Chapter 4
Thermal considerations
4.1 Introduction
This chapter introduces basic thermal considerations that must be considered when designing an i.MX 8/8X series processor-
based system. PCB’s should be designed with the thermal requirements factored in early, because only remedial actions are
possible after that. Factoring thermal management at the end of the design cycle will increase the cost of the overall design and
delay productization. This section provides a few key design considerations to improve the thermal management of the final i.MX
8-based system/product.
The Thermal Design Power (TDP) represents the maximum sustained power dissipated by the processor across a set of realistic
applications. The activity profile of the application can have a significant impact on the thermal management techniques used and
on the TDP.
If the customer application requires high performance for extended periods of time and/or if the product is required to work in high
ambient temperatures, the usage of passive thermal management techniques, such as a heatsink, becomes necessary. For very
high ambient operating environments, active thermal management techniques, such a cooling fan or forced convection, may also
be required in addition to the heatsink.
For less demanding applications, it may be feasible to consider the PCB as one of the primary heat dissipation media if good
design practices are followed. In such cases, NXP recommends cooling fins as a minimum to be mounted to the lid of the processor
using thermal paste or appropriate Thermal Interface Materials (TIM).
4.2 PCB dimensions
The dimensions of the PCB directly affect its capability to dissipate the heat. Typically, more than 80 % of the heat generated by
a high-power component is dissipated through the system board when no thermal solution is implemented. The bigger the board,
the larger the surface area through which heat can spread away from the source component and it can be also transferred more
efficiently into free space.
An NXP conducted PCB sensitivity simulation shown a 50 % reduction in PCB x-y dimensions results in an increase of between
44-65 % in package thermal resistance due to the loss in conductive volume to dissipate heat. System designers must be careful
when designing smaller form factor boards that have multiple high-power components.
4.3 Copper volume
Increasing the heat dissipation (reducing thermal resistance) can also be achieved by increasing the metallization in the system
board. PCBs are made up of copper and dielectric material with the copper being orders of magnitude more thermally conductive.
Copper volume influences the heat capacity of the board. With higher copper volume, the board can accept more heat, so an
i.MX 8 processor-based system can operate in a high-performance state (or near the maximum TDP) for longer time periods. The
copper volume can be increased by increasing the dimensions of the board, by addition of ground layers, or by increasing the
thicknesses of the layers on which power and ground planes are located. The PCB stack-ups of the 8QXP and 8QM MEK boards
shown in Figure 33 can serve as examples.
NXP Semiconductors
i.MX8 QM / i.MX8 QXP Hardware Developer’s Guide, Rev. 2.4p, 06/2021
User's Guide 75 / 89
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