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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?

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  • \$\begingroup\$ Do I correctly understand that you are using multiple glass sheets smaller than your PCB? \$\endgroup\$ – JRE Dec 27 '16 at 22:52
  • \$\begingroup\$ no. The whole picture frame unit is larger than all PCBs I use and in all my tests, I have used one single glass sheet. \$\endgroup\$ – user116345 Dec 27 '16 at 22:55
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    \$\begingroup\$ Do you agitate while developing? Sometimes centres don't develop as fast because developer hugs the surface, and becomes tainted. Quartz is one of the few that passes UV. Many glasses & plastics attenuate deep UV, even near-blue UV. \$\endgroup\$ – glen_geek Dec 27 '16 at 23:01
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    \$\begingroup\$ What are you using for a UV source and how even is it. \$\endgroup\$ – RoyC Dec 27 '16 at 23:58
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    \$\begingroup\$ Build or buy a "Vacuum Frame". That will easily hold your PCB in contact with your negative and frame glass. That will eliminate that as your problem. The printing industry uses them for exposing lithographc plates. An example of diy network54.com/Forum/258889/thread/1368747248/last-1368830059/… \$\endgroup\$ – Optionparty Dec 28 '16 at 3:00
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Glass can most certainly bend.

A standard method to ensure good glass to art to board (or film) contact is to use a vacuum pump (might be able to improvise sufficiently with a vacuum cleaner) to get them all "pressed" together nicely. This is common for contact prints on a large scale and for PCB exposure.

Super-thick glass is not required. Typically there is a gasket larger than the largest working area needed, a glass a bit larger than the gasket, some layers of perforated material to aid in even vacuum pressure.

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  • \$\begingroup\$ a gasket? So I should buy something called a gasket? I'm trying to understand because I never used a gasket for pcb production before \$\endgroup\$ – user116345 Dec 28 '16 at 20:42
  • \$\begingroup\$ See Optionparty's comment above. A gasket is part of a "vacuum frame" which is the same thing I'm describing. It allows the glass to seal so a vacuum can be pulled which presses the glass to the art to the board. \$\endgroup\$ – Ecnerwal Dec 29 '16 at 3:30

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