# why did my pcb not expose properly

I sandwiched my pcb artwork onto my pcb using a photoframe then placed the frame on top of my leds. I exposed it for like 4mins. I was going to expose for 2 but I over ran abit lol. Anyway bits of it didn't even transfer. When it did transfer I could clearly see the traces but it was faint when I developed it. Admittedly I did staple two copies of my artwork together which meant the artwork wasn't pressed flat aginest the pcb . Does that matter?

Have I even brought the right type of pcb, along with some positive developer?

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what did you use to block the UV light? Many people assume that UV light penetrates as well as normal light but there are some things it passes through more easily and some less. A clear transparency actually only allowed around 50% through and had some scattering affects(putting light onto what you want to not expose). Exposing for twice as long is also a problem due to the light bleeding through the inked paper. We found a printer that really layered the ink on helped a lot. After much investigation we found someone had incorrected mixed the etchant. This all allowed 3-4mil traces. – Kortuk Jun 18 '12 at 14:43
@Ageis Can you post pictures of the PCB? Were the areas that were undeveloped exposed to the UV light as well as other areas? What did you use to print the artwork on? If its normal paper, try rubbing it with some sunflower oil and leaving it for a while to dry. The white parts of the paper will get more transparent this way. Also this depends a lot on the actual UV source. With my set-up, I need around 10 minutes to get good exposure. Make an experiment. Take start exposing a PCB, but place opaque film between the PCB and the UV source. Every 2 minutes uncover a part. – AndrejaKo Jun 18 '12 at 14:55
Calculate the number of parts so that total exposure time is say 16 minutes. After that develop the PCB and see which areas are underexposed, which are overexposed and which are correctly exposed. – AndrejaKo Jun 18 '12 at 14:56
@AndrejaKo - What? – Rocketmagnet Jun 18 '12 at 15:02
@Rocketmagnet What What?? I don't see anything too strange in my comments. If you do, please let me know. – AndrejaKo Jun 18 '12 at 15:03

You want transparency sheet pressed on PCB as flat as possible, no bubbles or gaps and you want it toner side to PCB. What i mean is you want toner to be as close to UV sensitive coating as possible, otherwise youll get shadows with blury edges.

Overexposure might be a problem too as UV does get thru toner in the long run. I usually lay two copies of a print for one side of PCB (as ideally aligned as possible of course)

Another opposite thing - you mention you are using a photoframe. Does it have a glass or plexiglass in it? UV is filtered out by glass, you need a plexiglass as it's more transparent for UV.

Edit: I usually use two copies of artwork and water - wet transparencies want to stick togeather because of water surface tension. The same for "glueing" it to PCB. It's just a matter of pushing air bubbles from under transparency - i use a couple of cotton balls - one hand to hold board in place and other to swipe bubbles to the sides.

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 interesting I didn't know I need plexi glass. I'm not sure if it's plexi glass. Probably not since it's a cheapo 1 pound frame. Thanks for the info. I need some proper gloves anyways. How do I get hold of plexi glass? – Ageis Jun 18 '12 at 16:10 plexiglass feels like plastic, glass feels like, well, glass. you can get plexiglass in a hardware store, i don't know where you live, so i'm not sure. – miceuz Jun 18 '12 at 16:12 ah ok and should I have a gap between the leds and my photofrmae. At the moment I just soldered the leds onto a custom pcb then placed the photoframe directly on top of the leds. – Ageis Jun 18 '12 at 16:17 i'm not experienced with UV leds, but my guess would be it all boils down to angle of cone of light leds produce - it could be quite narrow, you have to check the datasheet. you may want to have some distance so that leds cover the whole area as even as possible. – miceuz Jun 18 '12 at 17:13