Homebrew PCBs

Steve wrote:

> 
> I'm really confused. Black areas are where copper is.

   Exactly.

  But black areas
> over the emulsion result in it washing away.

   Again exactly.

  This isn't making sense
> to me.


   If black areas are where the copper is, but instead you need black 
areas over what to remove, then you need a negative of the copper areas 
image to expose the emulsion.  You use a negative image, the light goes 
through where the copper needs to stay and hardens the emulsion to 
protect that copper.  Since the emulsion makes a reverse of what you 
want, you have to reverse the picture of what you want before you expose 
the emulsion.  The emulsion makes an inverted hardening resist, so you 
also need a negative image.  "-" once and then "-" again gets you back 
to "+" of what you want..

   Just think of a black circle in the middle of a tranparency.  If the 
circle will make a hole where you wanted a copper dot, then what you 
really need to make is a black page with a clear circle.  A negative of 
the original.  That's what you use to expose.

   That is assuming a negative emulsion, there may be some positive 
acting emulsions out there.  The usual stuff is negative though.