Peter L. Peres wrote:
> On Mon, 13 Jan 2003, mark@MBCC.CO.UK wrote:
>
> *>the image to be blurred.  I guess what I'm interested in finding
> out is *>if anyone has had any practical experience of focussing an
> LED's beam to *>close tolerances.  Maybe then I'll give this a go!
>
> If 1.6 microns is small enough then look at the optics of a CD head.
> My probably very wrong calculations show that a 1mW UV led painting a
> 0.5x0.5mm image should move at 15mm/sec to achieve the same exposure
> as my mercury arc lamp does. At this rate it would take ages to plot
> even a medium size board.
>
> Semiconductor manufacture technology does use photoplotters but these
> image an aperture from a wheel onto film or photoresist. The light
> comes from a specialised arc lamp ($$$) that often runs 1kW and more.
>
> Peter


Of course that TIME should be taken in consideration.
A photoplotter film requires low light, so a bright light can travel fast
over the film, and plot while it moves.

If the PCB photoresist requires one minute of a regular UV fluorescent
lamp, then the UV LED will also require to stay one minute parked over each
dot it will produce over the photoresist. It can take forever to plot a
complete PCB.

Of course, that speedier photoresist can be used, and powerful UV LEDs can
improve the speed.

A focus lens system can helps to make a tenth off mm bright dot over the
PCB, and if required, two disks can be installed 10mm apart, in the path of
the beam, with a hole with diameter a little bit less than the beam, so it
will cutout the beam feathers.

The major problem is related to ticker traces and wide copper areas.  For
those I sould implement an inteligent system that could change the UV dot
size, changing focus, and illuminating a wider area.  Of course it is not
totally necessary, since the multi-pass can solve the issue. Regular pen
plotters use to do multi-pass, since there is no other way, but I am afraid
that if the pens tip could be widen, some plotter would be using this
feature already.

This kind of photoplotting is being used in newspaper companies for long,
the final metal plate is done directly from a computer file, using
photoresist over a foil of stainless steel.

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