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'[EE] PCB Positive Photo-resist'
2006\11\09@111526 by Rolf

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Hi all.

I am doing some hobby-related playing with building my own PCB's... I am
using the positive-resist coated PCB's, and am pondering a really basic
solution to being able to see what I am doing while working on it
(drilling register holes, aligning transparencies, etc).

The instructions call for a photographic "safe-light" (red), or a yellow
25Watt "yellow" bulb at least 15 feet away (or a yellow-sleeved
fluorescent bulb?)....

Since the resist is responsive to UV, I figure I can use Red LED's.
LED's have a very narrow spectrum emission, and I figure it must be far
more narrow than a safe-light.

My point is that I have a red LED brake-light cluster (originally) from
a truck that is *very* bright (as well as a 24V DC power-supply), and I
would be easily able to work the board with it's light, but, would it be
safe?

Anyone see a flaw in this reasoning?

Rolf

2006\11\09@121035 by Howard Winter

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Rolf,

On Thu, 09 Nov 2006 11:18:02 -0500, Rolf wrote:

{Quote hidden}

I think you'll find it's *most* sensitive to UV, but that its response against wavelength of light is a curve of some sort,  so it will be *somewhat*
sensitive to red, even a single red frequency.  Note that they specify the distance, so there is some effect from a safelight, but by keeping the
distance up / light level down, you are reducing it to a negligible level.

> My point is that I have a red LED brake-light cluster (originally) from
> a truck that is *very* bright (as well as a 24V DC power-supply), and I
> would be easily able to work the board with it's light, but, would it be
> safe?

Maybe, maybe not!  :-)  You don't know how sensitive it is to the colour of your chosen LEDs so you don't know what light level is completely safe.  I
think you'll need to experiment to find out.  At least shine the light upwards or against the back wall, so you are getting reflected/diffused light on
the worksurface, rather than a direct beam.

Cheers,


Howard Winter
St.Albans, England


2006\11\09@121742 by Orin Eman

picon face
On 11/9/06, Rolf <spam_OUTlearrTakeThisOuTspamrogers.com> wrote:
> Hi all.
>
> I am doing some hobby-related playing with building my own PCB's... I am
> using the positive-resist coated PCB's, and am pondering a really basic
> solution to being able to see what I am doing while working on it
> (drilling register holes, aligning transparencies, etc).
>
> The instructions call for a photographic "safe-light" (red), or a yellow
> 25Watt "yellow" bulb at least 15 feet away (or a yellow-sleeved
> fluorescent bulb?)....
>
> Since the resist is responsive to UV, I figure I can use Red LED's.
> LED's have a very narrow spectrum emission, and I figure it must be far
> more narrow than a safe-light.
>
> My point is that I have a red LED brake-light cluster (originally) from
> a truck that is *very* bright (as well as a 24V DC power-supply), and I
> would be easily able to work the board with it's light, but, would it be
> safe?

What make of board?  The MG Chemicals boards that I use aren't that
sensitive.  It takes a 6 minute exposure using inkjet transparancies
and the official MG chemicals UV tube... a little non-fluorescent
light doesn't do them any harm.  Still, they have a peel-off plastic
cover that can be left on until ready to expose the board, meaning
only the step of aligning the transparancy really exposes the board to
light.

Orin.

2006\11\09@122634 by David VanHorn

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Almost certainly ok, but you can test with the light.
Do a simple pattern, and expose it with the LED light say a foot away, for
an hour.
If it dosen't work, then you're ok.

The resist may be entirely unable to work with red light due to not enough
energy per photon.
If so, it dosen't matter how many photons you have.
That's what got einstein going on the photoelectric effect.

2006\11\09@130937 by Peter P.

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Rolf <learr <at> rogers.com> writes:

> My point is that I have a red LED brake-light cluster (originally) from
> a truck that is *very* bright (as well as a 24V DC power-supply), and I
> would be easily able to work the board with it's light, but, would it be
> safe?

