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Soldermask ink for homemade PCB's (yeh, the green stuff)

PostPosted: Sat Jun 04, 2011 7:18 pm
by erdabyz
I think we all hate how oxidized homemade PCB's get over time, and I also think we all love how easier it gets to solder SMD parts when you have a soldermask.

I myself tried to find a source for low quantities of soldermasking ink, better if photosensitive or UV curable, but I was unsuccesful. All I could find were buckets of the pro stuff for €€€,€€ in strange distributors.

Until I found this:
http://cgi.ebay.com/UV-Curable-Solder-M ... 2eb527251d

I found it like a year ago, and I thought I had to give it a try. Bough a syringe and... my life has changed since then. Once you know how to use this stuff properly, your homemade PCB's will never look the same.

It's a bit hard to learn how to use that stuff, however.

The recommended way to apply it is to squeeze some drops in the centre of the PCB, cover it with a plastic film and extend the product with a squeegee all over the board forming a thin layer. But there is a problem: this stuff sticks to nylon, acetate and polyamides, so you can't use transparency films with the photolite already printed on them, because you'll mess up the board, so I'll tell you how I apply it.

The first thing you must find is a sheet of any thin but still quite rigid plastic. It must be as thin as possible but it must NOT form wrinkles, and be as smooth as possible. And it also must not stick to the stuff. Seems easy to find such plastics, but it isn't. I get mine from paper sheet packages. When you buy 500 sheet packs, they usually come wrapped in a kind of plastic that is just perfect for this job. You have to remove the prints from the plastic, I do it cleaning the plastic sheet with common ethil alcohol, until it's totally clear and transparent. Don't worry, plastic sheets are reusable and you won't have to buy a new 500 sheet pack every time you want to make a board.

Then, I apply a few drops of the product to the board, I put the plastic sheet on top and I extend the product all over the board with a plastic squeegee . The layer of product must be thin. If it looks too green then it's probably too thick. Once I have all the board covered with the product as uniformly as I can (a sandwich of board, layer of product and plastic sheet), I place the transparency film with the mask printed on top of the stack, and I align it with the board. Then I sandwich everything between two layers of glass and insolate the board for about 30 minutes with a 8 watt blacklight tube. Once it's done, the stuff that has been exposed to UV light will be hardened and stuck to the board, while the stuff that was covered by the mask will still be liquid. I peel off the plastic sheet that shouldn't be stuck to the ink (the plastic sheet shoudn't have rests of hardened ink attached), and the liquid ink that remains in the board can be easily removed wiping it with a towel and some alcohol or something. Then I clean the plastic sheet (which will have some rests of liquid ink) so it can be reused.

The results are pretty amazing, achieving very very high resolution masks that are really useful to protect the boards and facilitate soldering. Usually the coating won't be as uniform as one would want, and the texture and appearance of this solder mask isn't the same of an industry applied mask, but does its job perfectly.

I'll post pictures of the process if requested, but here's a sample of some homemade boards I did with the stuff applied:

http://lulzimg.com/i22/47ea9d.jpg

(I took the same image I used for my presentation thread, obviously the OLS and the BP boards aren't homemade :P) I don't have other pics right now. The robot you see has three LGA or QFN -packaged IC's (a buck voltage regulator, a dual H-bridge and an electronic potentiometer) and the mask was perfect for them.

Re: Soldermask ink for homemade PCB's (yeh, the green stuff)

PostPosted: Sun Jun 05, 2011 3:57 am
by ian
Nice work, thanks for sharing. Please post some pictures! This would be a great wiki tutorial/blog post too!

Re: Soldermask ink for homemade PCB's (yeh, the green stuff)

PostPosted: Sun Jun 05, 2011 8:15 am
by erdabyz
Okay, i'll post pics the next time I make a board. You'll have to wait a bit because I'm on my final exams period and I can't play with boards in this moment, but I promise I will deliever.

