I bought the following exposure unit to make my PCBs:

and instead of transparencies, I use vellum paper for the art. I tried a small strip of PCB (1/2 inch by 8 inches) and it came out decently. Now I tried a 4 inch by 8 inch PCB and the corners seem to be OK but the center looks as if someone scraped tiny bits off of the artwork in a uniform fashion. I set the exposure time in both cases to 15 minutes.

I begin to wonder how and if I should throw heavy weight on top of the exposure unit lid while its running,or should I change the paper and/or printer settings? If throwing weight on the unit is beneficial, how much weight? I dont want to go too crazy or I think I'll break the inside glass.

Also, how beneficial would it be if I added extra padding inside the unit on top of the circuit board?

When I have developed both boards I used the same technique of constantly having the board in motion so that the developer doesn't eat away at one spot for too long. The developer I use is MG chemicals (sodium hydroxide) and I use 1 part developer to 12 parts water so I can see the results gradually appear instead of over-developing..

so is weight the answer? or changing the paper the answer? or should I just limit the size of the circuit boards I should try doing?

 

Typically, the exposure level varies over the surface, it's really hard to design something like that yo make it even over the surface.

I would hazard to guess that your process of using velum does not provide 100 % opacity to the exposure light, or the velum attenuates the light too much in the open areas. This exposure variation degrades the crisp edge that is required for good PCB's.

The fixes might be:

1) Increase the contrast and opacity of your artwork.
2) Control the overall exposure more accurately.

 

My unit comes with a timer so #2 is possible. #1 is difficult because my printer (like most lazer printers) doesn't print black the best.

 

Do a test strip- print a grid of lines, expose the strip progressively - in XX second exposure increments using an opaque sheet.

Process it and look at the results, this should answer the question.

 

So is my problem then under-exposure? The paper i used is 120gsm vellum