conformal Coatings.
There are various types and methods of application, so you need to decide which is best for you. Here is a comparison of the various types (acrylic, epoxy, etc)
Some will protect the PCB from moisture, oxidising, etc, but you can solder through them if you need to alter something (e.g. spray lacquers)
More permanent and hard wearing solutions include potting compounds (e.g. put circuit in suitable potting box, pour compound in, leave to set)
Look on places like Farnell, RS, Mouser for "PCB coating" or "Conformal Coating" and you will get plenty of options.
Here is an example of a spray on conformal coating you can solder through.
Conformal coating is not just to prevent oxidisation, it also helps prevent problems caused by contamination (e.g. acids/alkalis) or moisture (important for e.g. sensitive/high impedance circuits), and also can protect against arcing in high voltage circuits (with suitably high breakdown voltage rated compound)

If you are looking for something to apply before populating the board to stop trace oxidisation, then see the tinning suggestions in the other answers.

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Better than any wrong chemicals on your board are no chemicals at all.

You are talking about spraying the board after soldering, which is actually no solder mask but something in the direction of conformal coating, except there's nothing conformal about it when you don't use a coating specifically designed for this purpose. Painting or coating a board after assembly is very critical and you have to pay big attention to any component that doesn't want foreign material somewhere inside - most prominently connectors, switches, piezo buzzers and potentiometers.

Keep in mind that the materials used for solder mask, lamination, coating and whatnot are usually carefully chosen to not interact with each other or with anything else on the board - flux, components, various metallic alloys, ...

Also, different thermal expansion properties are a huge issue: A coating that expands a lot when hot will kick away any components that happen to be in its way.

Expansion of your coating may also happen if it is hygroscopic (has the tendency to trap water molecules inside of its own molecular structure).

You may end up having a board with copper traces or component connections corroding away as time passes, and this effect may be worse compared to a board with naked copper traces.

Also, as you mention, flammability is an issue - and acrylic paint or glue is usually flammable to the max. (Ask a PCB shop that does flexible/rigid PCBs. They would love to use acrylic glue to attach the flexible polyimide layer to the rigid FR4 material, because acrylic glue has fantastic mechanical properties, but without adding tons of nasty, flame-retardant chemicals, the acrylic glue would be prone to start a hell-fire.) In addition, I highly doubt that the spray paint you mention is able to withstand temperatures you would need when repairing the board. The paint may become charred, and this means you release carbon from its organic, polymeric molecules. Carbon is conductive, so pretty much anything that looks burnt on a piece of electronics will create unwanted shunt resistors and thereby modify the original circuit, to put it in a careful way. Once the unwanted carbon resistors are so low-ohmic that they dissipate more and more power as current is continuously fed towards them, they are a good recipe for disaster.

With no solder mask, NiAu or Sn coating over all the copper on your board would be a good choice.

Here's an example of a board with no coating and NiAu on the traces: HP 5381A (source)

This is a piece of excellent HP test equipment, there's nothing cheapo about it. (Note how there's another board with solder mask in the picture.)

The process is called ENIG (electrodeless nickel, immersion gold). Immersion tin (Sn) is also not bad, and can be done by hobbyists.

I've searched some more, and here's an example of a board with tinned traces and no mask or coating: HP 5245L (source)

Again, from a piece of equipment built to last for decades (HP 5245L Frequency Counter, I happen to have two of them and they run like new after some fixing effort).

Alternatively, there are sprays for this exact purpose, often with material similar to flux.

If it's just a small board, you could also use a soldering iron and solder wire to carefully cover the traces with tin. Just be careful to not overheat the board - the traces might delaminate. A temperature below 350°C on the tip of your soldering is a very good idea! Excessive tin can be wiped away with silicone rubber (available in the form of insulation tubing) while still hot or removed with solder wick. The ugly remains of flux can be washed away using cheap alcohol from a drugstore.

Something quite similar is available on the market: You can order ready-made boards with a HAL surface. HAL, in this case, is not a super-computer on a spaceship, but means "Hot Air Leveling": The board is entirely dipped into hot solder and hot air is used to blow away excessive solder after the dipping process. These boards are made by the millions and millions, mainly for consumer products. Compared to chemically deposited surfaces like ENIG (NiAu) or immersion Sn, this is simple and cheap, but has the disadvantage that you won't get very even pads (important for BGA or QFN packages) and you apply quite a lot of thermal stress on your board because you dip the entire board in liquid metal: Through-hole copper vias are stressed and may break and the core may delaminate. However, I bet you have dozens of these boards in your home.

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  • \$\begingroup\$ Cool picture, I love the insides of old test gear... \$\endgroup\$ – Oli Glaser Jul 7 '12 at 7:16
  • \$\begingroup\$ +1 for tinning. I did a couple of pretty large boards using that process and found that very high temperature actually produces better results, if the board if fluxed. I sprayed the board with a flux spray, set the iron at 450 C and tinned the tip a bit more than usual. After that, I very quickly moved the iron tip over the area which needs to be tinned and got a nice, thin coating of tin on the traces. At high temperature and with flux, the process is so quick that there's no risk of lifting traces. Once again, the trick seems to be very quick movement of the tip over traces. \$\endgroup\$ – AndrejaKo Jul 7 '12 at 8:49
  • \$\begingroup\$ Thank you for this detailed answer zebonaut! This information is very useful. \$\endgroup\$ – Matt Cashatt Jul 7 '12 at 12:00
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A much better way to protect your copper is immersion tin plating the PCB before populating it. This product deposits a 1 micron protective layer after 5 minutes. And it eases soldering as well.

I've used this successfully before, though I'm not sure if it was exactly the same product. Anyway, I had to do it in a well-ventilated room because it smelled awful.

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  • \$\begingroup\$ It gives a nice result, but IMHO it is not really worth the effort for most one-off home etched PCBs. It's also relatively expensive compared to other solutions, and if you want protection against contamination, etc you will still need a conformal coating anyway. \$\endgroup\$ – Oli Glaser Jul 7 '12 at 7:01
  • \$\begingroup\$ +1. For a hobbyist, this is what I would recommend, too, if you want something better than "solder spray" (flux from a can). I've always used the latter for my projects. My homebrew audio amps, for example, still work after near-daily use ever since 1993. \$\endgroup\$ – zebonaut Jul 7 '12 at 7:02
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    \$\begingroup\$ @Oli - I don't know the price (I used this at work), but it's absolutely not much effort: make the solution in tepid water, let it cool down, throw your PCB in it, wait 5 minutes, done. \$\endgroup\$ – stevenvh Jul 7 '12 at 7:05
  • \$\begingroup\$ @Steven - Fair enough. I suppose if you have a little tub of the solution made up sitting next to your etch tank, then it shouldn't be too much of a big deal. I haven't looked in ages, but I think it costs around £2-3 per 100mm x 100mm board, which is quite a lot relative to the rest of the process. As a solution to oxidisation, it'll certainly do the job okay though. \$\endgroup\$ – Oli Glaser Jul 7 '12 at 7:13
  • \$\begingroup\$ Steven, thanks as always my friend! I am looking into the tin crystals, but not sure where to purchase. For now, it seems that all I need is to purchase some easy conformal coating at my local electronics store. I am mainly making toys and games for my kids with blinking LEDS; however, I am also designing a multicopter controller and for this I will strongly consider your approach when it comes time to etch! \$\endgroup\$ – Matt Cashatt Jul 7 '12 at 12:03

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