DIY solder mask

[homebrew]

Hi all,

I've a question regarding solder mask on home-made PCBs.
Of course I know that one would by proper dry-film solder mask and laminate it onto the board, but that has several disadvantages:
1) It's hard to get the material. (At least here in Switzerland or Germany)
2) You need a laminator
3) It's apparently really hard to avoid air bubles and make sure that the resist is properly laminated to the substrate between fine tracks.

So I was thinking of a much easier process.
Some people (has also been discussed in this forum) simply leave on the photo-resist by exposing it a second time using a mask covering just the pads. A subsequent developing step exposes the bare copper pads but leaves the rest of the traces covered up.

That's fine but it still won't act as a helpful mask, as there is still nothing between the pads. (Thinking of TQFP packages or so ...)

So here is the idea/question. You can here easily buy photot-resist as a spray paint. A product like this:
http://uk.farnell.com/kontakt-chemie/positiv-20-200ml/positive-resist-photo-sensitive/dp/801010?ost=Positiv+20&selectedCategoryId=&categoryId=700000004393

Why not spray the final board with the paint for full cover? Then expose the pads. Hence in the end, the board would be fully covered (as with any solder mask) just exposing the pads.

This stuff is apparently also fairly temperature stable so it should survive the reflow process without any major damage.
Of course this would not result in a proper MIL spec coating, but I would assume that it could well serve the purpose of helping during the soldering process for DIY projects.

What are your opinions/objections?

[sleemanj]

First, my down to earth, boring, realistic and derived from personal experience opinion.  Don't bother with trying to do soldermask on home-etched PCBs, it's not worth the hassle.  Home-etching is great for quickly knocking together prototypes.  But once you want or need soldermask, pay your $15 and get them made in China.

With that said, here are my experiences in DIY soldermask, briefly, I've said all this before so I'm sure google will bring it up.

  1. Liquid Photo Imageable Soldermask (Soldermask Ink/Paint/whatever you want to call it) - this is readily available from eBay, Aliexpress etc, easy to get and cheap, it's the real stuff used in industry.  It's a bitch to work with though at home.  In short, paint it on your board (spray gun is recommended, yes, it's a lot of hassle!), you should then preferably "tack dry" it in a temperature controlled oven (I forget what the temperature is, but it's somewhat critical, too hot and it kills it, too cool and it will never tack dry), or less ideally put a piece of cellophane (many other types of plastic will bond to it which is not useful!), expose, and use mild organic solvent to "develop" (as in, wipe it clean), then re-expose for longer time to harden.  Totally not worth the problem.

  2. Vitrea 160.  This is a "glass paint" available from your local art shop - take care specifically that it's Vitrea 160, other similar products do not work.  First you must toner-transfer your pads to mask them (I did not find any better way), then paint the PCB in Vitrea 160 (again, spray gun or air brush recommended, thin it slightly), air dry the PCB, then bake it in the oven at 160-180 degrees or so for half an hour (you are supposed to wait 24 hours before baking, but I never did, as soon as it was touch-dry I baked it).  Once baked use acetone and scrubbing to remove the toner.  This produces a very good DIY soldermask and is a bit less of a bother than (1) above.

 3. After etching, laminate another bit of dry-film etch resist (dry film etch resist is plentiful and cheap, compared to dry film soldermask which is harder to get and expensive) onto the PCB (or you could even do two more layers), mask the pads and expose, develop to remove the resist from the pads, re-expose under UV for an extended period (say half an hour) to ensure that it is fully cured.  This works more or less as a soldermask, the resist will stand soldering heat (I don't know about reflow) and as long as there is flux solder doesn't readily stick onto it so helps with avoiding bridging, however it is not very abrasion resistant, that is, it's easy to scrape.


I've tried all 3, (1) is horrible to work with, I got reasonable results once with Red on a small PCB, it's just too much of a pain, (2) isn't so bad, as long as you have an air-brush and manage to get the consistency right for spraying, (3) is of course the easiest method but the results are not as good.

It's a lot of work for little reward, unlike just the etching process which is a little work for a lot of reward.

