Homebrew PCBs

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


--- In Homebrew_PCBs@yahoogroups.com, Alan King <alan@n...> wrote:
> grantfair2001 wrote:
> > 
> > By positive, I mean that the copper tracks will appear white when
> > printed on paper, and etched portions will be black. My PCB software
> > does the usual "negative" print - copper tracks are black, etched
> > areas white, but not the positive print.
> > 
> 
>    As someone already pointed out, you have your terminology
reversed by 
> trying to label it what you want it to do.  On screen is a positive, 
> what you're describing is a negative, and then the exposure process 
> negates it again to make a positive PCB with correct tracks.  Note that 
> your reversed terminology will make you want a 'positive' and when you 
> print a 'positive' it'll be backwards to what you want.  What you
really 
> need to find is the program's way to print a negative image.
> 
>    Keep in mind that you can always reverse the sense of positive and 
> negative in anything, electronic or mechanical or photo or anything 
> else, and it'll still work logically as long as you keep the senses 
> consistent.  That means you are REQUIRED to correctly determine and 
> match the normal conventions, or you will be thinking the wrong thing 
> and be getting results that you think don't work correctly when really 
> you just chose the wrong result by being backward to normal polarity. 
> It's not that the program's 'positive' output or 'negative' output 
> aren't correct.  You just chose their 'positive' when you really wanted 
> their 'negative' and have your choices reversed to the conventional 
> direction..
> 
> Alan