OK!
For silk screening first thing you need is Silkscreen. Use some intended for textile (fabric) printing because it has biger holes which we need because we want to use thicker PVC based color. Prepare print of your PCB on film for use on laser printers or (worse) kinde of paper used for drawing plans and technical drawings (white, yellow or greenish, semitransparent paper), sorry I don't know name for it in english. You first spreed thin coat of photo sensitive emulsion (commercial one) on bothe sides of silkscreen (done in dark). Then you dry it with hair dryer. When dry it's ready for next step. You take your positiv printed on foil and put it over silk and expose it to light something like 500/1000 W. at 40 cm distance. You acctualy need to know how much of light and for wath time which deppends of kinde of emulsion. After it is developed you just remove undeveloped thing (your PCB traces) with water under presure (I use car washing compressor). You dry it again with hair dryer and voila, it's ready. Then you print it with PVC color and after it drys it's ready for etching. As I said I use H2O2 30% and 12/15% HCl. For more infos see sites about silc screening on T-shirts and similar. I hope this helps!!!

__________________
Aude, Vide, Tace

 

P.S. you need first to clean copper and to be sure your boards are clean and dry. PVC layer can fall of if not clean and/or dry.

__________________
Aude, Vide, Tace

 

thanks, ill make sure i clean them properly in future.

The spinning idea sounds good.

I have been experimenting and have found out the following things:


SPRAYING

Vacuum out your spraying box as suggested on the website (http://web.telia.com/~u85920178/data/pcb2.htm).

Instead of fighting gravity, it seems that spraying the stuff on verically works nicely.
Stick the board to one of the walls of your box, and then spray.
Graivty pulls the laquer downwards, and ensures an even coating (appart from the bottom cm, which is too thick, but just make your pcb 1cm longer than it needs).

Spraying vertically you can use less spray as you can spray closer to the board, and waste less on the surrounding paint box.



CURING
80C for 40 minutes seems to work very nicely. Any hotter and the stuff gets cooked onto the board and refuses to devlop.

DEVELOPING

2.5 Teaspoons of caustic soda into 1 litre of water.

__________________
Chris

 

Sounds about right.

That commercial board is the stuff i bought. Its really good, and so much better than spraying your own (if you can afford it!).
If i was rich, i woudnt bother spraying, i would just use the precoated stuff.

__________________
Chris

 

Most of my boards are small in size even if it was large around 8x10in i would still use just 2 4x5 boards. Saying that i would recommend buy this in bulk like :

PHOTOETCH PCB SS 100 X 160 (RC) - 1.99 x 50 = $100 (add shipping = tax)

but you get a nice large supply and save money at the same time? I guess just having a safe place to keep them all would worry me

 

Thanks. We uses a similar process back in the Z80 days. The ink was a special PCB ink. It is a good process for production runs. But it is more difficult then either photo resist or toner transfer. Unless a person has previous silk screen experience the chance of success is very very low. And it is a lot of work for one board.

The only reason I asked twice was because the people having trouble with either toner or photo are looking for another process when they fail with the current one. Suggesting that silkscreen could be used does not help them.

Sharing is a good thing. But it needs to be in context.

__________________
search engine for electronic parts
Junebug USB PIC programmer kit., USB Bit Wacker, 3v0's Homepage
The 15 Minute Printed Circuit Board! (+drill time)

 

No reason to worry! Keep the PCBs in a fairly dark and dry place, e.g. clothes locker, and use a black plastic bag for additional light protection. The boards are well usable even after two years of storage. Stored a longer period than that they are still usable; they just require a slightly longer exposure time (from 240 seconds to 270 seconds with UVA incandescent lamps)

 

From Rapid online that would be £100 or ~$200! I could not warrant spending £100 just on boards.

I think that using the method i described above and spraying your own is likely to be the most cost effective solution.

__________________
Chris

 

Let me trace your 1st post back,. You already have the spray with you. Did you try little more diluted solution of the developer to prevent the stripping effect by trial and error by using few 1" size layout. this will confirm at what concentration it works for the spray used by you.

Already you have received suggestions that you might need to clean the board to prevent speckling. Hope to see the results of a fresh trial.

__________________
Regards,
Sarma.

 

It has been a long time since I used the spray resist but I want to make a few comments regrading some of what has been mentioned.

