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I'm currently working on my first PCB prototyping by using some pre-sensitized PCB board, purchased from Maplins UK. However, I'm having terrible trouble exposing and developing the board, it just never seems to turn out right. I haven't even reached the etching stage yet because I'm not happy with the presensitized developing results. I'm looking for general tips and tricks, and how to follow the right 'path' to success — I seem to be getting worse at it for some reason! ;-)

I am using inkjet transparency sheets with the darkest tone applied for the mask, and after noticing you can see light thru the printed areas on the mask, I decided to double up, and sellotaped two identical transparencies aligned on top of each other. You can hold it up to the light and still see a kind of dull purple glow, but it is mostly opaque. I acknowledge this may be one of the causes of my troubles!

I started off using a normal 100 watt lightbulb, with little success; exposures seemed to take 40 minutes and beyond to have any effect at all, and got very warm. I used to hold the mask down onto the board using glass, but after realising glass blocks a lot of UV light, I decided to switch to a sheet of clear acrylic.

I then found a cheap portable UV sterilizer, which actually claims to emit UV-C light; I know UV-A is the type required to expose the board as far as I can work out but it seems to be having a much quicker effect, I see an effect even after just two minutes exposure. The manual states it uses a "Long life 4 watt germicidal UV lamp", and yes, it's not what you'd call bright. The longest exposure I have done so far is about six minutes, and I still saw the same problem (see below).

According to their site, the boards are supposedly developed using a caustic soda solution.

On all of my attempts, I am seeing the circuit pattern appear, but the developed copper which is to be etched away ends up nowhere near as shiny as I'd expect it to be — I am expecting the photoresist layer to completely dissipate resulting in exposed shiny copper, going by a lot of those reference PCB etching sites on the internet. Instead I end up with varying degrees of random areas of shiny copper but mostly dull copper (partially disolved photoresist), and other areas complete bluey greeny photoresist layer. I haven't tried etching yet, but its not difficult to see why I expect such a poorly exposed and developed board to poorly etch too. I can't imagine the 'mostly dull copper' areas will etch at all given the photoresist is only partially dissipated.

I'm cutting off small areas of my two medium sized pre-sensitized boards for testing purposes, and I'm running out quickly!

So I am unclear and am proceeding very slowly because of all these variables:

  • Not sure about exposure time (for my lamp).
  • Not sure about UV light position and height required.
  • Not sure if my mask pattern is opaque enough (to UV light).
  • Not sure about correct proportions of caustic soda to water.
  • Not sure about time to leave the board in the caustic soda — whether I'm going too far developing.
  • Not sure about dipping in water to stop the development — Can you put it back in the caustic soda to continue development if you didn't leave it in long enough? Also how can you tell!?
  • I also don't know whether a developed pre-sensitized board stays so, and won't fade as its left in daylight over time. How long before normal daylight affects a pre-sensitized developed board?

Any help anybody could give me would be appreciated. I haven't kept a decent log of my activities, to learn effectively, because I thought I'd have nailed it by now! However I can provide pictures of my efforts so far, if need be.

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    \$\begingroup\$ Stu, Chiphacker is just the place for this sort of question :) \$\endgroup\$ – mad_z Nov 16 '09 at 1:32
  • \$\begingroup\$ I am working on an answer, but how are you checking how "bright" your UV light is, a UV light should not be very bright to your eyes, but could easily hurt your eyes. The second thing is determining what power the material needs and determining your output power and the transparency of your material. \$\endgroup\$ – Kortuk Nov 16 '09 at 5:56
  • \$\begingroup\$ Yeah, Kortuk. I'm not really doing anything scientific like measuring brightnesses, gear to measure this would cost quite a bit I'm sure. Indeed I haven't looked into the light, and I'm wearing some prescription sunglasses, very good at blocking all kinds of UV light, whilst exposing the boards. The purple glow emitted is surprisingly bright for a 4 watt bulb. Maplins dont report any sort of exposure time or power but mention you should be using their own UV box - of which, may I add - costs around £80 I think, so I wont be going for that! Many thanks for your responses! \$\endgroup\$ – H3liosphan Nov 16 '09 at 21:45
  • \$\begingroup\$ @mad_z. Great, thanks! Except I noticed there are no pre-defined tags for these words-"pcb","etch","presensitized" etc etc etc, and it wouldn't let me submit until I put in one known tag. Perhaps when I earn enough kudos it'll let me add them... ;-) \$\endgroup\$ – H3liosphan Nov 16 '09 at 21:54
  • \$\begingroup\$ Have you considered order the PCBs in a PCB house? I have made homemade PCBs in the past, it's mostly not worth it. There are a few PCB House that made a really good PCB for really cheap price. I agree with @Gustavo Litovsky and his comment here electronics.stackexchange.com/questions/58369/… \$\endgroup\$ – Jesus Castane Feb 19 '13 at 9:17
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  • Not sure about exposure time (for my lamp).
  • Not sure about UV light position and height required.

With these you just have to experiment, I'd suggest putting the board fairly close to the lamp (20-50mm or so).

