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Hints, Tips and Solutions (Do NOT post requests for help here) If you have any useful general hints and tips for vintage technology repair and restoration, please share them here. PLEASE DO NOT POST REQUESTS FOR HELP HERE!

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Old 25th Jan 2010, 3:34 pm   #1
dominicbeesley
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Default Roll your own printed circuits

Hello all,

Just a quick tip for anyone who like me has been put off from making their own printed circuit boards by the price of UV light boxes. Last week I thought I'd have a go but instead of a fancy light box I built my own.
I got a 5W UV LED Star from Farnell (167-8962). And stuck this to half the carcass of an old computer PSU to use as a heatsink and stand, this holds the LED a few inches above the workbench and gets warm but not too warm after about 10 mins. The pool of light is big enough to cover a 4"x4" (10cm x 10cm) board a couple of small blocks of wood would lift it high enough to do a bigger board.

Adjusted for 1200mA current through a 1R resistor the exposure time at 6 minutes was possibly a tad too high (4 would probably do it) for my rather leaky inkjet negatives but a good board was made (after a couple of false starts due to getting the negatives the wrong way round).
As usual I dived in and designed my own double sided board and this came out ok, a few adjustments next time (bigger pads for less than accurate drilling!) and it would be spot on.

Whatever you do watch your eyes though! These things are very very bright and will probably do you some serious damage. I managed to briefly flash it whilst looking directly at it and had a dose of welders eye the next day! so a nice light-tight box over the top would be an good idea. I can just close the door to the workshop and switch on/off from outside!

One thing I'm considering trying at some point this year is the next time the sound panel in my Decca Bradford plays up (pretty ropey around the PCL82 valve base). Is to photograph the underside and use this to make a negative then etch a new PCB this will require a couple more LEDs as the board is quite long and narrow. However if developed onto a nice fibreglass board (instead of the Decca ones which seem to be made out of brown paper and vinegar) it should last a lifetime not sure if anyone would be interested in a board also, to help offset the costs!

Dom
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Old 25th Jan 2010, 7:50 pm   #2
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Default Re: Roll your own printed circuits

Hello there,
I wander if you can mount the UV LED in an old flat bed scanner with the guts pulled out. It is about the right size fairly lightproof and fairly cheap to get a broken one. It may be possible to mount an array of UV leds inside and add an interlock switch for the lid.
Christopher Capener
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Old 27th Jan 2010, 12:22 pm   #3
dominicbeesley
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Default Re: Roll your own printed circuits

That's not a bad idea - though it would want to be a fairly deep one so that the light footprint is wide enough (about 3" minimum). I finally chucked my collection of broken / unwanted scanners (after robbing the little wheels for tuning chords) a few months ago

Dom
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Old 30th Jan 2010, 10:33 pm   #4
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Default Re: Roll your own printed circuits

There's an alternative to UV boxes -but a bit expensive -Maplin call it Peel n Press- print to sheet and iron to board using dry iron -have seen it elsewhere cheaper than maplin at £15.99 for 5 letter sizesheets .Problem is that you need a laser printer .Have seen some discussions on using ordinary paper that works .Have toyed with idea .
On the software side -maybe preaching to converted ,but I find Express PCB (freeware from www.expresspcb.com ), good -it does schematics and PCB and links between the two.
It's an American company ,the software is free to encourage folks to design their own boards and send design off for boards to be made .
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Old 30th Jan 2010, 11:45 pm   #5
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Default Re: Roll your own printed circuits

Great idea to use UV LEDs for an exposure box as I was thinking of making my own rather than doing a frequent 50 mile round trip to borrow a friends. Half a dozen of these star LEDs would expose a decent size board.

I've used the press and peel sheets in the past with variable results, not so good with fine tracks but OK with tracks of 1mm+. A laser printer that won't eat the film and a hot iron are required. The only time I enjoyed ironing
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Old 31st Jan 2010, 1:12 pm   #6
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Default Re: Roll your own printed circuits

Hi,
I built my own UV light box prior to 2000 (I think), the 12 inch UV tubes were from Maplin and fitted into a cheap camping/caravan twin flurescent strip light fitted into a home made wooden box.
A reflector was made from cardboard coated with kitchen foil. Boards up to 5 Inches by 9 Inches can be accomodated. Did not have a timer so used my watch and my hand on the On/Off switch. Power for the Light (13 Volts required) was from a home made PSU 13.8V that I used to power my amateur radios.
In 2005 the box was modified (one of the tubes went so got 2 more from Maplin) and I decided a better distribution of light for the larger boards could be obtained by spacing the two tubes further apart. This was done by changing the camping light and using separate choke and starters obtained from RS and powering directly from the mains.
The UV tubes I believe are still available from Maplin, Farnell and RS (approx £16 - £19 + postage). This compares favourably with the LED which is priced at £19.57 + postage?
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Old 31st Jan 2010, 3:22 pm   #7
dominicbeesley
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Default Re: Roll your own printed circuits

Hi lads,

I've tried peel-n-press a few years back and have to say it was fiddly and the results were pretty marginal though this was at school I think but I recall the smaller pads and tracks tended to go awry.

