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Meet
the Problem
Solving
QFN Flux Entrapment
Designers
and packagers are increasingly utilizing QFN
(Quad Flat No-Lead) packages, taking advantage of
their small size and good thermal dissipation.
However, frequent QFN isues include: 1) flux
entrapment under the device; 2) shifting, which
reduces spacing; 3) reduced solder in the
joints.
The
above photo shows a typical QFN package, which
appears to be problem-free. Cross-sectional
analysis of this QFN revealed the presence of all
three issues. The component was 1mil off the board
surface, the solder in the joints was reduced by
85%, and the QFN had shifted, reducing the space
between the back of the power pad and the larger
ground plane from 10mil to 5mil. Within this gap,
gooey, conductive, moisture-absorbing flux was
trapped - a residue observed to still be gooey 6
months after application (reflow temperature was
above liquidous for 120 seconds).
Foresite
has, for three years now, advised clients on how
to fix their QFN flux entrapment and solder loss
problems - how to maintain QFN/board spacing,
protect against shifting and allow the flux to
vent and completely complex, creating a
non-conductive, non-moisture-absorbing residue.
The fix: plug the thermal vias (which eliminates
solder flowing down the vias, pulling the QFN
down); put a soldermask strip over the plug to
create a 3-4mil standoff; then, remove the
soldermask from between the perimeter pads, which
allows for ventilation of the flux and a benign
residue. This method has effectively worked in the
field for very sensitive, communication
components. Such components routinely failed in
high humidity testing prior to this fix - no test
failures have occurred with the lifted
fix. |
