Page 17 - PEN eBook July 2022

Thermal Management Thermal Management

For further improved Z and
thja
dynamic power dissipation for
TSC, the implementation of an

intermediate heat spreader is
a good option, as shown in
Figure 7. The thermal capacity
of this additional heatsink
can store for a certain time
(some seconds) the additional
heat and transfer it further Figure 7: Single-device heat-spreader mounting
Figure 5: Standard positive package standoff (left) and negative package standoff (right). The copper pad below the
package body is beneficial as mechanical support of well-defined height and high board-level reliability in case of a to the common heatsink
negative standoff and ambience. Depending on the system design, removing the common heatsink and TIM is also
possible for improved system Z , wherein the heat spreader is the primary heatsink and is directly
thja
In case of a negative package cooled by the fan airflow.
standoff (Figure 5, right), other
considerations are needed for
the PCB design to avoid system
reliability issues, which could
cause additional effort and

complexity for the system
design and manufacturing.
A negative package standoff
has the advantage of reduced
Z because of its reduced
thja
package height tolerance,
leading to a thinner TIM
thickness. However, when
Figure 6: Electrical isolation foil and gap filler between the external heatsink considering other tolerances
and the device
like PCB warpage, especially
with larger PCB size and multiple power devices using a common heatsink, the thermal advantage
of a negative package standoff becomes less important. Figure 8: Typical transient thermal impedance junction ambient (Z thja ) for multiple packages at forced convection

For the common heatsink approach, Figure 6 shows schematically the TIM stack between device
and heatsink, which consists of insulation foil and gap filler in this example. The gap filler is Thermal performance
used for compensating device-, heatsink-, and PCB-related manufacturing tolerances. Using only Figure 8 shows Z time-dependent plots for selected through-hole device (THD), BSC SMD,
thja
a gap filler for heat transfer, a reliable insulation between the device and external heatsink must and TSC SMD packages considering an FR4-based PCB design with forced air cooling. The same
be ensured. Additionally, the gap filler material must fulfill the necessary breakdown rating, and device inside all shown packages is assumed just as the same power losses. Comparing DDPAK
enclosed particles within the gap filler or blowholes during PCB assembly need to be prevented. (TSC package) with TO263 (BSC package) on an FR4-based PCB, DDPAK achieves 60% lower Z ,
thja
In general, a clean manufacturing ambient for PCB assembly can lower the risk of system failures although the effective cooling area of both packages is quite similar. DDPAK bypasses the bottleneck
caused by pollution during system manufacturing. “thermal vias,” as described in the section before. The graph also illustrates that top-side packages

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