Our experience is that there are "windows" of available viscosity for a given 
wt%; other factors have significant contribution by the way including the 
relative "size" of the particles, their shape, etc. as well as the thixotropic 
(thickeners) used.  Overall a lower wt% paste (say 85%) can achieve a lower 
viscosity mix than a higher wt% (say 92%) but there is a large amount of 
overlap.  Note also that even small increases in Wt% can make a big differnce in
volume& solder (e.g. 85% by weight is ~42% by volume; 90% by weight is ~53% by 
volume: C. Lea, A Scientific Guide to SMT, Figure 6.7).

I suggest that some of the "printability" attibutes that are "good" for fine 
pitch paste may work against you in paste-in-hole.  I suggest contacting you're 
paste vendor re a appropriate "mix."

______________________________ Reply Separator _________________________________
Subject: ASSY: Solder Paste Viscosity
Author:  [log in to unmask] at esdigate
Date:    5/1/96 3:21 PM


     I am trying to "optimize" a screen printing operation.  This 
     operation screens solder-paste for both SMD and 
     Through-Hole(Paste-In-Hole).  I am using a solder-paste that is 
     90% solids(Non-No-Clean) with a viscosity of 1M(cps)+-100K(cps).
     
     Whew!...Finally my question(s):
     
     I have found that a higher viscosity helps when doing fine-pitch 
     but I may be hurting myself when doing Paste-In-Hole. Is there a 
     relationship (weak or strong) between solids-content and 
     viscosity or do I keep the solids-content a constant and simply 
     ask for a lower viscosity solder-paste if I need it for (PIH)?
     
     
     Thanks
     
     Harlan
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