Cheapo DIY UV Exposure Unit for PCBs

[Yorkie]

Many hobbyists who would like to be able to make their own printed circuit boards are put off by the expense of the equipment, not least a UV exposure unit - even basic units consisting of two 8 Watt tubes cost well over £100. There have been some ‘home-brew’ excellent designs that have appeared in magazines such as Every Practical Electronics, but they’re invariably mains powered and fairly sophisticated designs. I made one such unit about 25 years ago and have used it many times since, but the average constructor only wishing to make the occasional board is likely to find that the work involved and the expense, is off-putting.

Fear not – there’s an excellent design by a member of the EPE ‘Chatzone’ of which I’m a member, which might interest members of this forum. The photo resist layer of either ready coated PCB material or aerosol self-spray lacquer is very sensitive to UV light in the 340nm to 420nm wave lengths. Traditional UV exposure units use special mains powered UV tubes that emit UV light at a wavelength of around 350nm. The exposure unit that I’m about to describe takes a different approach – it uses 100 UV LEDs. It doesn’t need mains voltages and can be built for around £15 if you do some shopping around for the parts.

UV LEDs have become very inexpensive, and are now available from several sellers on EBay for a fraction of the cost from more traditional electronic component outlets. By ‘inexpensive’ I mean £3.17 for 100 with free P&P! (From China – where else?). Sure, you’ll have to wait a few weeks for them to arrive, but just chill - what’s the rush?

UV LEDS from: e-bay; Item number 280834124496 (China)

http://cgi.ebay.co.uk/ws/eBayISAPI.dll?ViewItem&item=280834124496&ru=http%3A%2F%2Fsearch.ebay.co.uk%3A80%2Fsearch%2Fsearch.dll%3FGetResult%26fvi%3D1

You’ll need 50 x 56 Ohm 0.25w Carbon Resistors, which are cheap anywhere and can be had from this link, 50 for £0.99 + 89 Pence p&p: e-bay item number 260323928007 (UK):

http://www.ebay.co.uk/itm/260323928007?r...26_fvi%3D1&_rdc=1

The only other item you need is a piece of stripboard of the required size. This looks about the right size, at £4.75, first class same day post free:

http://www.ebay.co.uk/itm/Copper-Prototy...etTopBoxes&hash=item2a1899a8ee

Coincidentally, the designer’s name is Joe and the project can be found on his webpage – ‘Joe’s Hobby Electronics’ If any one interested in building Joe's project, it’s a bargain. Total cost? Under a tenner! If you go to this link, you’ll see full constructional details of how to build this very cheap but excellent UK exposure unit:

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

The spec of the Super Bright UV LEDs at the link above is:

Super Bright High Quality 5mm Purple/UV leds
* Emitted Colour : UV
* Size (mm) : 5mm
* Lens Colour : Water Clear
* Peak Wave Length (nm) : 395~410nm
* Forward Voltage (V) : 3.2 ~ 3.6V
* Reverse Current (uA) : <=30
* Luminous Intensity Typ Iv (mcd) :
Average in 2000
* Life Rating : 100,000 Hours
* Viewing Angle : 20 ~ 25 Degree
* Absolute Maximum Ratings (Ta=25°C)
* Max Power Dissipation : 80mw
* Max Continuous Forward Current : 30mA
* Max Peak Forward Current : 75mA
* Reverse Voltage : 5~6V
* Lead Soldering Temperature : 240°C (<5Sec)
* Operating Temperature Range : -25°C ~ +85°C
* Preservative Temperature Range : -30°C ~ +100°C

I’ve mentioned on the forum before where I get other materials from – I’ve long since stopped using ferric chloride as an etchant and have opted Sodium Persulphate copper etchant. This is a much cleaner effective alternative to ferric chloride which has a 6 – 8 minute etch time at 45C – 50C. It comes as white crystals and makes a clear liquid which turns light blue as it etches the board. It lasts several weeks and can be re-used. It has the advantage that you can see the etching progress through the etchant. I spray the boards myself with UV lacquer - not only is it cheaper than pre-sensitised PCB, if the exposure doesn't work out right, all you have to do is wipe the self-sprayed board clean with meths, re-spray with lacquer and start again, whereas with pre-sensitised boards, if it fails to turn out right all you're left with risks being a plain piece of PCB.

If anyone wishes to make Joe’s exposure unit and would like advice on where to source chemicals and how to go about making PCBs, please just ask, and I’ll say what works for me.

Hope that's of interest.

[Radio Fixer]

I keep saying that I will do some proper PCBs for projects but too many other things stop me getting around to it. I usuallu knock up on vero board or use a marking pen and ecth for simple crude boards. So I'm interested in this David and will copy it for later.

