Author Topic: PCB solder mask  (Read 529 times)

Stonemull

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PCB solder mask
« on: February 23, 2018, 02:33:02 AM »
UV curable solder mask, dab a little on the pcb, squish with transparent film and smooth out with a bearing. Use T2, import solder mask gerber and then invert, need a negative.
Tried a few exposures using my plotter with the 405nm 50mW laser and the 2.5W laser.
Speeds down to 1000mm/min and power 100 on the higher powered laser, no curing occured.
On the 405nm at full power, 5000mm/min left me darker patches over the pads (where there shoukd be no mask) however even acetone would not remove resist, increased to 10000mm/min and had first success, excess removed with isopropyl alcohol, acetone still too violent and removed exposed material easily.
So somewhere between 5000-10000 mm/min at 50mW 405nm will be the magic spot.
Will toughen it up with further exposure to UV later.

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Zax

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Re: PCB solder mask
« Reply #1 on: February 23, 2018, 06:36:42 AM »
That's pretty cool, do you need to do a bake after removal to ensure it's set or is the laser sufficient?

Stonemull

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Re: PCB solder mask
« Reply #2 on: February 23, 2018, 07:14:07 AM »
will zap it with extra UV afterwards, either leave it in the sun or hit it with my mercury lamp. just been wiping it off and trying again so far, I did have to use a paint scraper at one point, even steel wool wasnt getting it off.
The laser is not fully hardening it, however the pads were not clearing if I zap it more.
It says to use petroleum or petroleum based solvent to use to wipe it off, hence trying isopropyl, dry cleaning fluid and acetone so far..
Some people use thinners.
Still experimenting a bit. its only a little over $2 delivered for a tube of this stuff so worth an effort.

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ianchia

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Re: PCB solder mask
« Reply #3 on: February 23, 2018, 03:22:18 PM »
Nice! Are you experimenting with the intent of using it as a proper soldering mask or etching mask?

Stonemull

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Re: PCB solder mask
« Reply #4 on: February 24, 2018, 07:14:13 AM »
Solder mask, would help with 0.5mm pitch components.
Etched a board last night and spent the last few hours trying to get it right, no luck so far.
Mainly due to lining up the mask with the pcb on the laser, you have to add some lines on the mask layer at 0,0 and the diagonal corner to assist in alignment as T2 will trim the gerber to the pad extents if you dont, the 0,0 is no longer where you expect it to be.
Despite the misalignment I still let it expose and "developed" it, I got it partially right only once.
I think the issue is that the laser is on most of time and scattering from the lens is enough to partially set the resist.
My first attempts were at 0.05mm resolution, at 6000 mm/min the resist falls off, at 5000 mm/min .. none of it comes off unless you use acetone, then it all comes off.
Dropped to 0.1mm resolution to reduce the time spent over the board, still nogo, photo shows the best i managed it so far, simply not enough setting of the UV resist, I think I need a different wavelength, all I can find so far is it should be 350-400nm.
UV sunlamp set the resist hard in a few minutes though, I may have to use a film technique which I do not want to do or I might as well use it for the whole method.

Only a protype design so I did not really want resist on this one as it will have wires tacked all over it, also big pitch an no need for it, but good to experiment on, now I am depressed :(

« Last Edit: February 24, 2018, 07:17:55 AM by Stonemull »
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ianchia

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Re: PCB solder mask
« Reply #5 on: February 24, 2018, 04:01:56 PM »
Have you considered this technique of laser engraving kapton tape as a soldering mask? Not sure if it'll work with these lower powered lasers, but it's worth a shot. Unless that's what you meant by:
Quote
I may have to use a film technique which I do not want to do

https://hackaday.io/project/1554-making-a-solder-mask-using-kapton-tape/details

- Ian
« Last Edit: February 24, 2018, 04:05:14 PM by ianchia »

Stonemull

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Re: PCB solder mask
« Reply #6 on: February 24, 2018, 06:19:07 PM »
I have tried kapton to use as a solder paste stencil and the 2.5W does cut it, it had some issues that I can't remember off hand, being orange it absorbs the energy just fine. I think I needed to do more research, are nuked polyamide fumes dangerous ?
I was doing it in situ though on the pcb and the power needed burnt the fibreglass too, I then found that black insulation tape gave better performance with cleaner edges .. apart from being vinyl with nasty fumes.
I might revisit it with a mask in mind.
The main issue is that masks are larger than the pads, stencils are same size or smaller, so you often end up with only a few microns of material between pads on the mask with smaller pitch devices.
I will update after more researching..

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ianchia

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Re: PCB solder mask
« Reply #7 on: February 24, 2018, 10:04:04 PM »
so you often end up with only a few microns of material between pads on the mask

Ouch. That is a tricky issue. Good luck! I'll be interested to hear more of your experiments.

- Ian

Stonemull

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Re: PCB solder mask
« Reply #8 on: February 27, 2018, 03:58:09 AM »
I finally got around to assembling the board from a few days ago and this mask stuff is sooo unforgiving!
It needs a far more thorough stripping than I gave or the 'clean' copper is basically unsolderable. It looked clean but it must have had microscopic film or blobs of resist and about the only thing that helped was scratching away with a stanley knife.
That means the resist has to be exposed well to allow a good cleanup, something that will be impossible with the 2 wavelengths I have tried so far.
Maybe it would be possible if the laser had no side flare, as it is off when travelling over the areas to be unexposed, however it still partly exposes, enough to be the major issue. So it must just be speckle off the lens surface .. perhaps dropping the lens down really low ? I need a point source .. might be worth finding a more precision lens.
The other thing that might help is just using a much thinner layer of mask, clutching at straws here :(
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Stonemull

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Re: PCB solder mask
« Reply #9 on: April 24, 2018, 08:33:06 AM »
Finally some success, no luck at all using the laser to expose it but I can sure nuke it, so there are probably better masks out there like high temperature paint maybe since the UV cure side of things is useless (unless you want to make a film and expose it under a UV source as designed that is).

So I ended up with 0.05mm resolution, unidirectional, 400mm/min, laser 200

First image is straight from the laser, second after giving it a brush down with steel wool and the third after attacking it with solder.
Solder is still a bit hesitant on the surface, so some residue remaining still but not a great deal.
It also took 15 minutes to burn .. and thats just a 12mmx 12mm region so doing the entire pcb like this would be a pain, I dont need a mask on the entire pcb though, it is just for those annoying little 0.5mm pitch things and SOIC devices like this (pads mostly underneath) that need solder paste.
Question now is that the mask layer is larger than the pads, so if I lasered this over the top of an IC would it burn the fibreglass all around the pad.
Might need more tuning yet.


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