Electronics > Manufacturing & Assembly

UV Exposure times

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I am about to create my first home made PCB using photo resist board. I have a UV box, and a transparency with the artwork. All I need to do is place the artwork on the glass, then the board on top of it, close the lid, and set the timer for a few seconds.

I am not sure how long I need to set the UV timer for, does anyone know?

It is "system-dependant" (PCD-light source distance, light power, light type, ...) so you will have to test.
Try at 30s, 60s, 120s, 240s, select the best result and them fine tune exposure time.

The best way to identify the correct exposure is using a STOUFFER scale.
Here some more details on its usage:

The price of this scale is rather low and it simplify your life a lot!

It depends, you should make test board where you cover parts of it after some time of exposure. So you can test different exposure times on the one board. Start from 10-15s. It also depends on transparency contrast. If it is bad you will have a hard time as unwanted places will be also exposed. You would need very precise exposure time. If contrast is really good then exposure time is very tolerant and can be changed by couple of times without huge impact on board quality. Which photoresist do you use? From my experience, positive 20 and other positive ones are not worth to mess with. Especially if you print photo-mask on regular inkjet printer (laser is usually even worse). Negative dry films are the way to go as they are more process tolerant, have better resistance to etchant and overall result in better quality.

I used 3 minutes as a starting point because that is what was mentioned on the photo-resist PCB instructions.

I then developed it with mediocre results. As it can be seen the sides of the board have not got exposed adequately. The problem is the UV exposure - it is not uniform and it is stronger in the middle than on the sides. I am not sure yet how to fix this problem on my UV box.

I attach pictures of the problems:

1) The UV machine with the artwork on top in a dark room - now I can clearly see the uneveness of the light... Considering the UV box contains 6 tubes inside I am very surprised.

2) The board as it was being developed - you can see the photoresist is not coming cleanly off the edges of the board because it has not been exposed to UV light correctly

3) The board as it is being etched - again the copper is not coming cleanly off the edges - this is puzzling because much of the surviving copper is not at all covered by photoresist yet it is not coming off - I have no idea why this is happening.


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