On my latest board 1/4mm tracks (for writing) are not a problem so long as the negative is held against the board nicely. I think I might make something like in the recent Elektor - a plastic bag and vacuum pump to firmly hold the negative against the board. Rather than my current arrangement of placing nut and bolts at the corners to hold it down! I've not even found a nice piece of thin glass yet and even, then the boards aren't always 100% even...

As to making something with tubes I thought about that but never got round to it. The LED setup literally took 2 minutes - solder in a second hand junk box ballast resisitor and hook up to the bench PSU and stick the LED to a scrap frame. Also for the tubes you need the starters and ballasts and all that messing put me off for the past few years though I suppose one of those flourescent torch ballasts would do it...

The single led gives more than enough light output - I reckon my 5 mins exposure time is a bit too much I 2-3 would probably do it. Or the LED could easily be raised up on a larger frame to expose to expose boards twice the size (4x the area) and just use 10 minutes exposure time! Also no need for reflectors the LED gives a very nice even pool of illumination. I thought I'd have trouble with the edges being underexposed but everything works really nicely.

The other advantage of having a single "point source" is that if the negative and board aren't in perfect contact the tracks just move slightly instead of blurring badly as can happen with multiple tubes.

Anyway just thought I'd share the experiences of a lazy experimenter - I've wanted to develop my own boards for years but never got round to it and always thought it would be difficult. Turns out its a doddle and doesn't require any special equipment, though I'm in trouble for getting acid on the washing up gloves!

Below are a few pictures. As can be seen I've not yet found the setting on my printer to get the negatives to come out without the black being patterned. Apart from that all works well. As can be seen the LED throws a very nice even pool of UV. Shame the tinning came out so scruffy, I should invest in a flux-pen!

Dom
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Old 31st Jan 2010, 4:25 pm   #8
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Default Re: Roll your own printed circuits

Hi Dom,
Just a suggestion - presumably the pcb is at the bottom with the negative on top - is to put a small piece of glass on top of the negative and this will hold the negative flat against the pcb. Another thing is to have the ink side of the negative closest to the pcb but this means reverse printing the negative.
One last point is the film used for the negative, I use Jetstar for an inkjet printer as this is this gives a denser black.
Try looking at the Mega Electronics web site there is a load of information about pcb production.
Note that the exposure time for pcbs can vary considerably depending on supplier, I now try to stick to Photoboard 2.
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Old 31st Jan 2010, 4:57 pm   #9
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Default Re: Roll your own printed circuits

Hi Fred,

Yes, I have the ink side closest to the board. However flat glass is no good unless the PCB is 100% perfectly flat which it tends not to be after cutting down to size. The idea behind a bag and vacuum is then that the negative follows the curve of the board. Anyway The nuts and bolts on the corners seems to work ok for now!

I don't know what happened with my ink. The first one I did was really solid black but I messed up, then the ink ran out. Now with a new ink cartridge I just can't get it back...With this HP printer there's any number of settings I could have upset and as I have something that works and only 5 sheets of transparancy left I don't want to mess about. I'll get some more transparencies this week and have a play!

The boards I got are some generic Farnell ones, they seem to work well and have nice thick protective film on them. I've got some Maplin ones I bought years ago but suspect they will have gone off by now!

Cheers

Dom
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Old 31st Jan 2010, 6:48 pm   #10
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Default Re: Roll your own printed circuits

As a confirmed fan of solid-state lighting, I think the LED approach is really neat!

One word of caution, though: a 1R series resistor is asking for trouble. This type of LED has quite a strong negative temperature coefficient to its forward voltage. That means that as the LED heats up, as it will do, its forward voltage drops. With a very small series resistance, like 1R, the forward current will then increase substantially. So the LED gets hotter, and you know what happens then. Eventually the LED dies of exhaustion.

