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Developing and etching PCBs

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yorkie
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Username: yorkie

Post Number: 16
Registered: 09-2010

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Posted on Thursday, 12 January, 2012 - 05:58 pm:   Edit Post Delete Post Print Post    Move Post (Moderator/Admin Only)

I've been making my own PCBs for many years, using a home-brew UV light box. Most of what I've done has been to copy magazine artwork rather than to design my own - I'm afraid that I'm not vey far up the food chain when it comes to using PCB design software!

The following notes on my own experiences might be of interest - I'm not saying it's the best way or the only way for a hobbyist constructor - just what works for me.

I use UV aerosol lacquer, which I spray onto plain board rather then to buy pre-sensitised boards. That way, if somthing goes wrong - under exposure or too strong a developer mix, I can wipe the board clean with meths and respray it with lacquer - the only loss is my time, and it's a hobby anyway, so that's no big deal. I spray the plain board in as dust-free an area as I can, which for me, is the greenhouse! I cover it with a plastic box right away to keep dust from settling on the board, then leave it in a desk drawer overnight to harden off.

For masks of artwork, I use acetate sheet printed on an ink jet printer. The film that I use, which I've found to be economical and with excellent opacity of the printed tracks, is 'Q Connect Universal Inkjet OHP film'. It has excellent ink reception, quick drying times and is coated on one side. Fifty sheets presently work out at just 30p each. I try to print off several PCB designs on one sheet by copying and pasting them in 'Publisher', and moving them around on the page to make best use of the film. I get the film from here:

http://www.amazon.co.uk/INKJET-CLEAR-PRINTER-ACETATE-SHEETS/dp/B000KJO7BO

Price: £15.12 delivered FREE in the UK (RRP £69.00!)

Though the ink has excellent density on the film, to make sure, I print off two identical copies of the artwork onto one sheet, then sellotape them together to improve the density. It’s probably not necessary, but I might as well do it, or I end up throwing the rest of the sheet away anyway unless there are other layouts on the sheet. If I'm careful in placing the film in the printer tray, as an alternativr to using two masks taped on top of each other, I can run the same film through the printer twice and get it to print the artwork twice by overprinting the first 'run'.

Over the years, I've tried various chemicals as a developer of exposed PCBs. I've had little success using a weak solution of caustic soda in getting the right level of dilution - I've always found it either too weak, or too strong. Legend has it that it should be just strong enough to feel 'soapy' when the forefinger and thumb is dipped in it and rubbed together. I hasten to add that I'm not recommending this method! Ride at your own risk!! Nowadays, for developing exposed PCBs I use sodium metasilicate ('metso'). You can buy 1kG packs of lab grade sodium metasilicate pentahydrate developer for £10.56 + £6.99 at this e-bay link:

http://cgi.ebay.co.uk/1Kg-Sodium-Metasilicate-Metso-Laboratory-Grade-/230315371046?pt=UK_BOI_Medical_Lab_Equipment_Lab_Supplies_ET&hash=item359fddaa26

As with other developers, you need to experiment to get the concentration right – I start with it weak, then add a few more crystals if it doesn’t dissolve the developed lacquer on the board. Too strong, and all of the lacquer will dissolve, but then little is lost but your time – as I said earlier, all I do if this happens is to re-spray the PCB, expose it again, then try it again in a weaker etchant solution. It’s all a bit hit and miss – some I win – some I don’t! I save the developer in a bottle to use again. It lasts a few weeks, but becomes so opaque that I can’t see the PCB at the bottom of the container. I rock the container (just a plastic lunch box or whatever) gently back and forth to agitate it until I start to see the dissolved lacquer floating off the surface after perhaps 30 seconds or so. Then as soon as the board is fuly developed, I straight away put it in clean water to halt the process.

Etchant:

Ferric Chloride seems to be the most commonly used etchant, which I used for more than thirty years, but I gave up using that some time ago as it's so messy that it was almost grounds for divorce! Instead, for some time I've used sodium persulphate, which I find is a much cleaner and effective alternative to ferric chloride, and cheaper too! It makes a clear liquid, which has a 6 – 8 minute etch time at 45C – 50C. The bath life is a maximum of 4 – 6 weeks depending on use and operating temperature. As the PCB etches, the liquid turns light blue. You can use it several times over. For etching the board, I use an aquarium heater to warm up the etchant, and an aquarium air stone about 4” long, with a small aquarium air pump, placed in the bottom of a plastic cereal container to ‘bubble’ the solution to speed up the etching process. I suspend the PCB in a piece of plactic cut from one of those plastic mesh 'sink drainer' jobbies. I cut it to fit the cereal container and bend it in a 'U' shape then push it into the container. If I want to monitor the etching progress, I can just lift out the plastic mech, and if the PCB isn't fully etched, push the mesh back into the container, together with the PCB.

You can buy a 500 gram HDPE bottle of Sodium Persulphate copper etchant, supplied as a fine crystalline white powder, which will make up to 2.5 litres of etching solution, from the link below for £3.50+£2.24 P&P. (The instructions for use are printed on the bottle label:

http://cgi.ebay.co.uk/ws/eBayISAPI.dll?ViewItem&item=270746775207&ssPageName=STRK:MEWNX:IT&_trksid=p3984.m1439.l2649

I hope that all of this makes sense, and is of interest and use to someone out there. I know that there are other methods of PCB production, such as using a laser printer to 'iron on' artwork printed on glossy paper, then soaking the paper off, whic obviated the UV exposure stages.

For anyone wishing to use UV there's an excellent project for a UV exposure unit on Joe's blog, using 99 UV LEDs - a much cheaper alternative than a UV exposure box using UV tubes, and mains voltage:

http://www.hobbyelectronics.net/con_uv-exposure-unit.html

What's not to like about that!!?

Have fun!

David
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jgurley
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Username: jgurley

Post Number: 11
Registered: 12-2011

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Posted on Thursday, 12 January, 2012 - 07:48 pm:   Edit Post Delete Post Print Post    Move Post (Moderator/Admin Only)

I've often thought of getting into home brew PCBs, but I currently get prototypes from Sparkfun's BatchPCB resource, and two layer boards with solder mask and silkscreen are only $2.50/sq inch plus $10 setup in less than two weeks. Hard to beat when you consider the chemicals, angry wife, etc. I'm on a septic system so the chemicals are especially problematic!

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