Homebrew PCBs

DJ Delorie wrote:
> Adam Seychell <a_seychell@...> writes:
>> I'm interested in your exposure times. I get 60 seconds with negative 
>> dry film, grid density 0.28 LEDs/cm^2, BestHongKong 390nm UV LEDs at 
>> 20mA, about 100mm PCB to LED gap, measured with inkjet transparency + 
>> 5mm window glass in light path.
> 
> Just finished some tests.  With the glass and transparency I'm using,
> it needs 5.5 minutes of exposure to hit 8 on the step gauge.  That's
> with a ~5 inch gap and about 0.2 LEDs/cm^2, as reported before.  The
> step gauge helped - I got a perfect exposure on the second try, just
> by doing the math!
> 
> Note that for two out of two tests I forgot to remove the cover film
> before developing :-P
> 
> My etch tank seems to be a little pokey these days, though.  I suspect
> most of the H2O2 is just H2O by now.  It still etches, but takes
> longer.  Not that I'm in a rush ;-) It's still the right color green,
> but not as deep as others I've seen.
> 
> Still need to practice applying the film, though.  Lots of wrinkles
> and bubbles on my first try (good enough for exposure tests, not good
> enough for circuitry).
> 

Thank for getting back to us. I'm bit new to these step gauges. I 
ordered mine yesterday. I also got Stouffer $11 Transmission Resolution 
Guide to make the $10 air mail delivery to Australia order more worth 
while :)
 From what I can gather, the transmission gauges are logarithmic. With 
21 step gauge, each successive step has the transmission 1/sqrt(2) 
factor relative to the previous step. In general, transmission is given 
by 2^(-n/2) where n is the step number starting from 0 (100% transmission).

step 8 means 6.25% transmission. if you exposed 5.5 minutes, then your 
real exposure should be 5.5*0.0625 = 0.34 minutes (21 seconds).
Is my math correct ?


Are you using the hot roll laminator to apply film ?
I will update my wet lamination web page sometime soon, since I've 
worked out some improvements to the process.

Also I just discovered the importance of leaving the PCB stand after 
exposure for > 5 minutes. I've read several places that negative 
photoresists continue to polymerise after exposure , and that this 
polymerisation is inhibited by oxygen. So its really important you leave 
the protective polyester cover film on during this hold time. I was 
getting really puzzling results before I realised this. Often I used to 
just develop immediately after exposure, and would find that one side of 
the PCB would appear to be underexposed and get attacked in the 
developer. It turned out to be the short delay differences from exposing 
one side at a time was enough to show the problem. I also found I could 
reduce my exposure from 60 seconds to 30 seconds when the PCB held > 5 
minutes after exposure. The PCB process is much more consistent now.

Adam