4.4 PCB material selection
The PCB material selection is extremely important for systems with high-speed routing. The thermal properties should also be
considered when choosing PCB materials for multi-layer designs, in which the system is expected to endure excessive short-term
thermal steps. Specific attention should be paid to the fact that thermal properties of dielectrics are often different in horizontal and
vertical directions.
Material characteristics, such as the Coefficient of Thermal Expansion (CTE), should be considered. The CTE describes how
a material changes dimensions with temperature. Ideally, a PCB material’s CTE should be closely matched to copper, which is
about 17 ppm °C. CTE is a concern, because when the PCB expands during heating, it can elongate plated via holes and cause
fracturing. If the CTE is closely matched to copper, expansion of the PCB material and copper will be more uniform and the plated
via holes will be more robust during thermal cycling.
4.5 Thermal resistance
Reducing the thermal resistance close to the die and package is mandatory for good thermal performance. The actual
semiconductor die dimensions are relatively small when compared to the size of a typical PCB, which results in a very high heat
flux in the die, package, and its immediate vicinity. Therefore, thermal resistance encountered early in the thermal path causes
a large temperature gradient. The most effective place to focus resources to reduce thermal resistance is where the thermal
gradient is the highest. To efficiently dissipate the heat through the board, thermal resistance between the SoC and the board
must be minimized. This can be achieved by utilizing all the ground pads of the component and using underfill with good thermal
conductivity properties.
4.5.1 Heat spreaders
Thermal resistance can be reduced when a heat spreader is mounted on the top of a SoC package using a Thermal Interface
Material (TIM) with good thermal conductivity properties (thermal paste). If the heat spreader is also thermally connected with the
PCB, an alternative route for the heat is created, reducing the global thermal resistance. Spreading the heat at the beginning of
the thermal path not only reduces the thermal resistance near the source component, but it also provides a broader area to further
dissipate the heat.
The type of heat spreader to be used is dependent on the customer application available enclosure space and budget
considerations. Graphite heat spreaders are quite common, because they match the thermal performance of copper in two
directions (x, y) at a lower weight and cost. The high in-plane (basal) thermal conductivity results in spreading and evening out
of the hot spots. Due to its low cost, the area that the graphite heat spreader covers could be potentially larger, covering all
heat-generating components on the system board.
4.5.2 Thermal vias
Using a continuous low-thermal-impedance path from the processor to ambient conditions is important and a low thermal
resistance must be maintained throughout the PCB. Any small break in the low-impedance path is highly detrimental. System
designers should provide redundant thermal paths where possible. This can be achieved by adding an appropriate amount of
thermal vias to connect all the ground planes together and allow the heat to spread uniformly through other layers of the PCB.
System designers should allocate enough plated through vias around the ground and power balls of the i.MX processor and other
heat-generating components.
4.6 Power net design
Modern power electronics devices can have very low on-resistances. It is quite possible that the PCB traces and connector pins
that feed current to these devices contribute more ohmic losses to the system than the power transistors do. Such heating may
be avoidable if the traces are up-sized. Reducing trace ohmic losses may be the least expensive way to reduce the design’s total
power dissipation. Trace width calculators, which also predict the trace temperature rise, are readily available on the internet.
Using over-sized power transistors is a way to cut the total power and subsequent heat dissipation.
NXP Semiconductors
Thermal considerations
i.MX8 QM / i.MX8 QXP Hardware Developer’s Guide, Rev. 2.4p, 06/2021
User's Guide 77 / 89
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