Charles Barker@IO-US
10/03/97 08:50 AM

Good morning Martin,

In an earlier life, I was able to influence both the board design
parameters and the stencil parameters.  We were using .006" thick stainless
stencil material too. From what I have read, you want to have no smaller
stencil opening than 1.5 times the stencil thicknest. I also wanted to be
sure that there was no solder paste getting on the board itself. (We were
planning to go total no-clean and didn't want any solder balls forming
because of small amounts of solder on the board itself.)

Most designers were using about a .010" wide pad for the .020" pitch
devices. I wanted the stencil opening to be smaller than the pad at all
times.  Because of potential etch losses, we could not be assured that the
pad width would always be bigger than a .009" stencil opening. Also, even
with a .009" opening and a .010" pad, almost any mis-alignment would mean
solder on the boards.

The solution we were last working with was to increase the pad size
slightly, increase the stencil opening to .010" to get better paste release
(We too were seeing solder starvation , particularly at the corner pads.)
and then to specify to our board shops that the pads on all fine pitch
devices HAD TO BE no less than .012" wide.  With .0005" stencil alignmant
accuracy, we would always have a small amount of room to spare for stencil
misalignment.

This brought the pads closer together, but with all of the paste being
deposited on the pads and none on the board, we had no bridging problems.
The solder starvation incidents decreased considerably too.  I would still
recommend more frequent cleaning of a fine pitch stencil than a regular
pitch one.

You may also want ot investigate the newer stencil making technologies.  I
believe some give better paste release too.

Hope this helps.




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Hi,
I work with a SMT process which requires us to build boards ranging in size
from (6"*4") to (18"*14"). These boards can be single or double sided with
component counts ranging from 100 to 2,500. We palce large quantities of
20 thou fine pitch devices and BGAs'. Several of the boards are double
sided
reflow and are selective soldered at the wave solder machine.
We currently print using DEK and MPM machines with a 6 thou thick stencil
and a water soluable solder paste on to Hasl finish boards. Our biggest
cause of defects is insutticient solder and dry joints found both at the
end of the SMT area and at ICT. Has anyone done any work what the optimum
apperture size for a specific device type should be ? Are there any changes
which can be made to apperture design to allow better release of the solder
paste ? Any information that anyone has on stencil design would be greatly
appreciated.
We also use a View Engineering SVS 8100 solder paste inspection machine to
measure the area and volume of solder paste deposited. Is there a
theoretical
minimum volume of paste required to create a good solder joint for a
specific
pad size, component lead size etc. Our current inspection criteria on the
SVS
uses the theoretical Area and Volume of the specific apperture with a +/-
tolerance. Has any one come across a way of calculating  an actual expexted
value which would be close to the actual mean values measured by the
machine ?
This would allow us to use much tigher tolerances, and hence have a better
process window to work within.
Any information that any body can provide on the general screenprinting
process
and the inspection of same would be greatly apperciated.


Martin Bourke.
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