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[on:]

I want to know how much time should i let a photosensitive pcb under UV light.I have noticed that the time dependes on the Watts of the light. The problem is that i have been told that i will need 7-8 minutes for 100watts lamp and in some youtube videos i have seen people that place the light to 60Watts for 40 seconds. Is there a basic solution for this? I have a 36Watts lamp(4x9Watts) i want to use. How much time should i let the pcb under the light?

[Reply #1 on:]

Your best bet is to check the manufacturer's web site and follow their recommendations.  If that's not possible, the next best thing is probably a test board.  Just make a long thin board with a test pattern on it, and draw a bunch of lines across it to mark the different exposure times.  Then cover the board up to the last line with a card and start the exposure.  Every few minutes, move the card to the next line.  After developing and etching, use whatever time that gave you the best results.  That's my plan anyways...  I'm just waiting on supplies to arrive to begin testing the photomask method of making boards.

[Reply #2 on:]

Depends on the distance of the light from the board.
Is this in a proper box with a glass top and lid ?

[Reply #3 on:]

i used to use uv / light box, i now use the laser toner method much much easer and better end result have a look on utube lots of guides there

[Reply #4 on:]

You must do control exposures to find out the right timing. It depends on many factors. Just make 1,2,3..10min long exposures on a piece of pcb and you will see.. (it is doable in 10minutes, btw)
PS: UV light is dangerous radiation, do not expose yourself..

[Reply #5 on:]

no this is not a proper fix uv box so there is no datasheet and manual.Basically this is a friend uv box for nails, but it has a 15x15cm empty space that i can use and the distance is  3-4 cm away from the pcb. So i will have propably to try?

[Reply #6 on:]

3-4cm away might be too close.  You need it to be far enough away so that the light has room to spread out a bit and cover the whole board.  If you stick a board down there, does it look like the light is fully covering it?  Or are there dark spots?  Light and dark spots won't work.

Oh, the datasheet that I was suggesting you check is the one for the photomask that you're using.  If it's a pretreated board, check the manufacturer's web site for suggestions.  Depending on what you're using, you may not even need UV.  Datak's newer pretreated boards work with a standard 100 watt bulb.  A standard blacklight may also work, depending on what wavelength you need.  If nothing else, you can always pick up a few UV LEDs and make your own box.   

[Reply #7 on:]

no this is not a proper fix uv box so there is no datasheet and manual.Basically this is a friend uv box for nails, but it has a 15x15cm empty space that i can use and the distance is  3-4 cm away from the pcb. So i will have propably to try?

You are pretty much spot on.
With the tubes 10 cm apart and reflective foil behind a would suggest an exposure time of about 5 to 6 min.
Past few years I have had boards from a French maker (cif) which only require 2 1/2 min.
Check on the manufacturers recommendations.

[Reply #8 on:]

Well the first test shows that 2.5 minutes are fine for the exposure.Thank you very much but now i have another Question:

Transparencies are plastic and plastic alters in UV exposure. Should i change the transparencies after exposing them for 3-4 times? or there is no problem and i can make as many pcbs as i want with the the first Transparencies i make(i type in plural cause i use 2 transparencies to make black color more solid)?

[Reply #9 on:]

I have never hat a problem, using OHP film or photo, are you using something else ?