Homebrew PCBs

At least some usable data about led's.
So 10W led at 30cm need 60 seconds, That's seems little long for 
photoploting. But maybe isn't all lost as I thinking to put led way 
closer. Maybe just few milimeters (to have rom for aperture whell) or 
few centimeters if I need to put some lenses in betwen..
And 6mm thick glass should block a lot of light in that wavelength. I 
think it's worth to try.


On 09/17/2013 06:08 PM, Robin Whittle wrote:
> Short version:
>
>    10W ~380nm LED exposes Riston in about 60 seconds at 30cm.
>    A 3W one would be less expensive and require about 3 minutes.
>
> Previously I used a 500W quartz halogen linear incandescent lamp (with
> aluminium reflector) as my source of UV for exposing Riston.  However,
> it radiates a lot of visible and infra-red and so heats up the PCB and
> the phototool at the distances I was using it at: about 40cm, with a 4
> minute exposure.  This could cause problems with the phototool expanding
> at a different rate to the PCB and so messing up the image.  I described
> this on 2012-09-03:
>
>    http://tech.groups.yahoo.com/group/Homebrew_PCBs/message/30595?var=0&l=1
>
> including the ~7:1 contrast ratio phototools which I made with a Brother
> laser printer onto a particular laser transfer film.
>
> Riston, such as the MM540 I get from the Czech Republic:
>
>   http://www.ebay.com/usr/gaminn
>   http://www.tech-place.com/en/photosensitive-materials/23-photosensitive-film.html
>
> is exposed with near visible UV light.  According to the datasheet:
>
>
> http://www2.dupont.com/Imaging_Materials/en_US/assets/downloads/datasheets/mm500series.pdf
>
> the "peak response" of the material is 350nm (nanometre) to 380nm.  This
> is the same as in the other Riston datasheets I looked at, but I did not
> look at them all.  The shorter the wavelength of light, the more energy
> per electron is coupled, so 350nm light is more energetic than 380nm.
>
> UV LEDs are now obtainable in high power versions.  A 1 watt LED is a
> single LED chip and can be bought for a few dollars.  3W LEDs may have
> three chips in parallel.  10W LEDs have three in series, in parallel
> with two other sets of three in series.
>
> I found a wholesaler in China which evidently is source of many high
> power LEDs sold individually by eBay merchants.  This wholesaler has
> minimum order quantities such as 10.
>
> In their 10W High Power LED section they list the following UV wavelengths:
>
>   365nm
>   380nm
>   390nm
>   400nm
>   410nm
>
> The shorter the wavelength, the less light they put out, the more they
> cost and the harder they are to find.
>
> This is page for a 10W 380nm LED is evidently for the same LED I bought
> from an eBay retailer:
>
>
> http://www.leds-global.com/ultra-violet-380nm-high-power-led-modules-p-7.html
>
>
> This is item number G-P10UC140A1-XT.
>
> It is specified to run at 1050mA at which it will have 10 to 12 volts
> across it.  It will then produce 400 to 500mW of light in the 375 to
> 385nm range.
>
> The nine chips are arranged in a 3x3 array with outside edges of 5mm, so
> this is a nice small light source which should give sharp edge shadows
> on the photoresist even if the phototool is not pressed into direct
> contact with it.
>
> I bought the LED for USD$50 including shipping to anywhere:
>
>    http://www.ebay.com/itm/120896174810
>
> It arrived here in Melbourne Australia 9 days later with signed for
> delivery and a hand-written "Made in China" note.  This is a Hong Kong
> retailer:
>
>    http://www.ebay.com/usr/lucky_guy2010
>    http://stores.ebay.com/lucky4u2bid/
>
> It is titled "10W UV Ultra Violet 380~385nm High Power LED Light for
> Recognize Banknote".  There's no mention there of a part number.
>
> Running a few mA through it I see a rather broad spectrum with violet -
> like a pastel violet.  I assume I can hardly see the real peak of
> emission, so I won't be looking at it when it is running properly.
>
> I attached it a heatsink with small fan from a PC video display card.  I
> found an 18 to 19 volt regulated switch mode power supply and made up 9
> ohms of power resistors to wire in series with the LED.  This is one way
> of making a reasonably good constant current source.  I got about 1 amp
> going to the LED, including a few tens of milliamps for the small 12V
> fan motor.  This takes the LED voltage nearly to 10 volts.  So it is
> running pretty close to its recommended current and voltage.
>
> I found the ideal exposure time with 30cm distance, as before through
> 6mm of glass and the laser-printed phototool, was around 60 seconds.
>
> 20 seconds resulted in inadequate exposure - some photoresist remained
> but not enough.  40 seconds produced a good image.  So did 60 seconds
> and 100 seconds.
>
> This is at least a 2:1 exposure latitude.  Many people think that laser
> printed phototools are inadequate for exposing photoresist.  Perhaps
> with low-contrast photoresist (maybe some positive or non-Riston
> negative photoresists) this may be the case.  However, I find this
> Riston MM540 is high contrast.  This contrast enables the approximately
> 7:1 contrast ratio laser-printed phototool to work well, without being
> excessively fussy about exposure time.  "7:1" means that when I use a
> photodiode with visible light to see how much light gets through the
> dark part of the phototool, I find 1/7th the light than comes through
> the clear parts.  The dark parts are not entirely even, so the contrast
> between the clear sections and the lighter parts of the dark parts is
> probably 4:1, 5:1 or 6:1.  Anyway, it is still high enough to get good
> results with Riston.
>
>   - Robin         http://www.firstpr.com.au/pcb-diy/
>
>
>
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