Mel
How consistent is paste release.Currently,We are using Alphametal 6 mil
stencil for 20 mils QFPs. -mixed technology board .Stencil is  laser cut
with electropolish .Apertures are 1:1 unless you have specific
requirement for custom aperture for specific components..You may contact
Alphametal (Fernandoes:1-800-748-2033 ext 4439).

If solder is inadequate after reflow soldering, find out where? Reflow
profile ,Component dimensional outline etc--might be factors to look
at.in one of assy, we observed that lead form outline was such that we
had to go for custom aperture to spit extra solder paste.

If solder is inadequate/dry after wave, there could be secondary reflow
at wave.
DOE will reveal root cause of the problem.
> ----------
> From:         Mel Moschler[SMTP:[log in to unmask]]
> Sent:         October 3, 1997 8:40 AM
> To:   [log in to unmask]
> Subject:      Re: [TECHNET] SCreenprinting
>
> >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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> additional information.
> For the technical support contact Dmitriy Sklyar at [log in to unmask] or
> 847-509-9700 ext.311
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>

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