Regarding the RH in a PCB assembly facility, besides ESD, the RH (as well as
ambient temperture) affects the behavior of the solder paste in process.
Both No-cleans and water solubles are hydroscopic in that they absorb
moisture from the air.  Hence, too high RH will result in a change in solder
paste printing characteristics and/or results ranging from shorter work life
to dewetting and, yes, the ever-annoying solder balls.
Too low a relative humidity will adversely affect OA solder pastes in that
they are effectively being dried out.  Again, symptoms I've observed include
shorter work life, shorter tack times, dewetting, non-wetting and solder
balls.
Most solder pastes seem to desire a range of between 35 - 75% RH.  When I do
a Process Audit for a client, I specifically look for a means of reading and
recording RH on an ongoing basis.  Those chart recordings can be useful in
defect analysis.
Another reason to keep RH within  a decent level is for the comfort of your
workers.  Just a thought!
Regards,
Phil Zarrow
Process Consultant
ITM, Inc.
Durham, NH  USA
www.ITM-SMT.com

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