How to protect non-soldermasked boards?

[mikeb]

Hey all,

I've had a few boards made for a project without soldermasks and I'm planning on getting a slightly larger quantity made soon (10-20). I'm under the impression that the soldermask also helps the board from environmental problems? I'm assuming maybe moisture, or other problems with the traces oxidizing?

So I'm wondering if there's anything DIYers can do to their boards that aren't soldermasked to give the same protection from all of this?

I was thinking maybe after the board is totally populated, covering anything that needs to be kept bare and then maybe spraying it with clear-coat or something?

The alternative is that I'm overreacting and that the board will be perfectly fine without a soldermask...

Mike

[Randaleem]

Hi Mike!

Some Very good questions!

First answer is that there is a LOT of SDIY that has worked well for decades without taking any special precautions. But there is probably a similar amount that would be working better if such precautions HAD been done. So much depends upon your local environment over the lifetime of the project.

So...

It used to be pretty common to cold tin plate the copper traces of a PCB. This helps in soldering and protects the copper from oxidation. It's also fairly easy to do; and pretty inexpensive overall. (FWIW; many board houses will do a HASL (Hot Air Solder Leveling) or tin plating even when they're not doing a solder mask. If the traces of your boards are already silver colored, skip the tin plate.

Otherwise, get some TINNIT brand powder and follow the directions. Most specialty electronics stores and suppliers have it or the comparable product LIQUID TIN by MG chemicals. Benefit of the latter is that it is already mixed. Downside is that the powder has unlimited shelf life.

Okay, now that your traces themselves are protected, here's a few DIY soldermask options.

First, you can buy what is known as "Conformal" coating from electronics suppliers. This is a coating (usually spray, sometimes poured) which is designed to protect all the items on the circuit board from the environment. There are many different tytpes and you need to take the time to understand the tradeoffs in each one. Some are thick and protect the board from EVERYthing; But these can make board rework/repair VERY difificult, and also can have an impact on heating/cooling issues for the circuit and board. Some even can affect circuit operation by changing capacitance parameters. Thinner conformal sprays have some of the same issues; but are often used in professional electronic assemblies.

Then you get to the DIY "conformal" coatings:

First is the time-tested, well-proven Krylon brand clear spray. Cheap, simple, effective. One point to mention here; however. With environmental laws kicking in and baring their teeth; many MFRs' products are NOT the same as they ever was. (Apologoes to David Byrne) So do some testing to assure that you're going to GET those "time-tested, well-proven" results.

Next is the use of Testors brand plastic model spray paint. Same cautions as above; but I can attest to its formula for product purchased a year ago. The nice thing about using it is that you can spray first and the soldering heat will melt the dried spray and allow you to solder the component. (I've done that with high amp mixed mode circuits; not sure I'd wanna go there with Synth stuff; but it bears mentioning anyways.)

The second thing about this is that two of Testors colors end up looking quite a bit like a professional solder mask. These are the 1224 Gloss Green and the 1601 transparent Candy emerald Green. Most hobby and/or Craft stores will have these little 3 oz. spray cans.

Then there is the laserjet Iron on toner technique; but using the toner as a permanent mask; instead of only as an etchant resist. I've not done this laserjet method myself; but have read of others who have said nice things about the technique. Couple of caveats": Black toner may be conductive. And "Permanent" is an interesting word here. (Some people have used the TT technique to print component legends too; same cautions apply.)

I'm lucky to have an ALPS MD-5000 printer; which uses RIBBONS instead of ink or powdered toner. This printer is no longer made; but if you can get your hands on one; it will allow you to do many wonderful things that other printers cannot. See the web and Yahoo groups devoted to it (there are several) for details. Suffice to say, it can print white compoent legends, or solder masks, silver/gold foil front panel lettering, do dye sub printing, decals, or... It's a wonderful resource.

Finally you have the "cutting-edge" technique of printing directly to a board; using a modified inkjet. I have 4 C84/86 Epson printers that I'm currently setting up for this. It's still an experimental technique; but shows great promise IMO. I'm going the MISPRO inks direction; others are using stock Epson Durabrite inks. (I didn't get good results using those.) Search direct print PCB's online to see more.

Enough for now?

Now it's ME who is over-reacting! (Yes; having written all this, you probably don't need to do anything special.
As I said; it depends upon where you live. If you're next to the Ocean; I'd look more carefully into things than if you're in the dry parts of Arizona. I personally think tin-plating is worth the effort. But I've read "scholarly" treatises that say it "hurts" the solder connection. Never seen any results like that in 30+ years. knock on wood.

Randal

[etaoin]

I have always used Kontakt 70 which is a plastic spray designed, according to the can, especially to protect pcb's. I've never seen any adverse effects of it on my DIY modular boards (resistance is 10^16 ohm per centimeter). And if I need to change something, I just solder right through it. The layer is quite thin anyway.

[etaoin]

By the way, I've seen mention that tinned boards attract fungi which can cause shorts, but that was in a post by someone who preferred coating.

[mikeb]

Thanks for the reply Randaleem. These boards I'm getting done are going to be through expressPCB. When I first started this project, I read some good things about expressPCB after getting frustrated with Eagle, so I designed my board in their software and it grew into a monster. Now of course, I regret having designed the board in "closed" software and being locked to their manufacturing, but whatever.

So their board specs are:

> We plate an additional 3/4 ounce of copper (totaling 1 1/4 ounce), resulting in a copper thickness of ~0.0017".

> The final finish is tin-lead reflow. Lead-free is not available with our MiniBoard service.


So it seems that I don't have to worry about tinning the board.

These boards are inside things I'm going to be shipping out to people, so the environment is unknown and I want to make something that will last. Some sort of thin conformal coating as you describe that I could spray both sides of the boards with once they've been completely populated sounds ideal. I suppose I would have to make precautions for not spraying the pins of connectors or something as well.

I was actually just considering krylon clearcoat, since I have some downstairs from previous projects. I wast just worried maybe chemicals in the paint might do more harm that good.

I'll definitely look into the various conformal coatings though. Definitely seems like a "no harm in trying" situation!

And thanks for all the info!

Mike

[mikeb]

[etaoin]

[mikeb]

[etaoin]

[mikeb]

[Randaleem]

[jksuperstar]

Don't forget, if you use clear coat spray, to mask any pots, connectors, switches, etc that spray really *can* hurt.

I know, it sounds obvious, but for that person (sometimes me) who reads this thread and is just so excited to jump in & spray their project....think first!

[etaoin]