>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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>
Suggest : 1. check design of locations, does the land meet the IPC - SM-782?
          2.  check lead width, like lead to be narrower than Land.Otherwise can
                lead to solder volume problems ( fine pitch ).
          3. calculate an aspect ratio ( As =Aw/St , Aw=Width of aperture in
stencil                 , St= stencil thickness,  As >2 is target.) for fine
pitch locations.                  Assuming Stencil Aperture width is much
smaller than  aperture length.         If  As (aspect ratio)  too low, paste
will stay in the stencil rather than on         the land.

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