Homebrew PCBs

--- In Homebrew_PCBs@yahoogroups.com, "grantfair2001" 
<grant.fair@s...> wrote:
> In message 352, list member Tom stated:
> 
> "BTW, the screen printing emulsions are negative acting so you will
> need positive films emulsion side up."
> 
> So I guess you are saying that my laser printed PCB patterns (on
> plastic sheets) are positive as is, and I have my terminolgy
> backwards. So I can use these with "negative acting screen printing
> emulsions" to get the silk screens I need to put resist ink on the
> copper which is not to be etched?
> 
> Grant



Grant,

Most screen printing emulsions are negative acting. Expose them using 
a postive image of your circuit board where opaque black is the 
copper and clear areas are etched away. The black areas on your film 
positive will prevent the UV exposure from making the emulsion hard.  
The areas that didn't get UV light will wash out leaving an open area 
in the screen for ink to go through during printing. The film 
positive should also have it's emulsion on the side that contacts the 
screen emulsion when UV exposing. This is really important if you are 
doing fine line work.

Tom