In total darkness a single red led (high brightness type) will be plenty to work
with. It is your eyes that need adaptation. As in any photographical work, do
not shine the LED directly into the chemical tray, shine it onto a wall so you
get indirect light. You can remove the board and hold it directly against the
led to check if it properly developed for a few seconds.

But for most commercial photoresists for home use, using subdued (indirect)
light, even fluorescent, will be ok. Again, do not allow the light to shine
directly onto the board, bounce it off a wall. If you are really nervous you can
buy red gelatine filter from a theatrical shop and wrap the fluorescent tube
with it (or tape it in front of a gooseneck type PL lamp).

good luck,
Peter

2006\11\09@132640 by Mike Harrison

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On Thu, 09 Nov 2006 11:18:02 -0500, you wrote:

{Quote hidden}

Normal photoresist board is not sensitive enough to normal light to need any special precautions -
just avoid direct sunlight and any unnecessary exposure.

2006\11\09@142828 by Mircea Chiriciuc

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I'm doing my photoresist boards myself for a while now (more than 2
years) and I can tell you that I coat the board using a photoresist
spray, I dry the coating in a oven (a microwave oven with grill - not
using the waves, just the grill to heat up the air ;-) 140 degrees C
for 4 minutes and let it cool down in the oven for another 10
minutes), then I do the film alignment and expose the board (I expose
the board using two 18W UV tubes at 15 cm from the board for 12
minutes). Finally I develop the board and etch it. Al the above are
done in daylight, incandescent light or even fluorescent light with
no problem as long as you don't allow the board to stay exposed for
too much.  And as others have said, do a small test: try to expose a
small piece of a board direct to the leds light from 10-20 cm for
15-30 minutes, or even an hour. Try to develop the board after that.
If the exposed pattern doesn't come up, you're free to handle the
boards as you may see fit using the leds as a light source.

My best,

Mircea Chiriciuc

2006\11\09@153457 by Dwayne Reid

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At 09:18 AM 11/9/2006, Rolf wrote:
>Hi all.
>
>I am doing some hobby-related playing with building my own PCB's... I am
>using the positive-resist coated PCB's, and am pondering a really basic
>solution to being able to see what I am doing while working on it
>(drilling register holes, aligning transparencies, etc).

If I can interject something here - you shouldn't need to use
register holes.  I've written about this many times previously (check
the archives) but will repeat it again:

Grab a chunk of un-etched copper-clad PCB material and cut it into a
"L" shape - something like a small version of a carpenter's
square.  The ones I make usually have legs 4" and 6" long; the legs
are about 3/4" or so wide.  I use copper-clad PCB material because it
is the same thickness of the board you are going to expose.

Tape the bottom transparency to the legs (both legs).  Now flip the
whole thing over and tape only the long edge of the top transparency
to the long leg, taking care to get the drill holes lined up exactly.

Now: just slide the sensitized PCB material into the pocket formed by
the 2 transparencies, stick into your vacuum frame (you DO use a
vacuum frame, don't you?), expose.  My vacuum frame is made from 2
similar pieces of glass so I simply flip the frame over to expose the
other side after the 1st side is done.

The reason for taping one transparency to only one leg is that I find
that the transparencies stick to the photo-sensitive emulsion during
the expose cycle.  Taping only 1 edge allows easy peeling of the top
transparency from the board; once it is free, its easy to peel the
bottom transparency from the board.

Quick, easy, accurate.

dwayne

(Dupont Riston 4315 photo-sensitive film laminate, Kepro laminator
(much modified GBC unit), mercury-vapor expose lamp with timed
shutter, potassium carbonate developer, ammonium persulphate bubble
etcher, sodium hydroxide stripper, Gordon Robineau's really cool PCB drill).

--
Dwayne Reid   <.....dwaynerKILLspamspam@spam@planet.eon.net>
Trinity Electronics Systems Ltd    Edmonton, AB, CANADA
(780) 489-3199 voice          (780) 487-6397 fax

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2006\11\09@213410 by Rolf

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Thanks all, Dwayne, Mircea

great information from everyine, and I like that tip about the 'L' frame
for alignment.

Rolf

Dwayne Reid wrote:
{Quote hidden}

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