Homebrew PCBs

UV transmission of glass and acrylic.
UV LEDs for photoplotting direct to Riston.

Hi Slavko,

I think this exposure time is short.  Looking at the way various items
in the room lit up with flurorescence, I guess this LED (really 9 LED
chips in a single package) is putting out 50% at least of the light
produced by an old (they have been around for decades) 40W linear
fluorescent "black light", though I haven't fired mine up for years.

"Ultra violet" covers many wavelengths.  The UV at the central emission
band of these LEDs is about 380nm, which is very close to the short
wavelength limit of human vision, variously quoted as 390, 400nm or
whatever.

The transmission curves at:

  http://www.rayotek.com/techincal_info_glass_sapphire.htm

show that soda-lime glass (the basic mass produced glass I am using - it
was the protective cover on the 500W floodlight) does not reduce
transmission until the wavelength gets to 300nm and below.

The transmission curve for acrylic (PMMA) falls of much closer to the
visible edge.  This chart:

  http://www.goodfellow.com/larger-quantities/polymers/tpx-characteristics/

shows acrylic (PMMA) falling off around 400 to 405nm.

I think you you could use a microscope objective lens and a 1W single
chip ~380nm LED to expose Riston.  However, I guess the exposure time
would be something like a second or a fraction of a second, which is
probably too long considering that you need to expose tens of thousands
of ~10um to ~30um pixels one after another.

If you were using an aperture wheel then you would be exposing more of
the PCB at a time, so such exposure times might be OK.

For small pixel exposure of the Riston, I think a UV laser of the
correct wavelength - I guess 360 to 390nm - when focused by a microscope
objective lens would be much better, since the laser delivers all its
light in a beam, some or most of which can be used by the objective
lens.  Then, I guess, depending on the power, you might get the exposure
time down to a millisecond or so.  This would make it suitable for
direct photoplotting onto the Riston photoresist.  However, you would
have some challenges keeping the focal point of the lens at exactly the
right height.  If your lens had a very wide angle, the depth of field
would be too small to get good resolution unless your PCB was flat down
to 20um or so (which it surely won't be, unless you vacuum-held it to a
genuinely flat base) and if the geometry of your plotter was totally
even and aligned to this plane with similar accuracy.

I am perfectly happy with the resolution I am getting with laser printed
phototools and with the ease of getting sharp images in the Riston from
these.  Since I am only doing small hand-drilled PCBs - single sided so
far - I don't need to worry about slight scale errors which are no-doubt
present in the laser printed phototools.

If I did want to make larger PCBs for NC drilling, then absolute
accuracy would be important and a laser and lens based exposure system
would be required.

  - Robin

On 2013-09-18 3:00 AM, Slavko Kocjancic wrote:
> 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.