I don't understand it...
When I tried developing a presensitized PCB, I never get perfect results for large boards.
If I use a small board (less than 6cm by 6cm) in a picture frame and use the picture frame clamps and expose the board for one hour, I get good results.
If I use a large board (15cm by 20cm) with the same method, then the middle section of the artwork on the board looks like it has disappeared, yet the rest of the artwork seems fine. It makes me wonder if picture frame glass can bend.
Now this time, I took the traditional method of using the same picture frame for a medium size board (9cm by 12cm). Like the professionals, I decided this time to not use the picture frame clamps and leave the glass just sitting on top of the artwork (because people assumed the glass presses the artwork down) and this time, I reduced the exposure time from 60 minutes to 30 minutes (where as in the last two cases, the exposure time is 60 minutes).
The problem with the last method is that when I examined my board, quite a few tracks were faint and some tracks looked like they're cracked to the point where an open circuit was present where there shouldn't be one, so I had to use a super-fine marker to fix the mistakes (a pain!).
I'm curious, does super-heavy glass exist where UV light can pass through it, because it seems in all these scenarios I have difficulty in keeping the artwork down (where every line touches the PCB).
Yes I have sanded the sides of the pcb prior to exposing it in order to elimate the copper bumps produced from sawing it to size (yes I saw smaller pieces of board with a utility knife from a larger board as that method is cheaper than buying multiple small boards).
I have also tried adding extra transparency sheets without much success. In fact, those transparency sheets seemed to add a few random copper dots to my board which don't wreck the circuit (thank god).
So does a super heavy/thick sheet of glass exist or should I stick hundreds of sheets of glass together in hopes enough UV light passes in one hour?
What can I do glass-wise?