We're a PCB manufacturer and have requests to build boards that have two or more metal finishes.  Many times these finishes are electroplated onto the traces and pads.  A problem we encounter is when we use these multiple metals as the "etch resist".  For those who aren't familiar with the process, basically we photo-define traces and pads then plate up copper onto those areas.  Underneath the photo-film is the base copper which acts as continuity for the plating process.  After copper plating we may plate tin, tin/lead, or gold (or combinations in selected areas).  We then strip the film and have to etch the background copper.

Therein lies the problem.  We use an ammonia etchant to etch the background copper.  When we use only one "etch resist" then we have no problem.  When we use two, say gold and tin/lead, then the copper is etched faster in the areas where these metals meat than anywhere else.  The industry defines this as electromechanical corrosion and has no good method for dealing with it.  

The problem is twofold: 1) background copper is etched at a faster pace in bi-metalic areas, and 2) the traces we're trying to protect with the resist are "undercut" due to the enhanced speed of the etchant in those areas.

Are there any corrosion experts here that can offer some solutions?  Like all PCB manufacturers we have band-aids for the problem, but no real solution.  Any help will be appreciated.  

Thanks

Mark Mazzoli
MMazzoli@protoeng.com