Most solder pastes state that they can only be run at certain humidities, 
for the water soluables that I have used in the past, it is 40-60%  Below 
40% the paste starts to dry out, above 60% it starts to absorb moisture and 
becomes really soupy.  As you have already seen, you can run the paste 
outside the stated conditions for a short period of time, but it will start 
to change over time.  Some advanced screen printers have an airconditioning 
unit in them to help prolong stencil life of the paste.  Humidity control 
is very important for the solder paste process!!!

Your question brings up something else.  You stated that you are at 14% 
humidity

---------------DANGER WILL ROBINSON!!!!!!!!!!!----------------

Running at 14% humidity level increases the ESD problems.  The suggested 
humidity for an electronics facility is 40-60%.  Low humidity increases the 
ESD concerns, high humidity could create moisture absorbtion problems in 
components.  
-------------
Original Text
From: C=US/A=INTERNET/DDA=ID/TechNet-request(a)ipc.ipc.org, on 2/20/97 7:51 
AM:
Hello,

        We are having problems with our paste tack time not lasting as long
this time of year.  We are running at 14% humidity now, normal is about 40%
to 50%.  We are using a water soluble paste, 90% metal content, with a
quoted tack time of 6 hours on stencil, and 6 hours on board.  We are 
seeing
problems after 3 to 4 hours.  Does anyone have experience to say this is
normal for the humidity, or ideas as to what we could be doing wrong.(I 
know
of plenty myself, but still would appreciate more).

Thanks 

Steve Quinn
Heurikon Corp

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