# UV lightbox with UV leds physics?

Discussion in 'Electronic Design' started by [email protected], May 17, 2007.

1. ### Guest

I saw an discussion at:

Claiming an rebuilt scanner with 40 UV leds will properly expose an
pcb with the speed of 2cm/minute.

Known:
2 cm/minute
40 UV LED in line.

Assumed:
Scan area = A4 width = 210 x 297 mm
P_led = 3.8V*.04A (mentioned in the text)
t = 297/20*60
lambda = 400 nm

Math:
Q = 3.8 * .04 * t * 40 = 5417.28 Joule
A = .210*.297 = .06237 m²

PCB needs 86857 Joule/m² to be properly exposed.
Ignoring inefficiency of the LED, distance to pcb and it's beam angle.

If a board with each LED responsible for an 30 mm² area on a 100x160 mm board
is built it will need to shine for:
(.100*.160*86857)/(3.8*.040*60*((100*160)/(30**2))) = 8.57 minutes

Correct?

Especially how much energy is actually needed to properly expose an pcb board
photoresist..?
I'm interested in knowing practical numbers for this type of application.
And if the boards gets sharp?
Would pulsed UV led be more efficient?

http://www.elektor-electronics.co.uk/default.aspx?tabid=29&forumid=5&postid=1369&view=topic

2. ### D from BCGuest

Sacrifice 1 photoboard.
Make a test jig with masking and expose areas with various exposure
times.
From that you can derive a scan rate.

Cameras takes pictures with a flash.. If the photo resist acts like
film then pulse overdriving the UV leds may reduce the exposure time.
That will increase the scan rate too..

Damn..I should have kept my old paper scanner...I could have converted
it into a PCB exposer
D from BC

3. ### Don LancasterGuest

And the problem with using the sun is...?

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Synergetics 3860 West First Street Box 809 Thatcher, AZ 85552

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4. ### D from BCGuest

The sun is not as fun as UV LED's...

However...I should mention..I use a 500W halogen for PCB exposure..
Quick, cheap, simple, easy to get...

Some people just order the PCB from the PCB fab house.
Heck..some people (managers) don't even order a PCB...
"Hey you! Yeah... Order a PCB for me.."
D from BC

5. ### Michael A. TerrellGuest

I like to work at night when its cooler, and no one s bugging me on
the telephone?

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Service to my country? Been there, Done that, and I've got my DD214 to
prove it.
Member of DAV #85.

Michael A. Terrell
Central Florida

6. ### James ArthurGuest

Do they have sun in Sweden this time of year?

But seriously, we have to save all that solar energy for Al Gore.

Best,
James Arthur

7. ### Guest

Sacrifice 1 photoboard.
I wanted to avoid experiments initially so I can just do the math and figure
out how to dimension the build.
I think the photo flash is more like send all the energy in one go to
exploit short exposure times. While the LED pulsing would be to exploit high
peak values. Question is if the peak energy is more efficient than using
mean energy ..?

8. ### D from BCGuest

I think junction temperature, LED current and LED voltage affect the
lumens/watt efficiency.
See the datasheet.. I dunno what wacky things a UV LED will do..

I'd guess taking an LED beyond it's spec current will reduce its
lumen/watt efficiency..
The junction heat spoils the performance.
But it still will be brighter!..for the moment..and that might
increase the exposure efficiency.
Exposure like this..
POOF...move...POOF...move...POOF...move
The LED's get to cool down during the moving period..

But if the exposure time is long (more *flux/second) then I think
continuous max UV LED current would be better.

*flux/second <<Just made that up..dunno if it's a real unit..
(The brightness of a spot light that moves.)

Here's an idea..
Take a fiber optic cable, couple it to a UV led.
Then use a converted ink jet printer to move the fiber optic cable
around..
This would print negative photo PCB's. No mask required.
D from BC

9. ### Guest

The junction heat spoils the performance.

Maybe mount them in a heat sink then. Without obscuring the light..
Though LED body is usually mad up of plastics wich is a poor heat conductor.
Have three sections of LED's, then light them in sequence?
Why not just mount some LED's in the printer head.

10. ### redbellyGuest

Since you now have a ballpark figure of 8-9 minutes to work with, and
this Nordic person at diyAudio was able to make an exposure in 2
minutes, it's time to move on to the next phase: experimentation, as D/
BC suggested.

Try using a single uv led and see how much area it will expose on a
board. Vary the distance and exposure time. Then you can figure out
how many led's you need, how close to space them, and how long to make
the exposure.

Good luck,

Mark

p.s. did Nordic (from diyAudio) ever say what wavelength his led was?
There is almost certainly a strong, nonlinear wavelength dependence
here. What somebody did with a 390 nm led could take orders of
magnitude longer (or be impossible) to do with a 400 nm led. At this
point, the best way to find that out is to try with what you have.

11. ### joseph2kGuest

How about converting a 24-pin type printer, far less issues with the
existing drive electronics.