[wraper]

That's fine but it still won't act as a helpful mask, as there is still nothing between the pads. (Thinking of TQFP packages or so ...)

If the pin pitch is less than 1.27mm, cheap Chinese proto boards won't have it too. But for a low quantity why would you bother at all. It just prevents solder bridging during the reflow, therefore less possible rework needed. Bubbling happens only near the pads if there are dense SMD or some long tracks at the right angle towards the laminator. For through hole boards it's a non issue basically. Personally I probably would bother only if I need some big board in single quantiy which are expensive to order from China. Here is a photo of what I've made in the past with a dry film solder mask.

[homebrew]

Well it certainly is not about the price. It's rather a question of fast turnaround times when prototyping.

THT is pretty much over, at least for me. Whenever I can I do everything in 0603 or 0805 where needed. It's a good compromise between board space and my hand assembly techniques.

Normally I use something like PCB-pool (http://pcb-pool.com) to get my boards done.

But as I get away with double sided boards most of the time.
Hence I just want to try to make them at home, just as in the good old days :-) But with higher standards.

I'm not so much scared by the resolution of the board image or film alignment. I think I can handle this sufficiently. The only problems I see are VIAs (for the moment I'll just poke a piece of copper wire through the board and solder it on both ends) and the missing solder mask.

Having solder mask makes soldering so much easier and also prevents oxidation afterwards.

I really like the suggested approach:

3. After etching, laminate another bit of dry-film etch resist (dry film etch resist is plentiful and cheap, compared to dry film soldermask which is harder to get and expensive) onto the PCB (or you could even do two more layers), mask the pads and expose, develop to remove the resist from the pads, re-expose under UV for an extended period (say half an hour) to ensure that it is fully cured.  This works more or less as a soldermask, the resist will stand soldering heat (I don't know about reflow) and as long as there is flux solder doesn't readily stick onto it so helps with avoiding bridging, however it is not very abrasion resistant, that is, it's easy to scrape.

However, what would be the downside of my suggestion using the spray paint etch resist? Basically it's the same thing, right?

[wraper]

However, what would be the downside of my suggestion using the spray paint etch resist? Basically it's the same thing, right?

Pain in the ass to use and not a real solder mask. Not resistant to alkali unlike true solder mask after exposure in UV and baking. Positive 20 is not cheaper than dry film solder mask too. And get a good room ventilation first if you don't want to get high.
If you want some good results without too much hassle, then a dry film photoresist is a good choice. But then you need a laminator. When you have a laminator, you can use dry film solder mask as well.

[madires]

I simply use SK10 and don't add any solder mask. Seems to be sufficient to protect the copper. Even 20+ years old PCBs are still looking fine, some are used in areas with higher humidly. And soldering SMT components for a few DIY boards without solder mask is ok.

[sleemanj]

If you want some good results without too much hassle, then a dry film photoresist is a good choice. But then you need a laminator. When you have a laminator, you can use dry film solder mask as well.

I use my trusty clothes iron, up to 7x10cm is easy enough to handle with it, saves luggging out a laminator and waiting gor it to heat and setting up a carrier etc...

Key is having the iron not too hot, about 50 to 70 degrees C, too hot and blisters will form, too cool and it won't stick.

http://sparks.gogo.co.nz/dry-film-tips.pdf

[Mechatrommer]

not wanting to create a new sparse thread so, concentrating all relating subject in this 1++ yrs old thread is a nice idea imho...

got pissed off with my previous home etched pcb due to alot of solder bridging, wasted alot of time to de-bridge them, the time worth for doing other jobs... so re-searching for homebrew masking, here is not so bad not so hassle-full as mentioned imho... you just need laser printer, transparencies, nail uv lamp curing light, and this liquid uv curable mask, 15 minute exposure then its done...

http://www.ebay.com.my/itm/10cc-PCB-UV-Curable-Solder-Mask-Repairing-Green-Paint-Used-for-Soldering-PCB-/263013049428?var=&hash=item3d3ccca454:m:m5ol4MHDd4vqYc4r8U5dIvg
then i put it in a little jar like this...
http://www.ebay.com.my/itm/100g-PCB-UV-Curable-Solder-Mask-Repairing-Paint-Anti-Corrosion-/162099746233?hash=item25bde5d5b9:g:4ZQAAOSwzJ5XWucM
so i can take out with paper knife and apply uniformly on the etched pcb. they are now commonly available in online store, that a good news. other color other than the boring green are also available.