Gary

[Joe]

There's obviously more to his PCB lark than I thought. I thought all you did was paint the tracks on the copper side and dunk it in ferric chloride.

I have enough bother following diagrams to build things on Vero, or floorboards , and think PCB design would be well beyond me.

How about one of your 'Numpties' Guides,' David?

- Joe

[Yorkie]

You can indeed paint tracks and dunk it in ferric chloride, but since you asked Joe, for what it’s worth here are my own comments on using UV techniques based on experience gained over many years using a home-made UV light box with four 12” 8W tubes and a built-in timer.

I'm not suggesting that anyone wade through this diatribe, but it might be useful for reference. (No numpties on this forum BTW - only class acts! Kwality!!) Nor am I saying that my ways are the right ways, I’m just saying what works for me, and sharing my experiences with others, which I hope may be of interest. (You can expose a PCB to UV simply by rays from the sun by placing the PCB and mask on a window sill or in a greenhouse, but it takes much longer).

Back in the 1970s before we had computers and home printers, and before I made a UV light box, as most of the projects I made were from magazine articles I used to take a photocopy of the artwork, sellotape it over a piece of blank PCB, then centre punch through the paper to mark the PCB at all the points where components were to be fitted. I then removed the paper and used it a pattern to ‘join the dots’ on the PCB using rub down transfers directly onto the PCB. I then etched the PCB in ferric chloride, then drilled all the holes. You must not drill any holes until after the board is etched or the etchant will start to etch away copper around the holes and will also impregnate the holes with etchant which risks corroding wires of components that are later fitted in the holes and soldered to the PCB, however well you think you’ve cleaned the board after etching. Occasionally I’d design my own PCBs on paper then use rub down transfers to copy that design onto plain PCB, the after etching, drill the holes. This rub-down transfer technique was time consuming and if you wanted another board, you had to repeat the process. I never found ‘Dalo’ pens a success – the board always ended up threadbare and offensive to the eye.

To make PCBs using UV techniques, admittedly there are some setup costs but by far and away the most expensive has been a UV box. The splendid UV LED design I mentioned earlier in this thread overcomes that problem – what’s not to like about a UV exposure unit for under a tenner?

Basically, the UV technique is to place a mask of the artwork for the PCB layout on a sheet of glass above the UV tubes, place the PCB on the mask held tight to the mask, then expose the PCB to UV light for several minutes. All of the areas of lacquer which aren’t obscured by the layout on the mask will – when the PCB is placed in developer, dissolve off the PCB leaving the layout. The exposure period isn’t critical but can only be found by trial and error. For my set-up, it usually takes about five minutes. The way to ascertain the correct exposure time is to use a test piece of coated PCB say five inches long, with four inches covered and exposed for one minute, then two inches exposed, then three inches, two inches and finally one inch, all at one minute intervals. The strip of PCB is then placed in developer to see which period of time has correctly exposed the lacquer to UV. EG, if all of the lacquer which has been exposed longer than two minutes dissolves, then three minutes will be adequate, but if the only area that dissolves in the developer is that which has been exposed for five minutes, then with that UV exposure unit and the lacquer that you’ve used, it will take five minutes – maybe longer for a large PCB. You don’t actually see any visible signs on the PCB at the end of the exposure – it’s only at the developing stage that the design become visible. (Hopefully!).

Making a mask:

I’m not adept at designing my own PCB layouts - I generally scan magazine artwork or designs from websites, then clean it up in Photoshop to improve track densities if need be. Often, the layout looks fine until it is enlarged and examined, when it is sometimes full of holes due to the limitations of the printing, and there’s no point in starting off with a layout that will look threadbare under UV light, so I make the threadbare areas opaque. Occasionally, I use Photoshop or MS Paint programs to modify the layout to suit my needs. For example, the PCB layout for the ESR meter project that I made has been much modified by me over time from the original layout.

I use an ink-jet printer to print the mask onto good quality universal inkjet transparency film. The type I use has an RRP of £69.00 for 50 A4 sheets but I get it from a reputable source for £14.52p with free P&P (about 29p a sheet). The print is dense and dries quite quickly. (I do a test print first on plain paper).

Q-Connect KF26074 Universal Inkjet transparency film. Clear, A4, OHP film suitable for use in inkjet printers. Features include: excellent ink reception, good drying times and coating on one side.

http://www.amazon.co.uk/INKJET-CLEAR-PRI...B000KJO7BO

£14.52 Delivered FREE in the UK

(RRP £69.00!)

