Search the Google

Friday, March 14, 2008

PCB Part 3 UV

Part 3
Before I continue, Let me tell you what you will learn and need in this chapter.
  • UV PCB (Photo sensitive PCB)
  • a little about the tracing paper
  • constructing a simple UV exposure box
  • constructing a PCB exposure unit
  • mixing and using the developer solution
  • doing a test exposure



The UV PCB

The photosensitive UV PCB comes in this packing.

The UV PCB can be obtained from Maplin at Pasar Road or any respectable electronics shop. This PCB has a special chemical coating that is sensitive to UV. This come is Paper Phenolic, FR4 Glass epoxy and CEM3 Glass epoxy. Also comes in various sizes. One board of 150mm x 300mm should last for quite a lot of small to medium size projects.


Tracing Paper

The paper we will be printing on for UV transfer is tracing paper. This is not the normal tracing paper, but a thicker kind. Used for plan printing. 1 A1 sheet of this paper cost me RM$2 and cuts into 8 A4 sheets. Remember print at 600dpi or higher resolution.



Tracing paper cut into A4 sheet.

UV Exposure Box

This you have to construct. You can opt to get a UV specially made for PCB transfer, more $$$.$$ or get the cheaper version. The difference is the time taken for exposure, 60-90 seconds compared to 5-7 minutes. Installing a 2nd UV tube, about 2 to 3 inches apart decreases your exposure time to 2-4 minutes depending on your setup.
WARNING: Avoid yourself from exposure to UV.

My simple UV exposure box
Dimensions: 24” x 10” x 8”

Inside lined with aluminum foil for reflecting back UV downward. This actually decreases the exposure time.

Warning: Exposure to UV can damage you eyes. When lit, this is what it looks like when on.


PCB Exposure Unit


This is actually a glorified term for something that holds the PCB and the printout together. You can buy this, again at Pasar Road. To build one, you need a rectangle wooden chopping block, a photograph frame with glass, some small hinges, sponge and some kind of locking mechanism.


Top view of the unit. The green stuff is the sponge. My unit measures 8” x 6”, with an exposure area of 6.5” x 4.5”.


I lined the inside of the unit with aluminum foil.


The locking hook. This is closed when PCB is in the unit so that the printout pushes down on the PCB.






Back view. Small hinges required. Also notice that I have added legs to make it closer to the UV light.


Mixing the developer

Before I proceed any further, you will need to make a developer solution. What the developer does is, it washes away the photosensitive coating exposed to UV. You will then be left with the pattern that you printed.

The Developer

WARNING! WARNING! WARNING! The developer solution is extremely caustic. It is made from NaOH (Sodium Hydroxide) or in English, caustic soda. Keep out or reach of children and store in high place. The ratio required for developing is 7gm NaOH to 1 liter of water. My solution is 10 times more concentrated, 70gm to 1 liter. I use 20ml of 10X solution and add 180ml of water. This way I don’t have make the solution every time is need to use it. Keep the solution in a thick plastic container like empty milk or fabric softener bottle and label it! Store the flakes in an air tight container and label it. I honestly don’t know where to get NaOH flakes from. I know someone who knows someone who got it for me. If you want them, and eyeball often enough, I will help you get some.




NaOH Flakes



10X Developer. 70gm:1liter ratio. Remember to label the container.


View on the other side



Make sure the label is large. In case someone mistakes it as drinking water.


Your first exposure
OK, I have done my schematics and printed your PCB foil on tracing paper and made my developer. But for how long should I expose my PCB under the UV? To find this out, you need to do a test exposure. Download this test pattern. Remember, paper is placed with printed (toner side) on copper. By doing this you have mirrored your image. Cut PCB and paper to size and tape edges of paper to PCB. Normally you would not need to tape, but for this exercise you do. Once you have taped the test pattern to the PCB, use a business card, to cover numbers 1 – 9. Place in your PCB exposure unit. Expose No 10 to UV for 1 min. Then move to card to cover 1 -8, and expose for 1 minute. Repeat until No 1 is exposed for 1 minute.

Now you have a 10 minutes exposure test. If you have a high end UV, expose in blocks of 10 or 15 seconds.
This test determines you best time of exposure. Next time use this time to develop your PCB projects.



Printout of my old test pattern


Taped to the PCB on the edges.



Excess tape on the back.

Exposing No 10 to UV and covering the rest.


Exposing No 9 and 10 to UV and covering the rest. Continue exposing every column for 1 minute.

Developing

Pour 20ml of 10X developer into a plastic container large enough to hold your PCB. An old ice-cream container will do. Then add 180ml of water to the developer. Remember always mix chemical to water and not the other way around. Place your PCB in the developer bath and in 10 to 15 seconds it should be developed. If not, try lightly shaking the container. If still nothing happens, remove the PCB, either by hand or if you have sensitive skin, you latex gloves or plastic tweezers. Take some 10X solution using a straw. Put 2 or 3 drops of it in the bath. Stir the bath and place the PCB back in. But if you measurement for the developer was correct, you should not have any problems.



Using an old mixture bottle for measuring.


Place the PCB in the bath and lightly shake the container.

1-5 minutes is underexposed. Tracks are not visible or blurred.
6-8 minutes is good 6 being the fastest.
9-11 minutes is overexposed. Tracks are getting thinner and dimmer.

That’s all there is to it. Developing a PCB should not take more than 7 minutes. If you want faster results, use transparencies. Exposing time is much shorter; I got 3 minutes for best results compared to 6 minutes. But are more expensive. Remember to use transparencies for laser printer and not ink-jet printers. Those are not heat resistant and will damage you laser printer. Also 2 UV's decreases exposure time. The bath solution is not re-useable. So dispose of it, flush it down the toilet or the drain.
Now I have to go and prepare the next Chapter, Etching. As always, should you have any queries, leave me comments.


2 comments:

9W2BSR said...

9w2gu,

Nice write-up... just to let you in on a small mistake... Always add ACID to WATER not the reverse... I remember it as A&W (the fast food chain) thus acid-to-water...

Check details here:
http://antoine.frostburg.edu/chem/senese/101/safety/faq/always-add-acid.shtml

9w2gu said...

OK sir. I will do the required changes. And thanks for pointing it out.