I've had this happen in a product of mine, in which I had economised by substituting multiple constant-current supplies for power LEDs with one constant-voltage (5V) one and low-value series resistors. The LEDs started to die mysteriously in the field. The solution was to run the LEDs at lower power by increasing the value of the resistors.

The 'Right Answer' is to use a constant-current supply for your LED. The quick and dirty answer is to use a higher value of series resistor (say 10R) and crank up the supply voltage, letting the resistor get very hot. That way the effect of the temperature coefficient is dramatically reduced.

Chris
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Old 31st Jan 2010, 9:11 pm   #11
dominicbeesley
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Default Re: Roll your own printed circuits

Hi Chris,

Thanks for the pointers! I did use a 10R resistor - just a typo (or brain fade) above. I usually set the supply to about 25V and bring the current limiter up to set it to run at about 1200mA, running from a bench supply. If I was going to make this into something that plugged in I'd take your advice and use a proper constant current driver.

However, the LED doesn't get too hot using just the PSU carcass. I've used other start-LEDS before and they seem to survive getting too hot to touch but like you say they tend to run away! If I were to run it for more than 15 mins though I think I'd add a bigger heatsink.

Dom
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Old 31st Jan 2010, 10:08 pm   #12
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Default Re: Roll your own printed circuits

Excellent - sounds like you're miles ahead of me
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Old 9th Feb 2010, 12:06 am   #13
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Default Re: Roll your own printed circuits

Have seen reports of ordinary copy paper being used with a laser printer and the result ironed on -though never yet tried it .Just some food for thought etc
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Old 3rd Apr 2010, 3:48 am   #14
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Default Re: Roll your own printed circuits

I use a laser printer and gloss paper....really thin paper, like cut out pages from the Argos catalogue seem to work best for me.

I just iron them onto a very clean bit of copper clad (pressing down very hard) quench in water, peal and etch....here are some results of boards i did for other people last week.

http://i624.photobucket.com/albums/t...pechat/222.png
http://i624.photobucket.com/albums/t...pechat/pcb.png
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Old 19th Jun 2010, 11:57 pm   #15
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Default Re: Roll your own printed circuits

I've tried peel-n-press a while back it would have been cheeper to buy a uv light , the sticky stuff stuck itselfe to teh inside of my coulor laser printer , i was not a happy puppy , nesather were maplin as they ended up buying me a new printer but thats a difrent story ,


you can buy 8w uv lamps and 13w from fish shops and they do the job sweet
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Old 20th Jun 2010, 8:47 am   #16
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Default Re: Roll your own printed circuits

Quote:
the sticky stuff stuck itselfe to teh inside of my coulor laser printer
Really? You are the first and only person I have heard say that and I spent a long time searching for this exact issue.

Press n peel is OK, not so good for thinner tracks but the UV method seems to be the most reliable.
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Old 20th Jun 2010, 10:43 am   #17
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Default Re: Roll your own printed circuits

A couple of months back I completed a full design for a low voltage UV LED lightbox - safety interlock, powered by wall-wart, simple timer etc. EPE accepted it for future publication but this might be some time because, unknown to me at the time of submission there's a different design for a light box coming up in the August issue (on sale in July, I believe) which uses a converted scanner. Don't know much about it, but in the same issue is my item on 'How to make your own PCBs', using the Seno range of kit.
Might be of some interest to forum members.
-Tony
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Old 24th Jun 2010, 1:04 am   #18
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Default Re: Roll your own printed circuits

Hi Dom,
I like the idea of your light box made out of the carcass of a an old power supply case. Have you thought of having glass panels cut to size either side of the box? That should shield you/anyone else in the immediate area from the effects of uv radiation.

Just a thought.

Rob
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Old 24th Jun 2010, 10:29 am   #19
dominicbeesley
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Default Re: Roll your own printed circuits

Thanks Rob,

Now I've read up on this I'm not too worried about the side-effects as this LED only puts out UVA, like a fly-zapper or black light so shouldn't cause too much harm.

Dom
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Old 23rd Jul 2010, 11:33 am   #20
dominicbeesley
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Default Re: Roll your own printed circuits

Another cheap idea I've spotted is "GEL LAMP LIGHT NAIL DRYER" - I think these are for drying out your nail varnish...anyway for less than £20 and containing 4x9W UV lamps should be good for making PCBs...

BTW, I still thing the point source led lamp is a better idea, being a point source it's dead easy to get the narrow tracks required for SMD work. I've now made a dozen or so SMD boards with 1/4mm tracks and no problems...

Dom
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