The ink has excellent density on the film, but to make sure, I always 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 you might as well do it, or you end up throwing the rest of the sheet away anyway! You need to make sure that the slightly rougher side is face down in the paper feed tray of your printer, so when it comes into the printer it’s the right way up on which the ink will appear.

I usually print two layouts onto one acetate sheet, then cut them apart and tape together one behind the other to make sure the layout is nice and opaque, but one alone is generally fine. Printers do vary greatly in the quality of the mask that results. (I had an HP printer that made excellent masks with very dense black lines, but it gave up the ghost. I recently replaced it with an Epson Stylus SX535WD which in mediocre whatever setting it’s on).

PCB material:

I’ve long since abandoned using pre-sensitised boards, which are expensive and leave you with lots of useless offcuts (albeit they can be used for test exposures). The lacquer coating is thin, and I’ve found that exposure duration can be critical, as can the strength of the developer. Too weak and it doesn’t develop - just a little too strong and the design disappears before your eyes, leaving you with a plain PCB. All in all, a bit hit and miss.

I prefer to use spray-on lacquer, which must be applied in as dust-free an atmosphere as possible. I do mine in the greenhouse and quickly cover the PCB with a plastic box before any dust has chance to settle. I do it on a level surface – just a couple of quick flashes across the board to get a nice coating. (I make sure I’m not wearing a woolly jumper which would liberate dust onto the board). I leave it overnight in a warm place to harden off - usually in a desk drawer. The next stage is the UV exposure which I’ve mentioned above, followed by developing.

Developing:

I don’t buy developer from the usual sources, (EG Maplin, from whom it currently costs £8.99 for 250G - enough to make 2.5 L of developer). Instead, I buy laboratory grade sodium metasilicate pentahydrate in 1kG packs for £10.56 + £6.99 P&P at this e-bay link(or I could get it from Maplin and pay £36 for the same amount):

http://cgi.ebay.co.uk/1Kg-Sodium-Metasil...upplies_ET&hash=item359fddaa26

I save the developer in a bottle to use again. It lasts a few weeks, but becomes so opaque that you can’t see the PCB at the bottom of the container. I use a shallow plastic container about 30 cms long and 20cms wide so I can see the board through the developer, and rock the container gently back and forth. You start to see the dissolved lacquer floating off the surface which starts to happen after perhaps 30 seconds or so.

I start with a weak solution, using less than the recommended amount of crystals dissolved in warm water and if the exposed lacquer doesn’t float off after two or three minutes, I add a few more crystals until it starts to work. It should take two or three minutes, gently rocking the plastic tray back and forth before there are any signs of the lacquer coming off. As soon as the exposed lacquer has all gone, I quickly rinse the board in clear water as there’s a risk that the design will be dissolved away if left in developer any longer.

If there are any mishaps, you can clean off the board with meths, then repeat the process, spraying it again with UV lacquer. Little is lost apart from time, and as it’s just a hobby, times isn’t a major consideration. The solution can be used several times and lasts for a few weeks. 0.5L is plenty – I save it in a clearly marked plastic container.

Caustic soda as a developer?

In the past, I’ve used caustic soda, (sodium hydroxide - NaOH) as a developer, which is cheap and plentiful. (EG £2.45 for 500g from the well-known High St Chemists). It’s also sold for use as a drain cleaner. It is a strongly alkaline compound, used in the manufacture of chemicals and soaps and in petroleum refining. It must be used with extreme caution.

I’ve found it difficult to get the concentration right. You need to add just a few teaspoonfuls of crystals in a Litre of warm water to make a very weak solution, and dip your finger and thumb into this very dilute concentration to see if it feels soapy when you rub your finger and thumb together. If so, it’s strong enough. I’m not recommending this – caustic soda is nasty stuff and sensible precautions should always be exercised when using chemicals.

Etching the board:

I’ve abandoned using ghastly ferric chloride. It’s only merit is that it’s fairly cheap and lasts quite a long time in solution. Instead, I use Sodium Persulphate clear PCB etchant. This is a much cleaner effective alternative to ferric chloride and has a 6 – 8 minute etch time at 45C – 50C. (above 50C it loses its strength). The bath life is a maximum of 4 – 6 weeks depending on use and operating temperature. Unlike ferric chloride, it leaves no residue – the solution turns pale blue in use. I warm it up with an aquarium heater and thermometer, but you can stand the plastic etchant container in a washing up bowl of boiling water to warm up the etchant. You can use it cold – it just takes longer to work. As the PCB etches, the liquid turns light blue. You can use it several times over. I use an aquarium heater to warm it up, 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, but you don’t have to.

