PCB prototyping with Presensitized copper boards

Question

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.

Answer 1

• 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.

Answer 2

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/

Answer 3

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.

Answer 4

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.

Answer 5

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.

Answer 6

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