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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