Design for Low PCB Thermal Resistance
Keep your PCB thermal resistance low with these design tips.
Heat management is very important for keeping components in your board within a safe operating temperature. FR4, arguably the most popular substrate material, has terribly low thermal conductivity, causing heat to remain confined near hot components. This is where a comprehensive thermal management strategy is needed to dissipate heat away from critical components and keep them within their operating temperature.
Fans and heat sinks are generally a part of any thermal management, but you should also design for low PCB thermal resistance. This requires selecting the right materials or copious use of extra copper to provide a low resistance path for heat away from critical components. Here are some ways you can reduce the thermal resistance of your PCB and ensure your board is in a safe temperature range.
What is PCB Thermal Resistance?
The term “thermal conductivity” is sometimes used in place of the term “thermal resistance,” but the two quantities are not the same. PCB thermal resistance is the thermodynamic analogue of electrical resistance. It depends on the thermal conductivity of the substrate material, components, and copper features, as well as the geometry of all these elements. A board with higher thermal conductivity allows heat to move from a warmer region to a cooler region at a faster rate, thus the board will have lower thermal resistance.
The various materials and components in your board will have different thermal conductivities, so they will conduct heat at different rates. The overall thermal resistance of a board requires considering the thermal resistance in each element. If you like, you can construct circuit models that can be used to find the total thermal resistance of a board using the thermal resistance of each component, just as is the case with electrical resistance. In this way, the combination of a high thermal resistance substrate (generally FR4) and low thermal resistance conductors (copper) determines the effective thermal conductivity and total thermal resistance of a PCB.
Design to Lower Thermal Resistance
If it is not obvious from the above discussion, the best way to lower PCB thermal resistance is to use more materials with high thermal conductivity. This is one reason that boards with hot components should use interior plane layers. The copper used in plane layers has high thermal conductivity, so it provides a low resistance path for heat to move away from hot components. If you are designing a board for use at high speeds or high frequency, you should use interior power/ground plane layers anyways as this aids isolation and provides shielding against radiated EMI from external sources.
Placing copper pads below hot components is another way to direct heat away from the surface layer. These pads normally contain vias that connect to an internal ground plane, which provides image shielding for these components. Components with a die-attached thermal paddle should be soldered directly to the thermal pad to maximize heat transfer away from the component. Be careful when designing these pads as placing too large/too many vias will allow solder to wick through to the back of the board during assembly. It is a good idea to check with your manufacturer assembly house regarding their capabilities.
Vias in a thermal pad. Be careful when sizing these vias and defining the spacing between them.
The other primary way to reduce the thermal resistance in your PCB is to use heavier copper. If you know your board must run at higher current, then you should use heavier copper anyways; the IPC-2152 nomograph (see page 6 in this PDF file) is one way to design traces to prevent excessive temperature rise, although it can be difficult to reconcile an IPC 2152-based design with impedance control requirements.
Supercharge Heat Dissipation with Alternative Substrate Materials
A sheet of FR4 has low thermal conductivity compared to other substrate materials, thus it has high thermal resistance, which motivates the use of thermal pads on hot components. Alternative substrates like ceramic and metal-core PCBs are an attractive option for thermal management. Both materials provide higher overall thermal conductivity, allowing heat to be moved away from a component quickly without the use of thermal pads and through-hole vias to the back of the board.
The thermal conductivity of FR4 is approximately 1.0 W/(m-K), and other high frequency-compatible laminates (e.g., Rogers and Isola materials) have similar values for thermal conductivity. In contrast, ceramic materials have thermal conductivities that range anywhere from 20 to 300 W/(m-K), making them ideal for use with hot components, or placed in a system near some other heat source. The high thermal conductivity of ceramic substrates can eliminate the need for bulky heat sinks or noisy fans in your board. Common ceramics for PCBs include aluminum oxide, aluminum nitride, boron nitride, and silicon carbide.
Ceramic PCBs have other advantages and disadvantages. Although ceramic materials have high strength, they are brittle and can fracture easily, whereas FR4 is quite flexible. The thermal expansion coefficient of ceramic materials is already closer to the value for copper than FR4 or other fiber weave substrates. This reduces thermal stress on thin traces and vias during operation. The material properties of ceramics can also be adjusted through the use of various additives; this remains an active area of materials science research.
Metal-core PCBs are another alternative to FR4 substrates. This type of substrate uses a metal sheet (typically aluminum) as the core. This core can be connected to a nearby ground plane, providing an extra layer of shielding against EMI. The metal core also provides higher mechanical strength and low thermal resistance while still being flexible; these boards will not fracture easily compared to ceramic materials. Aluminum core PCBs are often used for high power LED lighting systems, where the board is then attached to a large metal housing. This provides very high heat dissipation away from the board.
High power SMD LEDs on an aluminum-core PCB. The aluminum core provides low PCB thermal resistance and high structural strength.
Whether you are designing on FR4, ceramic, or metal-core substrates, you’ll need the right PCB design and analysis software if you want your board to have low PCB thermal resistance. Allegro PCB Designer from Cadence is ideal for creating your next layout on any substrate material, and Cadence’s full suite of analysis tools can help you identify hot spots in your board.
If you’re looking to learn more about how Cadence has the solution for you, talk to us and our team of experts.