We conformally coat our PCB assemblies using an acrylic and use Methyl
Ethyl
Ketone to strip assemblies if they require rework.
Has anyone carried out any work on the maximum exposure time you can expose
PCBs to solvents?  (I realise that it will depend on the laminate
materials,
component constuction etc.)

Michael,
The maximum time is "no longer than absolutely necessary".  We use toluene
to strip our acrylics and it seldom takes longer than 5 minutes.  We verify
removal using black light.   MEK is pretty darned aggressive.   The time is
more dependent on the solder mask you are using, since that covers most of
your laminate surface.  Most solder masks are only designed to take 4-5
exposures to reflow conditions before they start to break down.  By the
time you get to the rework stage, you may have already seen 3-4 reflow
excursions (HASL, SMT reflow, wave, touchup), so the mask becomes more
porous and more susceptible to chemical attack.

Methinks I hear echos in the background of someone who left the boards in
the MEK over a lunch break and wonders if the assemblies are still
good......

Doug Pauls
Rockwell Collins

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