John R. Kretsch asks,

    Why would "thickness" (as specified by the customer) be measured 
     before solder resist application?  The customer is designing around 
     and will receive a finished product (which includes solder resist) to 
     be installed in a higher-level assembly.
     
     What am I missing here?


A good question!  We design several versions of VME cards.  Some are single-
board designs which insert into the VME rack. They must be no greater than .070"
thick in the guiderail region, with or without soldermask.

Some have daughtercards mounted to them which have controlled thickness
in a small portion of the board (cleared of soldermask) where a "screwjack"
pulls the two boards together while engaging mother/daughtercard interconnect
plug/sockets and pushes them apart when disengaging.

Sometimes the overall height of the components of a two-sided SMT board is 
critical.  I then spec the board thickness from SMT pads to SMT pads front 
to back prior to soldermask.

Just depends on the application.  Of course I've never felt that +/- .003"
was required for any of our stuff.  Plastics/epoxies aren't that stable, are
they?  Certainly component heights aren't that good by the time you solder their
little wire feet to your board.

Gary P.
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                                  Gary D. Peterson
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