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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