Homebrew PCBs

----- Original Message ----- 
From: "Philip Pemberton" <ygroups@...>
To: <Homebrew_PCBs@yahoogroups.com>
Sent: Sunday, May 31, 2009 11:27 PM
Subject: Re: [Homebrew_PCBs] Setting exposure with a step gauge


leon Heller wrote:
>>> You put the gauge on some PCB and expose it for some time,
>> With the artwork or without?
>
> Without!

No need to shout! :)

I've spent this afternoon doing a few tests on some cheap Maplin pre-coated
laminate, with Seno "Develop 110" developer (the one that costs \ufffd7 plus VAT
from Rapid and looks like a bottle of shoe polish). I have a sneaking
suspicion the boards may have been sitting on a shelf for quite some time as
they were rather difficult to get even remotely close to step 8 on the step
gauge. It took half a dozen attempts, and a final exposure time of 960 
seconds
(16 minutes) and a developing time of 60 seconds (it says on the can that it
only takes 20-30).

My understanding is that my goal is to have a "step held" (the first step
that's <50% exposed) at step 8. In my case, with positive photoresist, that
means the first step that's less than 50% clear copper should be step 9.

I think I'm going to liberate a sheet or two of Microtrak or CIF precoated 
PCB
laminate from the storage cupboard and see if that works any better. Even if
it doesn't expose better, it should certainly cut better in the shear...


Buy the boards from Farnell or Rapid; the resist goes off after six months 
or so, I've found, and they could be old stock. I use sodium hydroxide for 
development - 12 g/litre.

Leon