I am thinking that when you spray the board you create a wet film on the board the must be dried.

If you spray with the board vertical the coating will sag or at least be heavier on the bottom.

If you spin the board the coating will be thicker on the edges.

Most spray processes have an optimal distance for applying the coating. If you move in closer to save on the spray you may get bad results too. Follow the directions for distance.

The things that have been mentioned that should help are:
A super clean board and spray chamber.
Perhaps better longer drying.
Work on your spray technique.

There is also the possibility that the spray is old and will not work regardless of what you do. Check with the maker to see if that can happen.

__________________
search engine for electronic parts
Junebug USB PIC programmer kit., USB Bit Wacker, 3v0's Homepage
The 15 Minute Printed Circuit Board! (+drill time)

 

mvs sarma,
I have perfected my technique now, and the washing thoroughly has helped
Also it wasnt that the developer was too strong, it was because my drying regime wasnt sufficient.


This is true, as i mentioned in my post above. However if you allow a cm at the bottom of the board, the surpless resist will build up in this cm, but will leave a perfect coating on the rest of the board. I have tried this a few times now, and the results are almost as good as the commerical stuff.


I disagree! Maybe if you are spraying horizontally, but vertically, gravity does a fine job no matter how close you spray.

Even though you waste a cm of board and spray, you actually use less spray, becuase you can spray almost a few mm away from your target. It doesnt matter about consistancy, gravity will pull the stuff down the board and leave a lovely coating instead of spraying loads on the box, becuase you have to spray 10-15 cm away.

the vertical method also ensures that only the thinnest of coating is left. Its also consistent all the way down the board (appart from that cm). This way, when you develop the board, its like using a commercially sprayed product, The resist gets eaten away consistantly.

Why perfect ones spraying technique, when you can be a lazy sod and let gravity perfect the coating?

__________________
Chris

 

A few years ago I called a manufacturer (Injectorall) and he made a comment about having just dipped some plates. If you can get resist in bulk (not a spray can), you may want to give dipping a try. I have used dipping with regular paint, and the only tricky part was to get it thin enough (about usual spraying consistency), but not so thin as to cause the film properties to break down.

I used a re-sealable plastic food storage bag for the painting -- put the paint and object in the bag, seal it, mix everything around gently, let it drain and the few bubbles settle, then remove. The bag gives you a saturated atmosphere so the paint doesn't start to evaporate during the process and allows use of very little paint. I was using primer and wanted to get the inside of some hollow steel doors primed. It was a non-sandable primer, and the finish coat looked fine. Of course, there is no over-spray and the paint in the bag can be saved and re-used. John

 

Chris,

It is good to hear you have developed a process that works. After you have perfected it, it would be a good thing to write it up in detail for others to use.

I am surprised the only the bottom cm of the board is bad. It sounds like a certain amount of spray will stick to a vertical board and any excess slips down and collects at the bottom, neat stuff. I agree regarding spray distance not mattering in this case.

__________________
search engine for electronic parts
Junebug USB PIC programmer kit., USB Bit Wacker, 3v0's Homepage
The 15 Minute Printed Circuit Board! (+drill time)

 

Thanks for your encouragement.

As soon as I get my hands on a digital camera, i will write a tutorial for the method, and post it here.

Im suprised that no one else seems to use this method, i havent seen any websites.

With regards to the bad cm, im sure that there is a solution to this minor problem. Perhaps I will have sorted this when I get round to posting the tut.
Maybe use a fan? The only problem with this is that you would have to make extremely sure that all dust was removed from the spraying box, and possible even the entire workshop. At the moment i seem to be able to get away with just vacuuming the box.

The real beauty of this method though, is that anyone of a non artistic disposition (like me) can end up with an almost premium quality coating, at a fraction of the cost.
Paying over £2 (more than twice the cost of the board itself!) just for the privelledge of a machine spraying the board, leaves a bad taste in my mouth .

__________________
Chris

 

I like the sound of that, though I bet it would cost a lot of money!

200ml spray = £10 (http://www.rapidonline.com/productin...moduleno=72547)

Even though it seems expensive, its nothing compared to the cost of the commericially sprayed boards. You can buy the cannister for the cost of about 3 160x100 SS pre-sprayed boards! 200ml would easily do many more than 3 boards, boards that cost less than a £ each.

__________________
Chris