Re: Soldermask ink for homemade PCB's (yeh, the green stuff)

PostPosted: Sun Jun 05, 2011 8:31 am
by arakis
I found this instruction with pictures from one ebay seller

Image
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Re: Soldermask ink for homemade PCB's (yeh, the green stuff)

PostPosted: Sun Jun 05, 2011 11:08 am
by erdabyz
Yeah, that's it. But what they don't say is that the stuff sticks to nylon, polyester, acetate and polyamides (learnt it the hard way) and most transparency films are made of any of these materials. Also, as transparency films are quite thick, it gets difficult to spread the paste correctly. So an intermediate layer other kind of plastic is required (say polyethilene or polipropylene film). There comes the paper sheet pack wrapping plastic trick ;). I screwed the first 2 or 3 boards I tried to soldermask because the stuff stuck to the transparency, and then screwed one or two more for not spreading the paste correctly. After that, all the PCB's have gone just fine. Don't worry, the PCB's I used for testing were small and cheap ;)

Re: Soldermask ink for homemade PCB's (yeh, the green stuff)

PostPosted: Tue Jun 07, 2011 12:57 pm
by metRo_
I just doesn't understand what stay in the pads, some soldermask? Or nothing?

I'm saying that because in the last picture i think i'm just seeing copper pads :s

Re: Soldermask ink for homemade PCB's (yeh, the green stuff)

PostPosted: Tue Jun 07, 2011 2:08 pm
by arhi
you need to add a fotoresistive foil with your solder mask on top of the green gue, after you ligt it up, the part that was lit hardens and the part where you had your mask stays gue so you can then clean the board and remove green gue from the board.

The green stuff is really mostly for presentation, if you want to really protect and "better" your home made pcb's use the chemical tinning compound to apply layer of solder to your copper. It will protect your copper + it will strengthen it and make soldering easier. You can find some info here: http://www.aliatron.pt/download/Sur-Tin_manual.pdf .. also multi compound chemical tinner are better then ready made stuff as they all degrade in time so with multi compound (usually 3 parts) you can make smaller batches as you need them.

http://www.youtube.com/watch?v=MGbxrRmP-Zs


or if you are american then you get something called "liquid tin" from MG Chemicals
http://www.youtube.com/watch?v=xnKRndxAo70&NR=1

Re: Soldermask ink for homemade PCB's (yeh, the green stuff)

PostPosted: Tue Jun 07, 2011 2:49 pm
by metRo_
Off-topic: the first link you post is from s shop in my country :)

Hmmm, ok. I tought that will make easy the solder process too.

Re: Soldermask ink for homemade PCB's (yeh, the green stuff)

PostPosted: Tue Jun 07, 2011 4:32 pm
by erdabyz
arhi wrote:you need to add a fotoresistive foil with your solder mask on top of the green gue, after you ligt it up, the part that was lit hardens and the part where you had your mask stays gue so you can then clean the board and remove green gue from the board.

The green stuff is really mostly for presentation, if you want to really protect and "better" your home made pcb's use the chemical tinning compound to apply layer of solder to your copper. It will protect your copper + it will strengthen it and make soldering easier. You can find some info here: http://www.aliatron.pt/download/Sur-Tin_manual.pdf .. also multi compound chemical tinner are better then ready made stuff as they all degrade in time so with multi compound (usually 3 parts) you can make smaller batches as you need them.

http://www.youtube.com/watch?v=MGbxrRmP-Zs


or if you are american then you get something called "liquid tin" from MG Chemicals
http://www.youtube.com/watch?v=xnKRndxAo70&NR=1


No, it's not only for presentation. In fact, the aesthetical part isn't important for me. The green stuff protects the board from oxidation and corrosion, and it's a really good protection. It's the same protection that industry uses for PCB's. And also, the biggest advantage you get with soldermask is that soldering gets much easier, specially for SMD's, as solder doesn't spread everywhere and gets confined to the pads, making things much easier and reliable. And I must say that this stuff is kinda "fragile" right after you apply it (with fragile I mean easy to scratch), but as it absorbs more and more light it gets harder and harder, and after a day or two it provides a very very good protection to the board.

I was trying to find chemical tinning compounds for a while, but they aren't easily available in Spain, at least not in small quantities. I wanted them for finishing the pads, but chemical tining isn't as good as people tends to think. It has some disadvantages. Tin melts at a higher temperature than solder, so the solder joint is no longer between copper and solder alloy, but between solder alloy and tin, which is chemically bonded to copper. Surely diffusion occurs and the intermetallic layer eventually diffuses through the solder joint blending with the alloy, but metallurgically speaking it isn't the best bond you can get.

That's something similar to what happens with electroplated gold finishes. As you can't directly plate gold to copper because it quickly diffuses, an intermediate barrier layer of nickel must be applied, so finally, what you get are solder joints between soldering alloy and nickel (gold diffuses almost instantly), and that nickel is plated to copper, and that's a weaker bonding.

Anyway, I'll post pics and document the process when I make my next board, which will be as soon as I finish my final exams.