The product is supplied as a fine crystalline white powder and instructions for use are printed on the bottle label. A 500 gram HDPE bottle of Sodium Persulphate copper etchant will make up to 2.5 litres of etching solution. You can buy it from Maplin for £19.99 a 1 kG tub, but I get mine in 500G plastic bottles from a reputable hobby chemicals source at less than half that price - £3.50 for 500g plus P&P at cost, £2.24. That’s £7.00 for 1kG, almost a third of the price of Maplin.

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

You can buy UV positive photo resist lacquer, tinning solution and other PCB materials from ESR Electronics, at Cullercoats who I find to be an excellent firm to deal with. The link to their PCB products is here:

http://www.esr.co.uk/electronics/products/frame_pcb.htm

A lot of experimentation is called for to get right what seems to work for you. Don't be discouraged - we're not putting rockets into space (well I'm not!) - we're just hobbyists!

(I don't have any connection with any of the suppliers mentioned above, or indeed, any commercial interest whatsoever, except as a customer).

To speed up the etching process, I use a 5” long aquarium airstone, which are very cheap, and a small aquarium air pump. I use a plastic cereal container as an etchant bath.

Drilling the board.

I don’t bother with tungsten carbide drills – I only use cheap HSS drills in a small PCB drill and drill stand, with a homebrew drill speed controller. For most holes for resistors, caps, IC sockets etc, I use 0.8mm, for some diodes - 1N400x series, 1 mm, and for Veropins, 1.2mm. I discard the drills after a couple of boards as they tend to wander when they get blunt and leave a burr. (0.8mm HSS drills can be had for ten for under £5.00. For those who only possess a normal pillar drill, you can get 0.8mm drills with a 2.34mm shank at a higher price).

I’ve attached a pic of my low budget etching tank, which consists of a cereal container from Poundland, an aquarium airstone (another pound), and an aquarium air pump for about a tenner. The digital infra red thermometer was on offer at Maplin for about £15.00 but is now listed at £39 (cheaper ones on internet, so no surprises there then). Not essential, but handy for checking the temperature of the etchant and knowing when to switch off the heater. I can’t recall how much the acquarium heater cost as I’ve had it for years, but here’s one for a tenner:

http://www.amazon.co.uk/100W-Aquarium-Fi..._7?ie=UTF8&qid=1331233965&sr=8-7

I hope these notes of interest and may provide a few useful pointers.

In closing, I should add that I do all this messy tedious stuff as a hobby and for enjoyment, but as can be appreciated, it's labour intensive and often you can buy ready made PCBs for magazine projects quite cheaply, or if a batch of PCBs are needed, can get them made at keen prices, so the cost-benefit case for DIY PCBs isn't a strong one.

I often make things I don't need on a whim, like why would I need a precision 10 Volt Voltage Reference? I can't sensibly answer that rhetorical question, but I'm making one, if only out of curiosity. I've just tarted up the EPE magazine artwork to make the mask, and have now etched and built the PCB. I've attached a copy of the mask to give an idea of what it looks like. (I could have bought the PCB from EPE for just £7.77).

David

[Yorkie]

For anyone thinking of making a UV light box using mains voltage UV fluorescent tubes rather the 'Joe's UV LED' project, Rapid sell an 8 Watt UV tube and starter set for £13 + VAT & P&P:

http://www.rapidonline.com/Tools-Equipme...et-34-0707

See - I can do short posts as well as long uns!

[AlanBeckett]

David,
I don't understand that. What about a ballast?
I made one years ago for erasing PROMS. I think I modified a caravan light
Unfortunately it's long gone.
Alan

[Yorkie]

Sorry Alan - I didn't want to imply that all that all that is needed to make a UV exposure unit is a tube and starter. A ballast and lamp-holders etc for the tubes would be needed, and I dare say that Rapid sell those too. I simply wanted to point out where UV tubes of the correct wavelength can be found at a reasonable price for anyone wanting to go down that route. There have been several excellent designs with varying degrees of sophistication in magazines such as EPE in recent years, using such tubes, but I think that were I starting out now, I'd give the UV LED design a whirl.

[AlanBeckett]

Ah, so, thanks.
Alan

[Aerodyne]

Hmm. Well over a year ago I submitted an article on the construction of a UV LED lightbox to EPE. They accepted it but to date have not published. From what you've said, David, I now doubt that it will be!
-Tony

[Paula A]

Avid reader here. I need to get into some quite large scale screen printing onto canvas type cloth.. I have been wondering for a while if I could use this uv active mask on fine net as a lower cost solution to commercial pre coated screens.

Thanks for giving me ideas xxx