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2.1.2
Thermal Vias
Thermal Via
Web or Spoke Via
NOT Recommended
Solid Via
Recommended
Exposed Copper –
0,05 mm Around Via
Board Layout
Inner or bottom layer copper planes also can be connected to thermal pad using vias and should be made
as large as possible. The thermal pad is usually tied to ground, and designers should verify the electrical
correctness when connecting the copper planes to the thermal pad.
Designers may leave the bottom copper plane exposed. However, studies have shown that this has
minimal impact on thermal performance.
TI recommends placing thermal vias in the solder mask defined thermal pad to transfer effectively the heat
from the top copper layer of the PCB to the inner or bottom copper layers.
TI provides the recommended layout of the thermal vias in most data sheets. The recommended via
diameter is 0,3 mm or less, and the recommended via spacing is 1 mm (see
Figure 4
).
Figure 4. Solder Mask Defined Thermal Pad
The thermal vias should make their connection to the internal ground plane with a complete connection
around the entire circumference of the plated through hole. Place a ring of exposed copper (0,05 mm
wide) around the vias at the bottom of the copper plane.
Do not cover the vias with solder mask which causes excessive voiding.
Do not use a thermal relief web or spoke connection which impedes the conduction path into the inner
copper layer(s) (see
Figure 5
).
Figure 5. Via Connection at the Bottom of the Copper Plane
SLOA122 – July 2006
QFN Layout Guidelines
3
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