Back in my PCB-making days, I used photoresist coatings
exclusively.  If everything is done just right, you get excellent
resolution and repeatability.  (If not, you get a lot of frustration and
wasted materials.)

In my experience (which is a few years old) there were two
fundamental types of photoresist: Negative-acting (Kodak and
Dynachem), and Positive-acting (Shipley).  These resists are
marketed under various names, so it's sometimes hard to tell
which brand you have. DuPont also makes a laminated-film
type of resist, but that was always beyond my budget.

In general, the negative-acting resists use some type of
hydrocarbon solvent as a developer, such as Xylene.  The positive
ones use variations of sodium hydroxide solutions, as the caustic
soda you mentioned.  Development is kind of an all-or-nothing
process.  The idea is to dissolve the unwanted resist, while leaving
the PCB resist pattern intact.  Both Shipley and Dynachem resists
were colored so you could get a clue when to pull the board out of
the developing solution.

Exposure time is not critical.  To get your process "right on", go to
the camera store and pick up one of those little 14-step gray scale
guides, and expose a few small boards with it using various times
like 5, 10, 15 minutes.  Pick the time that results in the bottom 1/3
of the scale leaving solid resist, and the top 1/3 being cleanly
washed away.  For Kodak KPR4 resist and a No. 2 photoflood bulb
at a distance of 10" from the PCB, six minutes of exposure was
just right for me.  Shipley resist wanted much more time, maybe
20 minutes or so.

Tip: I found it important to use as much of a point-source lamp as
possible for best resolution.  Those exposure frames with banks of
blacklights always blurred things out too much when I tried them.
If you're really serious, you can get *great* results from a carbon
arc lamp.

The final trick is to get an exposure frame that keeps the PCB in
*intimate* contact with the film!  (Yes, this is crucial.)  A vacuum-
back frame will give you perfect results every time, if you can afford
one.  I found those little wood-and-glass things with a spring on the
back never really did an adequate job for me.

email me if you want further useless info  ;-)

Brian Aase

> Hi Picsters, it's been a while..
>
> I am experimenting with these kind of boards to see if it is worthwhile
> for prototyping.
> What can I use for developing and exposing?
>
> Years ago a played with something called Positiv20. It's a spray on
> photosensitive (and a pain to use, so don't ask me about it, hehe).
> I exposed the board for 30 minutes in the sun and used Caustic Soda to
> develop it.
>
> will it work for pre coated boards?
>
> If I have to build an exposure unit, what wattage of lamp must I use?
> Will Metal-Hallide lamps work (used for printing plate exposures)?
>
> Oh, and BTW, are the boards positive or negative exposure?
>
> Thanks
> Quentin
>