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Copper Areas – make as large as possible
Solder Mask
Defined
Thermal Pad –
follow
dimensions
given
Thermal Via – follow spacing given and keep diameters less
than or equal to 0.3 mm
External Via
Rounded
Finger Pads
Voiding – keep
less than 50%
Only external
vias are used
Top copper
area covered
with solder
mask
Solder Mask Defined
Thermal Pad - exposed
copper area
2.1.3
Solder Loss and Protrusions
Board Layout
Figure 6. X-Ray – DRB Device
Vias may be plugged to prevent solder loss and protrusions. This often produces the best thermal
performances but is not necessary or recommended because of the increased cost of PCBs and because
solder tends to wet the upper surface first before filling the vias.
Vias also can be used in the copper area outside the solder mask defined thermal pad to help dissipate
heat through bottom or inner planes.
If thin PCBs or vias larger than 0,3 mm are used, designers may use only external vias to prevent solder
loss and protrusions (see
Figure 7
). Designers should note that this might reduce thermal performance
significantly and should be evaluated on their PCBs.
Figure 7. Example of Using Only External Vias in a DRB Package
Solder loss and protrusions result when excessive solder flowed through internal vias during reflow. These
usually happen when incorrect internal vias sizes and stencil openings are used.
Solder loss results in voiding and severely affects thermal conductivity. Designers are encouraged to x-ray
their reflowed boards to verify that at least 50% of thermal pad area is soldered (less than 50% voiding)
when using 0,127-mm thick stencils.
Protrusions might cause misalignment in stencil on the reverse side of the PCB (see
Figure 8
).
4
QFN Layout Guidelines
SLOA122 – July 2006
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