Author Topic: UV exposure box - microcandela needed?  (Read 2375 times)

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Offline fantis1337

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UV exposure box - microcandela needed?
« on: January 12, 2017, 01:26:52 pm »
The LEDs I'm using for double sided PCB UV exposure are rated for 2-300 MCD , 20mA at I assume around 3 - 3,4 V.

Are there any guidelines on how many of them I'll need? 

Google is giving me limited information or maybe I'm not searching for the right keywords.

Is it a trail and error thing where I should begin with maybe 20,000 MCD, expose some photoresist and see how long it takes vs how good it turns out?

Thanks
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Offline Buriedcode

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Re: UV exposure box - microcandela needed?
« Reply #1 on: January 12, 2017, 05:41:53 pm »
The trouble with the 'candela' rating is that is measured in a specific direction.  That is why many years ago now, we saw '20000mcd!' LED's, that had the same forward voltage, and same current (= same input power).  Sure LED's have got more efficient, but for the most part it was just a change of the housing lens that creating a narrower viewing angle - more light was focused to a narrower beam.

For UV exposure I would have thought uniformity would be paramount, otherwise hot-spots would over expose, and areas between them would under expose.  So I would say, use whatever LED's you have to hand, providing they are all the same.  Drive them at a much lower current to save your eyes, and shine them on a sheet of paper at the distance you're planning on having between the LED's and the glass for the exposure box.  If they're '20,000mcd' then I suspect you would have to hold the paper quite far away to get uniform light.

What I'm getting at is, more LED's = shorter exposure time, but also, with that shorter time, any non-uniformity could become a problem.  So experiment.  I am unable to provide a starting point because I did make a UV exposure box, but before it was finished, I realised it was just much cheaper to get boards made up (became really viable about 5 years ago, £20 for 10 boards, in 5 days, from China).
 

Offline fantis1337

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Re: UV exposure box - microcandela needed?
« Reply #2 on: January 12, 2017, 06:10:27 pm »
Thanks for all the information. I'll guess I have some testing to do  :)

The box is intended for use in a maker lab for students so it will get a lot of use I assume.

I might share the project on here on instructables when it is finished.
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Offline Buriedcode

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Re: UV exposure box - microcandela needed?
« Reply #3 on: January 12, 2017, 06:21:44 pm »
IIRC, I used piranha UV LED's, the ones without a dome lens, so the viewing angle was roughly 120 degrees. (bit like these http://lighthouseleds.com/media/catalog/product/ebayimg/5mm-3mm-flat-piranha-super-flux-led-uv-purple.jpg )  These were all the rage many years ago but I haven't seen many around, my point is, I would go for wide viewing angle to get a more uniform exposure in a lower form factor.  Using 5mm LED's I ended up using a white cardboard box that acted as a reflector (so the PCB got UV from all angles) that was rather deep (about 30cm..).  Perhaps PLCC2 would be handy, if you're ok with SMT.  I believe I've seen 3/5mm UV LED's with flat tops that have a wider angle, and are often available rather cheap.

Whilst most exposure boxes use UV tubes, it adds the complexity of a driving circuit that is often (but not always) driven by mains.  At least with UV's you can go with series-parallel banks and run the lot off 15V.
« Last Edit: January 12, 2017, 06:23:51 pm by Buriedcode »
 

Online Zero999

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Re: UV exposure box - microcandela needed?
« Reply #4 on: January 12, 2017, 10:48:06 pm »
The LEDs I'm using for double sided PCB UV exposure are rated for 2-300 MCD , 20mA at I assume around 3 - 3,4 V.

Are there any guidelines on how many of them I'll need? 

Google is giving me limited information or maybe I'm not searching for the right keywords.

Is it a trail and error thing where I should begin with maybe 20,000 MCD, expose some photoresist and see how long it takes vs how good it turns out?

Thanks
It's a trick question.

Units such as candela take into account the sensitivity of the human eye at different wavelengths, so UV leds have an output of zero cd. Lots of LEDs sold as UV output most of their spectrum in the shorter wavelengths of the visible spectrum, which does cause some fluorescence. The mcd figures are often exaggerated too.

In your case, it's the power output in Watts per cm2, over the part of the spectrum useful to you, rather than the brightness which is important. I believe 360nm is optimum for exposing PCBs but LEDs in this region are expensive. Fortunately, the shorter end of the visible spectrum will do (400nm to 440nm) although it's not quite as effective, as UV.
 

