Homebrew PCBs

Hi Russell,

In "Re: [Homebrew_PCBs] PCB light box - hf ballast / tube wattage - 400W
floodlight instead?" you wrote:

> I got a 380nm 10W LED. I haven't tried it yet.
> 
> http://www.ebay.com.au/itm/10W-Ultra-Violet-UV-High-Power-Led-Light-380nm-385nm-/230760896509

Wow!!!

> You can get a 100W one for "only" $999.99 ;)
> 
> http://www.ebay.com.au/itm/380nm-385nm-Ultra-Violet-UV-High-Power-LED-Light-100W-/360445882648

I think the 10 watt (power consumption) one would do the trick.  In
PCB-DIY I don't think we are in a tearing hurry for exposure time.

I think you would need to attach it to a fancy heatsink, with
appropriate heat conductive paste, to keep it cool and happy.

This would make a very small angle source of UV for exposing Riston and
the like.  This would make it less critical to get the phototool in
close contact with the Riston.

I don't know what the UV output of the 400W floodlight is.  However it
should be possible to guess an exposure time at a similar distance - in
my case 45cm for this LED.  Let's say this LED put out 70% of its light
on an area 50cm x 50cm.  Assuming it puts out 1 watt (as the eBay
listing suggests, for ~1000mA at ~10 volts) then this is 7 watts on 0.25
square metres, or 28 watts per square metre = 2.8 x 10^1 watts per
square metre.  This is 2.8 x 10^-3 watts (2.8 milliwatts) per square cm.

Looking at a Riston datasheet, for MM540:

http://www2.dupont.com/Imaging_Materials/en_US/assets/downloads/datasheets/mm500series.pdf

the exposure energy for 350 to 380nm is (table on bottom right of page
2) 25 to 55 millijoules per square cm.

So according to theory, this LED should expose the Riston in 10 to 20
seconds!

For larger boards, to get an even exposure, it would make sense to move
the LED to a greater distance, with exposure time quadrupling for each
doubling of distance.

A significant advantage of this LED compared to the 400W floodlight is
no significant heating of the PCB and phototool, and therefore no need
for a fan.

> I was going to try it for curing UV glue and ink, but it would
> probably do good for photoresist too.

I think it would be excellent for all these purposes.

> An array of 1W LEDs could be an option too.

Yes.

> Do not stare at these when illuminated. It could cause
> photopolymerisation (cloudiness) in the eye.

Unlike a quartz halogen floodlight, which is seriously bright, the UV
LED may not seem vary bright to the eye.  I guess it would activate our
retina to some extent, since it is so bright and since its spectrum
presumably has some width extending into our violet range:

  http://en.wikipedia.org/wiki/Cone_cell

I think eye protection would be essential for a device like this.  I
wear sunglasses when operating the 400W floodlight.  Maybe that would be
sufficient, since this is near-UV, not the same as shorter-wavelength
mutation-producing UVB which causes sunburn and cancer.

  - Robin          http://www.firstpr.com.au/pcb-diy/