Tuesday, November 29, 2011

Photo Etching: Part 2 - Exposure and Etching

We have a functioning light box, so, now its time to do some etching!


You will need:
  • An assembled light box from Part 1 of this blog entry
  • Small single-sided pre-sensitised copper-clad board 100mmx75mm
  • Craft knife
  • Steel rule
  • Scissors
  • Sodium persulphate etch powder
  • Universal photo-etch developer
  • Disposable latex gloves
  • Safety goggles
  • Empty bottles to contain the mixed chemicals (not the bottles should be clearly labelled POISON - DO NOT DRINK)
  • A bucket
  • Developer trays, I use recycled plastic food tubs

Everyone learns by making mistakes. Every exposure box is likely to have a slightly different intensity, and need different lengths of exposure. Also your inkjet printer, transparency film and copper-clad board are all likely to be different to mine. Although there is actually quite a wide margin or error, we need to try and find the optimal exposure for our set of equipment without wasting too much copper clad board.
So, to avoid frustration, just accept that you are not going to get anything except knowledge out of your first few dollars worth of copper-clad board and transparency film.
We are going to start by making a test strip to determine the best exposure for our printer / board / lightbox combination. This is a technique taken from ‘good old fashioned’ film photography. The idea is that we will have a test pattern printed onto transparency film that we will partially cover up with the first exposure and then gradually move the card to expose more of the board for the subsequent exposures. 
But first, let’s print the test strip onto transparency film.

First of all, we need to create a test strip pattern. The test strip below can be printed directly by printing the last page of these instructions onto transparency film. This film used to be used extensively for presentations on over-head projectors. 
I use an inkjet printer, but if you have a laser printer then you can also get transparency film for them. Be sure to get film for your correct type of printer. You cannot put inkjet film into a laser printer and vice-versa. Any major office supplier will sell transparent film. 
We need to print as boldly onto the transparency film as possible to ensure as little UV as possible leaks through to the coper clad board. To do this, make sure that you set:
the media type to ‘Transparency’
document type to ‘Monochrome’
quality to ‘Maximum’
The transparency sheets are relatively expensive, so I tend to print at the top of the page and then cut the top of the film and use the remainder for the next board, getting three or four boards out of one film.
When you have printed the film, try to keep it clean until you are ready to use it, as any little specs of dust will end up as copper.
A Test Strip
Firstly we need to cut up our sacrificial copper clad board into test strip sized pieces. You can do this with a craft knife and metal rule. Score the board top and bottom, where you want to cut it and then snap it over the edge of a table.
You will notice that the copper side of the board has a thick layer of light-proof plastic covering. This will need to be removed from the test strip before exposing it.
Here are the steps for creating your test strip.
  1. Mix water into the developer powder according to the instructions on the packet. Mixing them into a storage bottle. If the chemicals are designed to work at a higher than room temperature then place the bottle it in a bucket of hot water to bring them up to temperature.
  2. Do the same for the etch solution.
  3. Trim the transparency film to be about the same size as the copper-clad board.
  4. Disassemble the clip frame
  5. Peel the protective layer off the board and place is sensitized side up on the base of the clip-frame
  6. Place the test strip over the board
  7. Fix the glass lid of the frame back into place.
  8. Place the whole photo frame into the bottom of the light box.

Now we can begin making the exposures.

We are going to make four exposures, for the first three covering successively less and less of the test board. 

The sequence is:
  1. Cover sections D, C and B with a thick piece of card (on top of the glass) and expose for 40 seconds.
  2. Move the board to cover just the sections D and C and expose for 20 seconds
  3. Move the board to just cover section D and expose for 10 seconds
  4. Remove the board completely and expose for 10 seconds

By this time your chemicals will be at the right temperature and we can develop the board. If you are not quite ready to develop yet, then it does not matter, as long as you cover the board to prevent any further exposure from ambient light. Turning it upside-down is sufficient.
Put on your protective eye-ware and latex gloves and place the board copper side up in the tray that you are going to develop in.
Pour some developer onto the board enough to cover it with about half an inch of developer. Then gently rock the tray back and forth. You should soon see a picture starting to develop. Do not remove the board from the developer until the test strip image is clearly visible and well defined on the board. It is much easier to under-develop the board than to over-develop it, so if in doubt leave it in the developer a bit longer.
When its ready, it should look something like this:

When you are happy that the board is ok, then remove it and wash it in the water in your bucket. Pour your developer back into its storage bottle ready for next time.

Either use a separate tray from the developing tray or clean your developer tray very carefully, then place the board in copper side up again and pour the etchant over it. Again a depth of about half an inch is ideal. The etchant solution will gradually go blue as it dissolves the copper that is not protected by the photo-resist image, so it is much easier to see what is going on if you don’t fill the tray right up.

You may have to fish the board out of the etchant form time to time to check on it.
Almost immediately, you will notice the exposed copper areas to start to go pink. You know when the board is ready when all the pink areas have gone. The two pictures below show a board that is not quite ready yet followed by a board that is fully etched.
When the etching is complete, put it back into the bottle and put your chemicals away safely out of the reach of children. Especially the etchant, which turns an attractive blue color when used.
Wash the board again, and you are done!

Assessing the Test Strip
An example test strip is shown below.

On the left had side of the board, 10 seconds was not enough as most of the copper is still there. On the other hand 80 seconds was too much as the UV clearly went straight through the printed mask. 
40 seconds was still a bit much, but 20 seconds was just about right. Echos of Goldilocks there!

In the final part of the blog entry we will use our new toy to create a PCB for a timer to control the the UV exposure box.

Now I call that bootstrapping!

Part 1. Part 3.

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1 comment:

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