Online Benta

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Re: UV exposure box - microcandela needed?
« Reply #5 on: January 12, 2017, 10:55:09 pm »
Candela, cd or millicandela, mcd are units for visible light and useless for UV.
UV (and IR) light intensity is normally measured as W/sr (watts per steradian) for directional emitters, or just total radiated optical power (in W).
This again broken down per wavelength.

To give you an idea of the UV radiated power you'll need for making UV-coated PCBs, this is what I've used for 30 years with perfect results:
https://www.osram.de/osram_de/produkte/lampen/speziallampen/ultraviolettstrahler/ultra-vitalux/index.jsp

It's a 300 W incandescent UV spot.

My setup is a flat surface, UV-sensitive PCB-substrate placed there, transparent film held in place with a 4 mm glass plate.

The UV-spot is hung ~30 cm above this stack, which ensures that exposure is pretty much uniform for PCBs up to Eurocard size.

Exposure time is 5 min.


Now to your calculations:
The Osram lamp radiates 13.6 W at 315...400 nm and 3 W at 280...315 nm.
Around 50% of this power illuminates a circle of appr. 30 cm diameter, the rest is outside as stray radiation.
This corresponds to around 700 cm2 being hit with around 7 W / 1.5 W of each wavelength band.

That's the kind of UV energy density you need. Have fun calculating your LED demand :-)

Cheers.
« Last Edit: January 12, 2017, 11:05:50 pm by Benta »
 

Online wraper

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Re: UV exposure box - microcandela needed?
« Reply #6 on: January 12, 2017, 11:03:44 pm »
The trouble with the 'candela' rating is that is measured in a specific direction.  That is why many years ago now, we saw '20000mcd!' LED's, that had the same forward voltage, and same current (= same input power).  Sure LED's have got more efficient, but for the most part it was just a change of the housing lens that creating a narrower viewing angle - more light was focused to a narrower beam.
This is not true. Sure, with more directional lens you can get better spec in mcd, however not that better. Order of magnitude at most compared to opaque LED. Yes, LEDs actually became that much better in last two decades, power consumption the same but efficiency orders of magnitude better. Modern LED can be reasonably bright at 0.1mA current while old crap would be barely visible at best.
 

Online Benta

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Re: UV exposure box - microcandela needed?
« Reply #7 on: January 12, 2017, 11:07:56 pm »
Irrelevant. This is about UV.
 

Online Zero999

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Re: UV exposure box - microcandela needed?
« Reply #8 on: January 12, 2017, 11:15:13 pm »
Irrelevant. This is about UV.
Yes, candela is the unity for measuring luminous intensity. It only makes sense for the visible spectrum. Using it for UV, IR, microwaves, x-rays etc. is meaningless, like using torque to measure electric current.
 https://en.wikipedia.org/wiki/Luminous_intensity
 

Online Benta

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Re: UV exposure box - microcandela needed?
« Reply #9 on: January 12, 2017, 11:26:12 pm »
You already said that. Why repeat?
 

Offline sleemanj

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Re: UV exposure box - microcandela needed?
« Reply #10 on: January 13, 2017, 07:32:38 am »
I've built a few uv rigs now, the last one i used 5mm uv  leds on i think i had about 100 or 120, maybe a few less I forget now, spread over an a4 area, without diffusion but set at suitable long distance to provide even coverage, exposure time on dry film circa 3 minutes i think.  From memory I think they were set 25cm or so from the platen, trig can get you the minimum distance but you need to pull it back further than that in practice.

Current a4 capable rig  uses 8x1w leds, through two diffusers to cut down hot spots while keeping distance manageable, exposure time about 2m40s.

Currently building my...5th box, much smaller, planning on 2 or 3 1w leds only through at least 1 diffuser layer, the a4 size rig takes up too much space (size of a large laser printer) and i mostly do stuff 7x10cm or smaller, my instinct is exposure time will again be around the 2m40-3m mark, all the rigs i have made tend to work out about the same time.

End of the day, exposure takes as long as it takes, if you are exposing long time at low power or short time at high power, the total UV is the same.






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Online Zero999

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Re: UV exposure box - microcandela needed?
« Reply #11 on: January 13, 2017, 08:36:01 am »
You already said that. Why repeat?
a) Some people didn't listen.
b) I had forgotten to post a link in the other post.
 


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