Greg-
I would first experiment to see how much "dome" is tolerable before your
print gets out of hand. ie: Dome adjacent vias at varying heights, then
characterize the resulting prints.  Although we tent vias rather than fill
them, I've seen similar fine pitch printing problems caused by excessive
thicknesses of nearby silkscreen.

Larry Jindra
TRW/Avionics Systems Division
[log in to unmask]
ph (619) 592-3424
fax(619) 592-3940


> -----Original Message-----
> From: Greg Parke [mailto:[log in to unmask]]
> Sent: Friday, November 20, 1998 8:54 AM
> To: [log in to unmask]
> Subject: [TN] Screenprint problems
>
>
> Hi All,
>
> I recently switched screenprinting processes from an
> semi-auto machine to a full
> auto machine.  upon changing over we began to see an increase in
> shorting/bridging for fine pitch components such as QFP's,
> SSOP's, etc.  Upon
> further investigation it was found that the problem was
> confined to two sided
> boards.  Our test process dictates that vias are filled in
> order to pull the
> required vacuum during test .  We normally paste the vias on
> the circuit side
> mainly due to fact that many of our boards have BGA's and we
> can't paste them on
> the component side due to the obvious concern with shorting.
> For one sided BGA
> boards we have the extra process step of pasting via on the
> circuit side.  Two
> things are occuring which are contributing to the problem:
>
> Increased Volume - It seems that the switch to the automated
> printer has
> resulted in a higher volume of solder being deposited on the
> board.  The
> increase is approximately 15-20%.
>
>  Poor Gasketing - When the circuit side gets reflowed the
> result is a "dome" on
> the component side of the board which results in a poor
> gasket during the print
> stroke which in itself can lead to shorting.  When mixed with
> the increased
> volume being put down to begin with you end up with a mess.
>
>  The big question in my mind is the increased volume
> necessarily wrong? After
> all, the old process could have been depositing marginal
> paste volumes. I am
> working to put together a multivariate experiment to minimize
> the variation
> between the actual volume of paste deposited and the expected
> amount of paste
> based on the aperture size, stencil thickness, and some
> release efficiency.
>
> Now that you're all asking yourselves "just what the heck I'm
> talking about?", I
> have a few questions. Has anyone out there had a similar
> issue with these so
> called "domes"?  How did you solve the problem?  Has anyone
> gone through any
> experimentation similar to what is outlined above or do you
> feel that it is
> overkill?  If you would like to discuss this off-line feel
> free to call me.
>
> Thanks for the help.
>
> Greg Parke
> Teradyne, Inc.
> (617) 422-3192
>
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