A lightbox for exposing photosensitive copper clad boards for producing
high-quality single and double sided Printed Circuit Boards.
Having made a few PCBs using the Press-n-Peel
system (which is available from Maplin, CPC and other UK
outlets) with varying results I decided that I would likely
get
more consistent results producing PCBs using photosensitive boards
instead. Commercial lightboxes use cold cathode tubes, which can give
problems in
cheaper units because they only contain two tubes which does not give
good coverage
over larger PCB areas.
During a search for lightboxes on google I came across a guy who had
built
a small lightbox using ultraviolet LEDs. These must be a fairly recent
addition to the LED family because I had never come across them before.
A quick search on
eBay later and I had found a supplier doing 100 UV LEDs for
£9.95. Given that
a commercial lightbox would cost upwards of £100, and given
that second-hand examples
regularly fetch a good fraction of that, I thought I'd have a go
building one myself. With
any luck this would be my last Press-n-Peel PCB!
The printed circuit board I've designed uses 96 of the LEDs arranged in
an 8x12 grid. The LEDs
are spaced 25mm apart. This gives an effective exposure area of 325mm x
225mm
which allows a PCB to be
exposed up to A4 size (or slightly over if using a 300mm width board).
The LEDs I bought have a 4.5V forward voltage (this is typical). On the
PCB they are
arranged in groups of 3, which means that with a 12V power source
there is no need for limiting resistors. Hence the simplicity of the
board. In total the LEDS draw
approximate 1 Amp on a 12V/13.8V source.
The distance from the LEDs to the board to be exposed should be
approximately 100mm (a bit more is ok, less and the LEDs do not provide
overlapping coverage). Using
simple trigonometry this gives a small overlap between the beams of
light, which are 20 degrees
wide (if your LEDs coverage varies from this then you will need to work
out the distance required). Position a piece of standard 4mm glass
about 100mm above the UV LED board, place the artwork (on transparency
film or even better paper specifically designed for this task) on the
glass
then the photosensitized copper-clad board on the artwork (remove the
backing film first obviously). By experimentation I've found that
a two
minute exposure is plenty. It is recommended that the artwork be
created without mirroring so that the printed side can be placed next
to the PCB.
The board should then be agited in developer at 20 degC - 25 degC for
30 seconds before being thoroughly rinsed under running water. The
board can then be etched in Ferric Chloride at 25 degC. For a small
board this can take about 10 minutes. Scrub the board with wirewool,
drill then populate!
Total lightbox cost: £25.
Warning: if
you decide to build this board, do yourself a favour and take
precautions not to look directly into the LEDs, or indeed at the LEDs at all
for any more than is necessary. Ultraviolet light is dangerous to the
human eye, and even though these LEDs do not look bright remember that
most of their output is in the invisible ultraviolet light range - so
they're a lot brighter than they look!
My first project created using the new lightbox is a 556-based timer
circuit to control the UV LED board (strikes me as a bit recursive,
which is always fun!) The timer circuit runs off 12 volts (although
this requirement is really down to the relay used as the 556 will
happily run off anything between 5 and 15 volts). The time delay is
controlled by an onboard variable resistor which allows a delay of
between 1 and 10 minutes (approximately) to be preset. Alternatively, a
suitably calibrated external variable resistor would allow the delay to
be set as required each time the timer is operated. The time delay
starts immediately power is applied, and results in a green constant
LED being lit and a yellow LED flashing with just over a 1 second
interval. The relay is energised, thus connecting the external device
to the power source. When the time delay has elapsed, the relay closes,
the green and yellow LEDS extinguish and the red LED lights.