the dry film mask type, is currently sold by these guys...
http://www.ebay.com.my/itm/Dry-film-solder-mask-Dynamask-5000-0-3m-x-1m-/281908131503?hash=item41a308b6af:g:QeIAAOSwLVZVnQhN
http://www.ebay.com.my/itm/Dry-Film-Solder-Mask-Roll-of-12-in-x-42-5-in-bigger-lenght-by-request-/152191019782?hash=item236f4aab06:m:m_wESLBD3sDj3IdaQGpq02g
but i dont like this (i still have the very old stock film bought from china) because of extra step of chemical etching the film, so i have to keep an extra etching container and maintain chemical formulation, not very good for me. the liquid form above, i just wash/scrap with alcohol/thinner on a tissue after curing, then its done, easy.

here in this video, the solder mask seems sticky, but not in my case, mine is as liquid and as easy as water, only slight adjustment is needed before curing process...


so now i dont have to spend much time on de-bridging diy pcb, a happy customer albeit some imperfection such as not so uniform thickness, its still better than not having mask at all. and again to make it clear to avoid further arguments, yes for mass production, we should go to and pay the fab house, this homebrew is only suitable for prototyping, quick turnaround and a oneoff... and not bigger than what your laser printer can print, usually A4.. so not bigger than A4 pcb...

edit: white colored liquid mask is on its way from china, i will experiment with silkscreening later with the same method...

[M4trix]

Speaking of which, anyone tried to apply that UV curable soldermask ( Liquid Photo Imageable Soldermask) as Mechatrommer showed above with a rubber roller ? That squeegee coating technic looks very inconvenient with poor results. The coating is uneven and looks terrible. 



Edit: this LPKF video gave me an idea to use a rubber roller for soldermask and component legend ( white ink for silkscreen ).

[Mechatrommer]

with a rubber roller ?

yes i saw a youtube video using that just now but i closed it its lost already. or using silkscreen in fab house like in dave's video...

[M4trix]

@Mechatrommer, thanks mate. I already watched that Dave's video. The problem is that this UV ink sold on ebay is totally different than that ink used by the manufacturer in the video. This ink which I have (white and green), is always wet and stays wet for days unless you expose it under UV light. Well, I already have Dynamask 5000 dry film soldermask and that's not the problem. Silkscreen overlay is where I'm stuck. Using expensive silkscreening equipment is out of question. Not worth the hassle. That's why I asked if someone used a rubber roller for spreading the ink. Btw, Dave didn't show how the manufacturer is drying the boards after they are coated with soldermask and silkscreen ink.     

[sleemanj]

This ink which I have (white and green), is always wet and stays wet for days unless you expose it under UV light.

It's supposedly to be tack-dried in a closely controlled oven.

A while ago on the Homebrew PCBs mailing list there was a post by Andreas who had the name ctech, this was his results...

http://nicadrone.com/index.php?id_cms=24&controller=cms

There was discussion on the list about his drying method, because I wasn't able to get any success trying to repeat it with my inks.  Here was his reply....

>Firstly, that he managed to dry it before UV curing.
The trick is thin code (but still think enough that you can barley see throw the board) and dry it in a controlled temperature environment. Tosters do not work. The paint will start curing when you heat it up too much and the point between dry enough to apply the mask and curing is very close ~5-10deg C
(105-115deg it starts to cure, higher and it's all crap) when the color tone changes slightly its junk

Also I just dry the pain enough to apply the mask, it is a little bit sticky but the mask can be moved a little bit, and the paint does stick a little to the mask after curing so masks are one time use only.

>And I "baked" it in the oven at 180 degrees C for about 20 minutes and it "dried" ok, soft but tack-free.
No!!! way to hot. 105deg C 30 minutes, 115deg and I get crappy results. And index the mask when the board is just a little tacky

Later I tried baking at 110 C and didn't work either, but my oven was just that, a kitchen oven so not very closely controlled, I think 110 was the lowest I could "set the dial".

