The brick shape is determined more by the type of flux and the amount of
shearing back and forth during printing as well as temperature and
humidity, especially the temperature of the stencil. A few degrees and a
small change in humidity can greatly alter the characteristics and
printability of some pastes. Some solder pastes are much more
susceptible to slumping during printing than others. Much, much more
susceptible.
I was "lucky" enough to have had to qualify several solder pastes in the
past few years for different companies. What I learned about solder
paste characteristics was eye-opening, sometimes shocking, and I learned
an awful lot about different solder pastes. Boy, did I learn a lot about
solder pastes. Some of the "big names" performed miserably during these
exhaustive evaluations. Others did quite well.
I used both Cyberoptics and V.I. Tech 2.5 D and 3 D paste inspection
systems, and I used 4.5 to 7.5 mils (6 mils +-1.5) for UCL and LCL to
determine one of the printability performance matrices. The best pastes
were able to repeat themselves in even tighter ranges, had I chosen to
tighten them up. One of the leading "big name" pastes failed in this
category miserably, I re-ordered different lots of paste twice, same
results. One of the other big name pastes came to me with severe flux
separation inside of the Semco cartridges. It failed viscosity testing,
printability, etc. When I called the manufacturer to note the results
and the flux separation (the flux poured out of the cartridge upon
opening) they said I should have purchased their in-the-cartridge mixer,
and I would not have had any issues. So I did, and I mixed the paste
inside of the tube, and guess what; it failed the same way as before.
So, if you are having print issues that change from day to day that
cause the operators to request opening up the UCL and LCL, evaluate a
different brand(s). Some of the pastes are formulated to be stable under
many printing conditions, and they are. Some are formulated to reduce
voiding in BGAs, and they do. But only one paste performed well in all
categories (slump resistance, rheology changes due to shear forces,
rheology changes due to humidity, voiding, wetting, tack, etc.) for
Lead-free No-clean, and only two performed well for 63/37 no-clean and
63/37 water soluble. Don't ask me which ones they are, as different
pastes need to be qualified to your particular processing needs.
-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Phillip Bavaro
Sent: Monday, March 03, 2008 10:00 AM
To: [log in to unmask]
Subject: [TN] Solder paste print height tolerance
I am wondering how tightly others control their solderpaste print
height.
Assuming SAC305, type 4 paste, 5 mil thick stainless steel stencil, ss
blade, and a DEK printer, and Cyberoptics laser inspection system, what
is a reasonable process tolerance?
Is 4.6 to 7.4 mils acceptable?
My experience had always been successful with a tighter tolerance
(like 5.0-6.0 mils), but I am being hard pressed to open this up.
I realize the larger aperture permit for scooping to happen, but I try
to eliminate such occurrences by design.
I have always tried to dial in the print operation such that brick
shapes resulted, but as the limit of the stencil aspect ratios are
approached, the shape release forces alter the brick into a gum drop
shape.
Thanks in advance.
Phil
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