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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]
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