Homebrew PCBs

Adam Seychell <a_seychell@...> writes:

> So lets say your test expose for 120 seconds with the step gauge and
> full cure occurs at step 4.

If it were my film, I'd need to expose my boards for 480 seconds, the
equivalent of holding step 8.

> Exposure time for PCB fabrication will then be:
> 120 / (1.41 * 1.41 * 1.41 ) = 42 seconds

No.  I think you're misunderstanding what the step gauge is for.

> I cannot understand why you need a 5.5 minute PCB exposure based on
> your results. A 42 second PCB exposure agrees a lot more with my
> experiences.

Because the film's spec says so.  The film's spec says that, for
optimal performance, you must overexpose by 8-16x.  This may take into
consideration more than just "the film doesn't dissolve in the
developer" but also adhesion, edge geometry, resistance to abrasion by
spray tanks, etc.  It also may be the halfway point between "the film
is partially cured under the transparent areas" and "the film is
partially cured under the opaque areas" for your pcb pattern.

Note that my setup and yours are different.  Also, based on a 120
second exposure holding step 5, I could go as low as 30 seconds and
still cure *some* of the film.  For me, a 45 second exposure would
probably work some of the time, but I'd have a high risk of
underexposed portions.

Let's go by mJ/cm2 instead.  The spec for my film recommends 25-60
mJ/cm2, about a 2.4:1 range of times, or about two steps (2:1 time).
Let's assume 40 mJ/cm2 is our "midpoint".

Now, let's say that by trial and error we've determined that 2.5
mJ/cm2 is enough to START curing some of the film.  If you tried to do
a pcb with this exposure, some copper sections would be uncured, and
would wash off in the developer.  This 2.5 mJ/cm2 is the exposure at
which 50% of your exposed film cures, and 50% remains uncured.

So, you have to expose MORE THAN 2.5 mJ/cm2 to ensure that all your
exposed film cures.  But how much more than?

Ah, that's what the spec tells you.  Assume it tells you to expose for
16x more than the "bare minimum".  So if you determine that 2.5 mJ/cm2
is the bare minimum, you can calculate that 40 mJ/cm2 is the ideal
exposure.

So what's the step gauge for?  Each step scales the UV hitting the
film by some specified amount.  For the SST 21, each step passes
1/1.414 the UV as the step before it.  So each step passes some
fraction of the total UV.  Each step is thus a test for "Am I giving
it N times the minimum exposure?"  It's like performing 21 exposure
tests simultaneously.  The result is that you can determine how much
more than a minimum exposure your current time is providing.

If you almost hold step 5 (i.e. part of step 5's film is cured), for
example, you know that you're giving it 4x the minimum exposure.  You
also know that you're giving it more than 2.8x the minimum (step 4
holds completely) and less than 5.6x the minimum (step 6 is uncured).
If we want a step 8 exposure, we now know we need to give it 2.8 times
the UV to get there.  So we increase our exposure time by 2.8x and
test again.  Now we almost hold step 8, showing that we're giving it
11x the minimum exposure.  Step 7 holds, so we're giving it more than
8x, and step 9 is uncured so we're giving less than 16x.