To produce a good etch resist pattern, the toner side of the artwork MUST be in contact with the etch resist. This is easy to see when using the F100TDL film as it is translucent rather than transparent, but with OHP film it is not so easy to tell. Likewise you must ensure that the artwork is in contact with the board all over. I had a few failures before I worked out what was going on. If the artwork is not in contact, the UV leaks under the track pattern and you either get a very poor track, or none at all. I tried several ways to improve things, mainly by placing weights on top of the board. With a large flat base milling vice which weighed about 10 lbs. things improved, but this was not a very good solution. Also cheap single sided SRBP board tends to bow quite badly. To cure these problems I bought an off-cut of 10mm thick Perspex and made a press that I can sandwich the board and artwork between. That cured the problem fairly well. I do flex the cheap board by hand to flatten it if it is very badly bowed, this helps the press do it's job. The press is shown on top of a black carry-case in the image below.
How long you expose the boards for is a matter of trial and error. The obvious thing to do is to produce an exposure strip and have a look at the results, in the same way that you would do for photographic print processing. For my unit the optimum exposure turned out to be about 150 seconds for the OHP film, without the press. Using the F100TDL film, and the press I have had to increase this time considerably. However I am still fine tuning this.
Developing the Board
After the board is exposed, you should keep it in a light tight box or folder until you develop it. The sooner you develop the board the better. The developer is used in the same manner as the resist. Depress the bottle to get the developer flowing and then daub the developer all over the board. Let it sit for about 30 seconds and then gently wipe the developer all over the board. You will see the resist pattern form as you do this. Note that it is not a very dense pattern, it normally forms a a light green line. The total development time is about one minuite. Note that the developer is a little temperature sensitive if it is below 18C it tends to be rather slow acting, and may not work well at all. Above 23C it can be rather agressive and over develop the boards. Getting the development right, is the most important part of the process.
Etching the Board
As I do not make that many boards, I still use the simple tray method. The board is dunked in ferric chloride and gently rocked. I do warm up the ferric before use to speed the process up. I use the standard solid pellets bought from the likes of Maplins and Rapid, and mix them up as I need them. When the boards start taking a bit too long to etch I replace the solution. Do note that the solution does discolor quite quickly so you do need to lift the board out of the tray every so often to check on the progress. I find that it takes about 15 - 20 minutes to etch a board. Once the board is etched and washed, there is a etch resist stripper that can be applied to the board. This will dissolve the remaining resist easily, leaving the copper fairly clean. a quick scrub with the polishing block is then all that is required. SENO also produce a flux coating that can be applied to the board, to prevent the copper oxidizing, however I have never used this product.
The Results
This first test was with tracing paper, and I also used it as a test strip to get some idea as to the required exposure. The board was exposed is 30 second steps, and you can see that at about 150 seconds the best results are achieved. As the exposure gets longer, the tracks start to become thin and discontinuous. The other thing to note is the large number of copper specks covering the board..
The next test was with OHP film, and this proved to be much better. This was a single 150 second exposure. The tracks are fairly well defined, and this would be fine for general use, especially if track widths of 18 th. or greater were used. There is still a lot of copper specks over the board, so the tracing paper in the previous test was not to blame. The specks turned out to be contamination in the photo resist. When this bottle arrived, it was obvious that it had been leaking. I assumed that the valve had simply bee knocked during transit. Now I think that this was not the case, and that the resist had been exposed to the air for a long period and this was causing the problem. I discarded this resist and used a new supply.
The last set of tests is using the F100TDL and the press. The results of this are much better, the track edges are clean and sharp, and there are virtually no specks on the board. The image was scanned in at 300dpi. There is an area of excess copper between the last two 50 th. tracks in the middle of the board. I have identified this as a fairly deep score mark in the Perspex press. I will polish that out in the next few days or so. The second image is a smaller section of the same board, scanned in at 1200 dpi. here you can see that even the 8 th. tracks are very well defined.