I suggest you take one longish strip of photoresist PCB and put something on top of it that blocks UV light, then expose the strip while moving the UV blocker away at predefined intervals (say, every 10 seconds). What you will end up with is a PCB exposed in steps for different exposure times (10s, 20s, 30s, 40s, 50s etc.), then just pick the exposure time that gives off the best result.

  • Not sure if my mask pattern is opaque enough (to UV light).

If it's a normal transparency sheet it should be fine. Printing to these with a laser printer works but this depends on your printer. Large areas of black might get overexposed but this depends on your printer. If your quality of black is bad you can print two sheets and align them on top of eachother.

  • Not sure about correct proportions of caustic soda to water

I have been using a 3% solution of NaOH (KOH works too) which seems to work well.

  • Not sure about time to leave the board in the caustic soda - whether I'm going too far developing.

You just basically move it around in the solution until it seems developed (black stuff comes off the board), usually it develops quite quickly (under 30s) but this depends on your solution. If your solution is too strong or you develop it too long it will strip off all the photoresist if you keep it in the solution too long. A solution not concentrated enough won't successfully develop the board (again, 3% solution seems to work well). Washing the board under a faucet afterwards works well. You can continue developing after taking it away from the solution. You can actually even continue developing after etching the board a bit, just remember to wash the board. Putting the board in the etching solution shows you pretty well where the board has photoresist and where it doesn't, the copper exposed to the etching solution goes to this "dull" color/texture in under a minute in the etching solution.

  • I also dont know if a developed pre-sensitized board stays so, and wont fade as its left in daylight over time. How long before normal daylight affects a pre-sensitized developed board?

A board with the plastic shield SHOULDNT be affected by daylight but to be sure I'd keep them in a place shielded from light (like a drawer).

Btw. I'd suggest you try to expose your boards with a 11W fluorescent table lamp (the ones with a "U"-shaped lamp, they should be pretty common and cost like 10e from Ikea). Put the lamp quite close to the board (like 50mm close, and have a thin plate of glass over the board to keep the mask close to the board). You can get suitable plate of glass from picture frames (again, Ikea is a good place to get these), just make sure its real glass and not plastic. Expose for 12-15min (I've used 13,5min for my boards and setup). I have been using this method succesfully for a long time. Won't work for large boards due to the lamp being so narrow but for small boards it works well.

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  • \$\begingroup\$ Thank you so much Dago, there are some gems in there! It would have taken me ages to think up that stepped exposure trick, thats given me a great method to start working out exposure times! I'll try and find this 11W flurescent, I dont remember seeing any with a U shaped bulb though (I assume thats what you mean). My UV sterilizer does have a U shaped bulb and it seems to be giving me very quick results in relation to the incandescent I used. Again I thought glass would block a lot of the required UV light, even picture frame glass. Why not plastic or acrylic? Thanks again! \$\endgroup\$ – H3liosphan Nov 16 '09 at 22:04
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    \$\begingroup\$ Happy to help :) Plastic and acrylic are bad because they are pretty much opaque to UV light. I Believe "regular glass" like one used in picture frames is not perfect either but it is pretty transparent at UV wavelengths. I think the optimal solution would be quartz glass which is basically transparent to UV light but quartz is rare, expensive and unnecessary in this application. The table lamp type I was talking about is this one: [img]servicelighting.com/catpics/philips/26109.jpg[/img] Commonly used in table lamps. \$\endgroup\$ – Dago Nov 16 '09 at 22:35
  • \$\begingroup\$ The glass you normally have available will glow in UV light I believe as it absorbs and radiates the UV light. It has an absorption of 80-90% in the process, I believe. \$\endgroup\$ – Kortuk Nov 17 '09 at 9:32
  • \$\begingroup\$ Like I said, it might not be optimal but it's good enough :) I'm getting sub-2min exposure times with under 8W input to my UV-led panel (including power that is wasted in the resistors, most likely like a watt of actual light output) while using a couple mm thick picture frame glass (most likely regular soda-lime glass). \$\endgroup\$ – Dago Nov 17 '09 at 17:58
  • \$\begingroup\$ I had no idea plastics blocked UV light so much. Thanks for that. I know glass blocks UV pretty well though, because its designed to prevent rooms from being bathed in dangerous UV on sunny days - thats why you're curtains dont end up bleached after a time. I suppose you can say that about every kind of transparent sheet material available on the market as preventing dangerous exposure is a key requirement for most transparent materials. I'd wager the 'saving grace' for picture frame glass is that its so thin in relation to window glass, makes it less absorbant. Thanks all. \$\endgroup\$ – H3liosphan Nov 17 '09 at 20:21
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I use an Inkjet printer and it works great. You need 3M Inkjet transparency paper because it is textured. I have an instructable located at: http://www.instructables.com/id/Creating-Printed-Circuit-Boards-with-a-INKJET-Prin/

