The thermal properties of a PCB are critical to the operating junction temperature of a component.
To predict the heat transfer to the PCB, PCBs are modeled as a single unit with two effective thermal conductivities: effective parallel thermal conductivity and effective normal thermal conductivity.
The effective parallel and normal thermal conductivities of a PCB are dependent on the total thickness of the PCB as well as the thickness of the layers of copper and glass-epoxy.
As the power densities of electronic components increase, their operating junction temperature goes beyond the limit, dissipating more heat energy onto the package, PCB, and enclosure. A fraction of the heat dissipated by electronic components is transferred to the PCB through the leads and cases, and the share increases with the rise in power densities. The thermal conductivity of a PCB is a significant factor influencing the dissipation of thermal power loss in a circuit board. Optimizing the thermal characteristics of a component through the proper design and material selection of the package, PCB, and enclosure is critical. Let’s discuss thermal management and the thermal conductivity of PCBs in a little more detail.
PCB Thermal Management
Electronic components generate heat energy and dissipate a significant amount onto PCBs. Thermal management techniques such as heat sinks are often used to accelerate the heat dissipation rate from the electronic component to the ambient. Heat sinks offer low thermal resistance from the junction to the case and case to ambient, thus facilitating heat transfer. While specifying the heat sink for the electronic component of interest, heat energy dissipation through the PCB becomes significant. The thermal properties of a PCB are critical to the operating junction temperature of the component. The thermal conductivity property of PCBs is even more critical with the proliferation of high power density and high-speed electronic circuit design.
Thermal Conductivity of PCBs
PCBs are layered structures consisting of copper foils and glass-reinforced polymers that connect components electrically and support them mechanically using pads, conductive traces, and vias. High thermal conductivity copper foils are sandwiched between low thermal conductivity glass-epoxy layers. The copper forms the conductive circuit in a PCB, whereas glass-epoxy layers are the non-conductive substrate.
The most commonly used conductive material is copper. Other options include aluminum, chrome, and nickel. The non-conductive substrate most commonly used is FR-4 laminate. The thermal conductivity of copper is about 400 W/m/K and the thermal conductivity of FR-4 is 0.2 W/m/K. The copper acts as a thermal conductor and the laminate acts as a thermal insulator. There is a vast difference between the thermal conductivity of copper and FR-4, and this makes the effective thermal conductivity of the PCB anisotropic.
Effective Parallel and Normal Thermal Conductivities of PCBs
It is important to develop a heat transfer model within the PCB, as heat dissipation in PCBs heavily impacts the maximum board temperature and component operating temperature. To predict heat transfer to a PCB, PCBs are modeled as a single unit with two effective thermal conductivities. Assuming one-dimensional heat conduction through the composite layers of the PCB and ignoring the thermal constant resistance between the copper and glass-epoxy layers, the effective thermal conductivities obtained are:
- Effective parallel thermal conductivity: Describes the heat flow within the plane of the PCB board.
- Effective normal thermal conductivity: Describes the heat flow through the thickness of the PCB board.
The effective parallel and normal thermal conductivities of the PCB are dependent on the total thickness of the PCB as well as the thickness of the layers of copper and glass-epoxy. The thermal conductivity of a PCB is sensitive to the amount of copper present in signal layers. Similarly, the effective thermal conductivities vary under the presence of internal copper layers and their distance to the top layer.
Let’s look at a specific example: a PCB with copper on the top layer. The copper on the top layer causes heat to spread over a large area. It offers a low resistance path against heat dissipation into the PCB board. The value of effective parallel and normal thermal conductivities is different for PCBs with and without copper on the top layer, thereby making the effective total thermal conductivity of PCBs in both cases different.
It can be concluded that the placement of layers, component size, and operating conditions have a significant influence on the effective thermal conductivity of a PCB, making it a crucial element in PCB design. Circuit design engineers must consider thermal conductivity as a key factor in determining how heat could be dissipated in a circuit board during the design process.
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