Bottom Cooling - Host Pcb Design; Host Pcb Ground And Power Layout; Table 3. Natural And Forced Convection Comparison - ST STM32 Application Note

5.2.3 Natural and forced convection comparison
The table below details the main comparison factors between the natural and forced convection.
Possibility to control cooling efficiency based on needs
5.3

Bottom cooling - host PCB design

To provide enough 'thermal vias', as the host PCB is a secondary heat sink for the device, it is important to use
the recommended stack-up and to optimize the PCB layout beneath the package .
A significant power proportion is drawn from the package to ambient air through the host board by the paths
detailed in the next sections (sorted by importance order decreasing). The implementation of ground and power
connections design rule is necessary for cooling and power distribution.
It is not possible to produce a set of rigid rules for layer construction within a PCB because each design has its
own requirements
5.3.1

Host PCB ground and power layout

The ground (GND) plane must have a low impedance (thermal and electrical) for all PCB areas. This improves
signal quality and power dissipation. It also reduces EMI. The use of a ground layer and flooding of other layers
with multiple vias maintain the impedance at a low level.
To place as many thermal vias as possible on the ground and power planes just below the package, ensures that
there is rnough continuous copper between the adjacent vias for power distribution and thermal dissipation. A
compromise must be found.
The possible thermal vias are listed below:
• GND vias, connecting GND the unified GND plane (these are the most efficient thermal vias)
• Power vias, connecting power to the power plane
Thermal vias conduct heat from the device to the internal GND and power planes. These planes spread the heat
inside the PCB that acts as a planar heat sink.
AN5036 - Rev 3
Table 3. Natural and forced convection comparison
Comparison factors
Cooling efficiency
Cost
Reliability
Acoustic noise
Bottom cooling - host PCB design
Natural convection Forced convection
Small
(depending on air speed)
Low (no fan) Higher (cost of the fan)
Higher
(No moving parts) (Mechanical part moving)
None
(depending on air speed)
None Yes, by varying fan voltage
AN5036
Medium to high
Lower
Small to medium
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