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  • \$\begingroup\$ Ahh thats you is it!? Cool, you know it was your Instructable that got me started doing this in the first place! Your fault! ;-) Anyway, I printed it out on some Staples transparency sheets here - <a href="staples.co.uk/presentation-equipment/transparencies/… Inkjet Transparencies</a> This stuff is well textured too, but if I print it out once, I hold it up to some halogen light bulbs and can see a purple glow thru the black opaque areas, if I double up I see a much darker purple glow. Cant tell if UV light is blocked. \$\endgroup\$ – H3liosphan Nov 16 '09 at 22:12
  • \$\begingroup\$ Sorry, linky screwed up, heres another try - staples.co.uk/presentation-equipment/transparencies/… \$\endgroup\$ – H3liosphan Nov 16 '09 at 22:17
  • \$\begingroup\$ :) When I print, I set it for black and white (not even set to photo quality). When I hold it up to the light, I don't see purple, but I can see through the black a little bit. However, when I lay it down on the surface of the PCB its real dark looking and I haven't had any problems. \$\endgroup\$ – ArduinoFun Nov 16 '09 at 22:59
  • \$\begingroup\$ I am using a 15W bulb, placed about 5 inches over top of my work. The PCB is laying on the workbench, pre-sensitized side up then one sheet of transparency with the design over top and a clear piece of acrylic over both to hold the transparency down and keep it from moving. I expose it for 14 minutes then place it in the developer which I have mixed as a 1 to 10 ratio (heated, and I jiggle it back and forth wearing a rubber glove). Within seconds my design appears and then I put the board in cold water to stop the developing process. Then move onto the etching process, etc. \$\endgroup\$ – ArduinoFun Nov 16 '09 at 23:00
  • \$\begingroup\$ That's a nice instructable, but it is missing information on the inkjet printer. Setting the printer to BW and not photo seems to be an important detail to get the print dark enough. I also suggest, if available, setting the print density to the darkest setting (which I do on my Laserjet). Could it be that some brands of ink just aren't as opaque to IR as others? So knowing your make of printer might be useful, also. \$\endgroup\$ – SiliconFarmer Nov 17 '09 at 1:18
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I get excellent results with a cheap HP DeskJet 5940 printer and Mega Electronics JetStar Premium film, using positive pre-coated boards in a home-made UV exposure unit made from MDF and a few strips of wood. I use a sheet of glass in the exposure unit - it's transparent to long-wave UV. I keep meaning to add another tube to decrease the exposure time, it takes 11 minutes.

It's best to have the UV tubes some distance from the artwork/PCB, as it increases the resolution due to the collimation effect. I have done 5 mil tracks as an experiment, but usually use 10 mil.

exposure unit

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Inkjet on transparency is indeed weak for this application. You need to use a laser printer, or print it on your inkjet at 2X and positive on paper. Take it to a local print shop and they can reduce it 2:1 and make a negative for $10 or less.

Your supplier's web page clearly says 1 teaspoon sodium hydroxide (caustic soda) to 0.5 liters water for the developer solution. Since you say you don't know the concentration, it tells me you haven't read their directions carefully enough, so that is your next step.

Good luck.

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  • \$\begingroup\$ Well worded. Making sure your developer is good is very very important. \$\endgroup\$ – Kortuk Nov 16 '09 at 8:57
  • \$\begingroup\$ Hi SiliconFarmer I did indeed follow the instructions, except I didn't have a half a litre container, only some plastic cups, so I adapted the teaspoon measure by about 50% because I used about 1/4 litre water. It didn't seem to make any change to the board at all! I'm assuming thats because the exposure was all wrong but I cant be sure you see. Thanks for the info, but I have access to a laser printer, work photocopier that dont mind me using it for personal reasons, AND my inkjet printer, so no need for print shop services. However I will be trying laser Toner on xparency next. Thx! \$\endgroup\$ – H3liosphan Nov 16 '09 at 21:50
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I also use a cheap inkjet with JetStar film and get great results. For developing and removing the etch-resist, I use Seno's boot polish style applicators from Mega UK. Exposure takes me 3 minutes (per side) under 2 8W UV bulbs.

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Some additional information that came to my mind after reading @SiliconFarmer's answer:

  • Don't assume a laser printer will print better blacks than an inket one.
  • The resulting artwork will depend on a many factors, mainly on the quality of the printer itself (maximum resolution, registration precision, type of toner/ink) and the type of printing media used (transparency sheet, tracing paper...)
  • Check out the "blackness" you can get with your printer and printing media combination. Try different printing medias and printer settings combinations. Then check the printed artwork against an incandescent bulb and stay with the combination which results in a more opaque black for lines and pours.
  • Checking the resulting artwork under a good magnifying lens or (better) microscope will give you a good hint on the quality of the printed "blacks". You may be surprised that many laser printers out there will print "poorer" black pours than inkjets!
  • If any of the above gives you a good "black" quality, you can always resort to printing your artwork two times and then overlap and stick together both prints so as to get a 2x improvement in the opacity of the your printed black ink.

You can check this reference, which proved useful to me in the past, where some of the topics above are covered:

http://www.hobbyelectronics.net/tec_making-pcbs.html

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