Thermal vias for an IC

[FreshmanNewbie]

In this Application note recommendation for ground connection of the thermal ground pad of an IC on page 3,
(https://ww1.microchip.com/downloads...B-Design-Guide-QFN-DQFN-Packages-00001843.pdf),

I see that the ground paddle of the IC, is in square shape.

So, when I place it on a PCB, it is best, if I provide the same square shape ground pad on the PCB for good thermal dissipation.
But, there's an image on page 3, bottom left, where they mention to give 16 thermal vias, 4 x 4 arrangement, and this thermal vias to ground.

My question is, why should I place ground thermal vias when I can give a proper square shaped copper ground plane?
Why do I need to give vias which might be expensive that simply pouring copper?

Any advantages or reasons to go with thermal vias over square copper plane?

 

[FvM]

Without vias, a copper pour below the IC neither provides heat sinking nor ground connection.

If thermal vias are suggested or even necessary has to be calculated according to power dissipation and thermal resistance.

The "encroached" via option discussed in the application note doesn't involve extra costs.

 

[KlausST]

Hi

I agree with FvM.
"Transporting" the heat from the package just to the PCB ... does not help.
You need to "transport" the heat using copper to a much wider PCB area ... and further from the PCB to the ambient air.
--> The air is the target.

silicon --> package thermal pad --> PCB thermal pad --> spreading to wider areas on the PCB (*) --> air

(*) this can be
- on top side by using top copper area,
- inside the PCB using thermal vias to inside_GND_plane_copper
- on the opposite side using thermal vias to bottom_side_copper area (often GND_plane)

Like an aluminum heatsink that needs aluminum fins to extend the heatsink_surface to best pass the heat from aluminum to air.
In usual applications you rely on thermal capacity of air and on air convection (warm air is less dense and thus it rises).
In other words:
In space (no gravity) a heatsink does not work satisfactory, because of the loss of gravity you don´t get convection (air movement), thus you need a fan to provide the air movement.
And if there is vaccuum around, not even a fan would help, because there is no air to "transport" the heat. In this case you need to rely on heat radiation, which needs an even bigger heatsink area .. and the heatsink should be black.

Convection also has influence on PCB level. A PCB lying flat (horizontally) causes less air convection = less heat transport.
A vertically mounted PCB with free air flow around causes good air convetion and thus good heat transport.
And also the enclosure has influence: worst is a small plastics enclosre wihtout venting holes, best is a big metal enclosure with venting holes.

****

it is best, if I provide the same square shape ground pad

This application note is a general information.
But usually in the device datasheet you find the according PCB pattern to best adapt to the specific device´s requirements (regarding heat dissipation, in this case).

Klaus