PCB developing: How do I make the film stick to the PCB during UV exposure?

[Plecto]

Hi. My homemade PCBs are usually of OK quality, but I often get some traces merged together. I know that this is usually because of some distance between the transparent film and the PCB which causes the light to scatter, this is largely a problem on bigger boards. I wonder if it's possible to make the film stick to the PCB, some sort of oil or really light adhesive glue or something? Any tips?

 

[betwixt]

The image side of the transparent film should be directly in contact with the sensitive PCB. Ideally what you do is use a sheet of glass, place the transparency on it, place the board on top of that then apply pressure to the back of the board to squeeze out any air gaps. You project the UV through the glass side.

If you are preparing the transparency correctly, it should be a mirror image of the final track layout so when it is lying on the board, the printed side is touching the sensitive paint so there is no space for the light to scatter.

Brian.

 

[Mattylad]

As above, also masking tape works well.

 

[Plecto]

Well this is what I'm doing today. The printed side is touching the PCB, I always make sure of that, but the final result is rarely as good as it can be when doing bigger boards. I'm using two pieces of plexi glass and they are as fare as I can tell perfectly straight. I also often place something heavy on top during exposure, but still no good. Usually things work out okay, and the parts that are good (usually 90-100%) are perfectly good, but I would like consistent results every time. A saw a guy using 3 in 1 oil in a youtube video, he said that this would make a really professional result. I don't really know what type of oil this is exactly though. I am making double sided boards though, single sided boards give more consistent results (not sure why).

 

[betwixt]

3-in-1 is a trade mark for a general purpose oil with added solvents, it's for freeing stiff hinges and similar so I would not recommend you use it in case it contaminates or chemically reacts with the photo-resist layer. In any case, the oil could only increase the distance between the artwork and board, making the problem worse.

Plexi-glass is not a good choice, I'm assuming it refers to an acrylic 'plastic' glass. Some types are slightly fluorescent in UV light so it could provide a light source penetrating sideways close up to the artwork. When you say "two pieces of plexi glass" where exactly is the second layer? You should have it set up like this, starting from the bottom and going up:

light source
glass layer
artwork (printed side up)
PCB (sensitized side down)
pressure applied evenly.

Brian.

 

[Plecto]

I sandwich the PCB between the two pieces of plexi-glass, it's so I can turn it around without the PCB moving. I will get two pieces of regular glass then, see if that will make things any different

Thank you for the tips, I will be back if for some reason the regular glass doesn't work

 

[Plecto]

Two pieces of glass sheets did the trick, thank you