Hi Harlan,

Q:  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)?

A: Yes, but you need to determine whether that is the significant factor 
affecting your     process. ( DOE of PIH:  Refer to Journal of SMT 1995 PTH 
soldering with Solder     paste reflow by Bill Barthel, EAC) If you try to 
do fine-pitch and Paste-In-Hole at the same time, I think the thickness of 
you stencil will have more impact to the amount of paste desposite than 
paste  viscosity. Anyway, I think there are several  factors  that  you need 
to look into at the same time.

     1. Stencil Thickness
     2.  Stencil aperatures
     3. PCB Application
     4. Screen Print parameters
     5. FPD lead pitch (20 mil???)

Thanks

Michael Yuen

 ----------
From: TechNet-request
To: TechNet
Cc: hgvince
Subject: Re: ASSY: Solder Paste Viscosity
Date: Wednesday, May 01, 1996 4:56PM


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
     [log in to unmask]