>I believe there is also a significant set of boards made
>where the "resist" is actually the tin/lead plate, and the
>etchant is some ammonia based concoction.  (I'm not sure
what RHoS has done to this process.)  Naturally, a plated
>resist makes for the possibility of very fine lines...

The process as explained to me was that a tinning process was related to 
doing plated through holes (this was when the company I was with was looking 
at getting its first PTH PCBs done). I don't recall the ins and outs of the 
process. 

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