So I think that, if it's not certain ink (colour) specific which it may be, that pre-drying temperature may be really critical.

Now, once again to put it plain, if you need a soldermask, just get them made.  It's such a hassle.  With places like Seeed and DirtyPCBs and whoever else, who will produce proper factory made pcbs and deliver to your door for a few dollars.... just get them made.

[Mechatrommer]

A while ago on the Homebrew PCBs mailing list there was a post by Andreas who had the name ctech, this was his results...
http://nicadrone.com/index.php?id_cms=24&controller=cms

he's using car spray gun to spray the mask , well thats a hassle-full for me. i believe what he's trying to achieve is perfection for larger or mass produced board, this way i agree to get the board made by dedicated fab house, but cost is another consideration, if fab house charges high money, this diy method could be an alternative.

i think i dont need that kind of perfection, so oven step can be skipped for my case, i only want to avoid solder bridging during prototype soldering, and things like quick turnaround, cheap and flexible is on higher priority for this method to be usable. for my one-off, this mask albeit not as durable as ovened factory made, can give some cosmetic "better than nothing" appearance, avoid copper rusting and help with heat spreading on high current application, ymmv...

[M4trix]

@sleemanj, thank you for discouraging me ! 

* M4trix joking. 

Well, there is no stepping back down now. Things can't be undone. I've gone through all steps with dry film photoresist and soldermask. Yep, it was hard and a lot of pain the arse but I've managed to produce decent PCBs. Of course, the top overlay is the hardest task. I will try to coat the board with a rubber roller and dry it in an oven like you've mentioned. I simply can't stop now... I can't !   

 

[Mechatrommer]

Yep, it was hard and a lot of pain the arse but I've managed to produce decent PCBs. Of course, the top overlay is the hardest task.

i think its the same process as solder masking, ie cure green solder mask with uv + tranparency mask, when it cured and cleaned. then apply the white soldermask + silkscreen trasnparency, uv and cure again and wash out uncured part. this is demonstrated in the above dave's video, but... i'm waiting my white liquid mask for the experiment. if failed, i'll just use it as white solder mask. what i dont have and thinking right now is how to laser print negative polarity of silkscreen to transparency because the EDA i used dont have utility for negative polarity printing.

[M4trix]

@Mechatrommer, the problem is... I already bought this white UV ink from ebay ( the same stuff you're waiting as well) and made a mistake ! I thought, yeah no problem, I'll spread it with a rubber roller and wait till it dries. But it doesn't dry ! Ever !  I will try this drying method with an oven as sleemanj already mentioned but I think it's simply the wrong ink for this kind of application. Btw, does dry film soldermask exist in white color ?     

Edit: regarding the negative photo-tool, you can use this method ( jump to step 2 ) but I prefer Corel.

http://www.instructables.com/id/Killer-PCBs/

[sleemanj]

the wrong ink for this kind of application. Btw, does dry film soldermask exist in white color ?     

Yes, I believe so, but good luck finding any unless you are actually a factory using it.  It's been a while since lookng into it, but from memory there are a couple of dry film brands, one of them, Duramask is what you get on ebay from those handful of sellers, but it's only green.  There's another, I forget the name, which has a number of different ones. 

Trouble is, China doesn't use it I think, in china it's all liquid, so as a result, you won't find it for cheap on ebay or whatever.

The best method for silkscreen at home I believe, is, apart from an actual silkscreen of course, is white "toner reactive foil", you do a toner transfer of the silkscreen onto the pcb, then apply the toner reactive foil, it sticks tot he toner, and not to other stuff, you peel it off, and bish-bash-bosh you have a white silk.  Pulsar sells it (Pulsar WhiteTRF), and I think you can get it on eBay.  It's not so cheap.

White toner is available I believe for some printers if you wanted to just do toner=-transfer with that and no need for TRF then.  Or use some other colour.  Was there a thread here recently about that, or maybe it was on the Homebrew PCBs list, not sure.

