I just wasted about $30 in trying to create a large PCB. I made two attempts of removing the photo-resist correctly from the exact same circuit image but failed.
In the first attempt, the board was slightly bigger than the container with developer solution (sodium hydroxide) and water. I did each side one half at a time. The attempt looked wonderful until I noticed that in the dead center, all the tracks were missing and mostly faded.
I then made one more attempt. This time, I emptied the old solution and measured one part of developer to 11 parts of water (even though the bottle indicated 10) and as I was developing, things appeared normal at first as the image slowly appeared (which is what I want). As I examined the board, I noticed the exact same spot had the tracks missing AGAIN!
So both attempts produced exactly the same results.
Normally I do boards smaller than 11cm, but this one contained 3 circuits on it and is 15cm by 12cm. I was trying to save myself time by doing multiple circuits on one board.
Now what I used to hold the board together is a picture frame with clips on all sides and nothing blocked the light from reaching any spot on the board.
for the light, I used a UV blacklight and (call me crazy) but in the past I had success with smaller boards when exposing them for one hour, so I gave these large boards an hour exposure as well.
When I think about it I'm guessing the glass from the picture frame wasn't able to hold the board down 100%. Maybe I need to put heavy glass on top of the glass I got to force it down, but then again, will the light shine through?
I also attempted to make things better by adjusting the position of the board every 20 minutes so light can reach all of the board. My homemade exposure unit contains only one 7 inch blacklight and the roof of the unit has an aluminum pie plate covering the top so light that shines to the top can bounce back down (am I stupid to move the board during exposure?)
Can someone suggest the best solution? I feel I only have two options.
- buy multiple sheets of glass to force the picture frame glass down so that the artwork literally sticks to the PCB 100%
- waste time, lose mega dollars and buy a professional exposure unit
- be forced to break down my larger circuits and connect the smaller circuits together via wires.
but if someone else has a better idea on how I can make a PCB with the photographic method, I'd like to know.
The minimum track width I'm using is 0.25mm and I never had problems like this before with a 0.25mm track width, and in both tests, when I print my artwork out, I print two copies on transparency and line them up to get super dark black traces and the transparency image is what I expected to see on the board when its developed, not a patch on the board where it appears over-developed.