DJ Delorie wrote:
> 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.
>
Thanks, I get it now.
I have been doing it wrong all this time. Previously, I would estimated
exposure purely on the appearance of the film after development. If the
film lost its shine after developing then that means it was attacked by
the developer and therefore underexposed. Looks like I have to wait for
my step gauge to arrive.
So the photoresist professionals say you must expose the resist such
that when you put a 21 step gauge, the resist under step 8 should be
about "%50" cured. I assume "%50" cured means its gets heavily eroded by
the developer but not quite enough to be completely removed.
In this situation, the actual energy during PCB exposure will then be
11.3137 times the amount of energy needed for "%50" cure. I think this
is what your saying above.
Adam