Chris,

Manufacturability vs good RF performance are not good
marriage partners.  As you may be, I'm always sitting at a
different wave length with RF designers.  A good forced
convection oven won't cause tombstonings, provided with a good
temperature profile.  What might cause is micro-cracking of
solder joints, or cold, or stressed solder joints.  This happens
during the cool down process of boards where one pad is
solidified first compared to the opposing pad (pad connected to
a heavy ground plane).
This usually does not cause a problem at a lower freq, but hey, it
is an open guess at a RF/microwave freq.   Some of design
changes that might work:
- make cross-shape pad connections to ground plane
- narrow down the connector entering the pad from ground
plane, 25% down from the pad width.

best regards
Matthew Park
SMT Manufacturing Engineer
NII- Norsat International Inc.



>>> Chris Murphy <[log in to unmask]> July 19,
1998  10:32 pm >>>
Dear Technetters,

we currently design our PCBs so that the pads for chip
components are
thermally balanced, thus avoiding any tombstoning.  The
problem is that
ideally we need to connect the ground side of the components to
a large
plane for good RF characteristics at high frequencies.

Does anyone have any ideas on how to get around this problem
?  We
Heller and Quad forced convection reflow ovens, which should
reduce
any tombstoning rates.

Thanks in advance for any replies.


Chris Murphy

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