Of course, nobody said your silkscreen has to be white, if your board is light-ish coloured, a black toner silkscreen can look OK.

[M4trix]

@sleemanj,

toner transfer method is a no go for me. Since my good old HP 6L has died ( RIP my friend ) I use inkjet printers.

[tooki]

Hi all,

I've a question regarding solder mask on home-made PCBs.
Of course I know that one would by proper dry-film solder mask and laminate it onto the board, but that has several disadvantages:
1) It's hard to get the material. (At least here in Switzerland or Germany)
2) You need a laminator
3) It's apparently really hard to avoid air bubles and make sure that the resist is properly laminated to the substrate between fine tracks.

1) Easy to get on eBay, faster (and not especially expensive)  to get at http://platinenshop.ch/loetstopplaminat.php
2) I got a used laminator (a good one, not a cheap trashy one) on ricardo.ch for 1 franc.
3) Ok, this I can't comment on.

[M4trix]

i'm waiting my white liquid mask for the experiment.

You got the ink ?

[Mechatrommer]

i'm waiting my white liquid mask for the experiment.

You got the ink ?

i got the ink few days ago, here the ebay china link.. http://www.ebay.com.my/itm/100g-White-PCB-UV-Curable-Solder-Mask-Repairing-Paint-/162099749004?hash=item25bde5e08c:g:Iw4AAOSwtJZXWuft only this noon i have time to try it, conclusion = this thing is difficult, even as advertised solder mask, even more difficult as silkscreen overlay mod, it doesnt stick to green soldermask, it sticks more to transparency (toner printed), so it peels off the pcb, i overexposed resulting solder pad got cured, i got partial crack partly stick to transparency partly on the pcb and i have to scrap and redo many times etc etc. i gave up and get back to my green soldermask. the green soldermask is less hassle compared to this china white mask, i can make single exposure and its done (small size pcb, i havent try bigger pcb). the green mask is "Mechanic" solder "mast"... http://www.ebay.com.my/itm/1PCS-10ml-Green-PCB-UV-Curable-Solder-Mask-Repairing-Paint-NEW-L-/262904516121?hash=item3d36548e19:g:CzcAAOSwuLZY0P6F proven its better quality than 100g china mask. or maybe i'm too noob i havent figure out how it should be done, i'll experiment more later if i have time... fwiw.

[M4trix]

Yep, this ink is for silk screening equipment. I did some experiments these days with the green ink. Tried with the rubber roller. It spreads evenly and you can easily control the thickness. Then I pre-dried it in a toaster oven with the temperature around 80-90ºC and it took more than 3 hours to dry (almost) ! Then passed the board with cellophane over it through the laminator (cold settings). This is an important part coz the ink will then not stick to the photo-tool and it makes the surface shiny smooth. The UV exposure was really short, 30 seconds ( 6 x F20W/350BL tubes ). Finally, cleaned the uncured ink with cloth dipped in Marabu UR-3 cleaner and then cured the board for about 60 minutes under UV light. What I noticed is that this ink is more harder when cured than Dynamask 5000 ! Anyways, I will ditch this UV ink and stick to the Dynamask. Too many trials and errors. Conclusions:

1. This is the worst pain in the arse method since I started making boards. 
2. Even with the roller you could end up with uneven surface. Probably an airbrush would do a better job.
3. Drying is a very critical phase. Too long, it will cure. Too short, it will smudge and make pools or spots of ink under cellophane if pressed or touched with a finger.             
4. This stuff will stick to almost everything ! It will not stick to cellophane, polyolefine foil and polyester.

Btw, I ordered this white 2 compound ink for the component overlay. It's the same ink the manufacturer used it in Dave's video. I will post pictures when this stuff arrives. Cheers.

https://www.alibaba.com/product-detail/LED-board-photoimageable-White-solder-mask_60634500133.html

[M4trix]

*bump

Just a quick update. Do not buy inks from this seller or buy it on your own risk ! After paying to this scam seller, you will not see
the ink nor your money. They even stop answering to your e-mails ! Fecking Alibaba scum... 

https://ythengxing.en.alibaba.com/?spm=a2700.8443308.0.0.JuN6u4