Re: Soldermask ink for homemade PCB's (yeh, the green stuff)

PostPosted: Tue Jun 07, 2011 5:12 pm
by arakis
nvm

Re: Soldermask ink for homemade PCB's (yeh, the green stuff)

PostPosted: Wed Jun 08, 2011 10:20 am
by arhi
I used lot lack many times and yes it does protect the board... and yes tin does chemically bond to the copper but that bond is as strong (and as low resistant) as the solder to copper so it really makes no difference that you have a solder-tin-copper "sanwich", not to mention that you have more then 50% of tin in your soldering wire. If you are doing lead-free soldering your solder wire will melt at roughly the same temp as the tin coating the copper etc etc ... so that story don't really stand ...

As for protection, the tin is protecting the copper as well as the lot lack, is much cheaper and easier to apply. Does not look as nice but who cares :D (unless you are selling the boards but for that you'd like to have metalized holes too). I have boards that were outside exposed to elements (only protected from direct snow/rain but in open wooden box outside) for 10+ years that still work :D and there are no issues with the traces ...

Don't get me wrong, I don't say "don't use lot lack", but chemical tin is cheaper+faster+simpler protection + I assume you already did tinned the pcb before you put lot lack over it so when you do that lot lack is really only visual protection ... but if you don't do chemical tinning (beats me why) then yes lot lack gives you some protection ....

As for "not having chemical tinner in Spain" it is hard to believe that, I live in piece of sht god forbidden backwards country where I can't get most of the stuff I need, and I have 2 types of chemical tinning solutions available (ready made and 3component one)... You just need to check proper shops :)

Re: Soldermask ink for homemade PCB's (yeh, the green stuff)

PostPosted: Wed Jun 08, 2011 10:59 am
by sqkybeaver
i have tried conformal spray with vinyl masks and is too time consuming.

i like the the solder mask ink idea. ill have to try it when i finish my uv-BOX!

Re: Soldermask ink for homemade PCB's (yeh, the green stuff)

PostPosted: Wed Jun 08, 2011 11:26 am
by erdabyz
No, I do not chemically tin any of my boards. They are "just" protected by the green paste. I don't know why, but I can't find chemical tinners in Spain in low quantities (yeh, checked pro stores, I live 100 meters away from the pro'st electronics store of Madrid), and what is available is quite expensive. Much more expensive than the green paste for the same results. A syringe of the green paste lasts a lot of time, you'll spend about 1/10th - 1/8th of the syringe to coat a double layer 100x160mm board. The layer required is very very thin, and 10ml's will last quite a bit. I'm starting my second syringe, and my first one lasted a bit more than a year.

The thing about the chemical tinning metallurgy isn't made up, it's a real problem. Sure it's a bit paranoid and most times it won't be a real problem at all, but happens. it was just written for informative pourposes. Another problem that might happen with high frequency signals is losses due to skin effect, because tin doesn't conduct electricity as good as copper. Again, paranoid but it happens :P

I honestly think that protection provided by soldermask is better than protection provided by chemical tining. In fact, industrially made PCB's aren't chemically tinned (they actually use a chemical tin coat as etchant resist, but it's then removed, and the soldermask is directly applied over bare copper, and then the uncovered areas are finished by HASL, ENIG or other coatings). It doesn't only protect against oxidation and corrosion, but it also provides mechanical protection against scratches, and it facilitates soldering a lot, as your solder stays confined to the pads and doesn't spread everywhere, and it's a "must" for QFN's, LGA's or other packages like those.

Re: Soldermask ink for homemade PCB's (yeh, the green stuff)

PostPosted: Wed Jun 08, 2011 1:00 pm
by arhi
well the pro made pcb's get another layer of copper on top of what is originally there so they are different story :) ... but yes, if you can't get chemical tinner locally lot lack is the best solution. (locally ingredients to make 2.5L of chemical that last looooooooooooong cost ~30E)

Re: Soldermask ink for homemade PCB's (yeh, the green stuff)

PostPosted: Wed Jun 08, 2011 1:08 pm
by erdabyz
Yeah, pro made PCB's start with a very very thin layer of copper, and then they electroplate another layer on top, which also electroplates the holes, but the total thickness of copper is most times 35um, the same thickness of copper that homemade PCB's have. Here it costs 20 € for the same amount of tinner (I consider 2.5l a large quantity), that is quite expensive for me, considering that the shelf lifes of these products are very reduced, and that you have to make new stuff every time you want to make a new PCB. As most of the PCB's I do are small...it is expensive.