Using Thermal Analysis of PCBs to Design Rugged SystemsBy ZM Peterson • Jan 28, 2021
The goal in PCB thermal analysis and other electronics is to determine the steady state temperature of critical components and decide which cooling measures are needed, if any. The side goal is to determine how heat transfers through the system in order to design proper cooling measures. This sounds like it might be a simple matter, but there is no single equation for determining the steady state temperature and heat flux in every PCB.
Real PCBs have very complicated geometry and require simulations with 3D field solvers to get the most accurate results. The most accurate simulations involving active cooling components with airflow require multiphysics simulation packages or CFD packages. However, there are some simple steps in PCB thermal analysis that can give an estimate of the system’s equilibrium temperature, heat flow, and the effectiveness of cooling measures.
Thermal Analysis in PCBs for Rugged Systems
Products like backplanes, high speed SBCs, power amplifiers, and any other system that might be deployed in a harsh environment should always be designed with thermal management in mind. You shouldn’t wait until a PCB design review and DFM check to ensure your board can withstand thermal demands. The best strategy is to use a few simple calculations and some basic design choices to determine necessary cooling measures for a new system.
In examining equilibrium temperature, the system designer needs to select the maximum acceptable temperature for critical components, and then examine what the actual component temperature will be based on its power consumption. If the component temperature is at risk of being beyond the acceptable temperature limit, then additional cooling measures like fans or heat sinks should be added to the component. It is best to select an acceptable safety factor (maybe 20%) below the maximum allowed temperature for critical components.
When there are multiple components on the board that are at risk of reaching high temperature, our approach is to look at the minimum failure temperature among the group, and then base the maximum component temperature on that value (again, apply a 20% safety factor). Next, you need to use a component’s thermal impedance value to determine its temperature rise:
Definition of thermal impedance and temperature rise in integrated circuits.
Note that this is a temperature rise above ambient temperature, which I cannot stress enough… If the ambient temperature is closer to your maximum operating temperature, it’s more likely that your system will run too hot and will need some additional cooling measures. The thermal impedance value Z is normally found in the component datasheet for an integrated circuit. Z can be as low as ~20 °C/W for low-power amplifiers or ICs, or it can be as high as ~200 °C/W for microprocessors. This gives you a simple way to determine the appropriate power dissipation level in a component.
Power Plane Current Carrying Capacity
The PDN in a PCB is at risk of reaching high temperature when current carrying capacity is excessive. This is another area where the field solvers are needed to get an exact answer that shows the temperature field distribution for different values of current density in the PDN. However, the IPC has done extensive work in this area over the years and has developed an empirical model that can be used to determine the required cross-sectional area of a power plane or copper pour region for a given current density:
Cross-sectional power plane area vs. current and allowed temperature rise. This empirical model is defined in the IPC 2221 standards. Read more about this topic on one of my recent articles on Altium’s PCB Design Blog.
What’s important to note here is that the proportionality constant k is dependent on the PCB substrate material and laminate thickness. For thinner separation between plane layers, and for a material with higher thermal conductivity than FR4, k will be larger.
Cooling Techniques for PCBs in Rugged Systems
The two points above consider what can happen on the board to control temperature in the board and to ensure the operating temperature does not exceed system limits. In the event the component’s temperature is too high, there are some steps a designer can take to dissipate heat from a component:
- Heat sinks. This is probably the simplest way to protect critical components, but it’s very effective. Bonding a heat sink to a component with a thermal interface material or grease is a standard way to help dissipate away from a component. One option, which is used in newer 5G handsets and base station equipment, involves bonding the board directly to its enclosure to dissipate heat.
- Fans. Fans can be mounted directly on a component (such as a CPU), or on the enclosure to provide airflow across the system.
- Copper pour and plane layers. Plane layers and copper pour in a PCB can help transfer heat away from components. In fact, this is one reason thermal vias are used with a ground plane on a component’s die-attached paddle; they will carry heat away from the die and into the ground plane. Each plane layer and copper pour layer added will increase the total current capacity in the PCB.
- Component arrangement. It’s not always possible, but components that generate a lot of heat can be spread out across the board to prevent localized hot spots from forming. When used with planes and copper pour, this helps create a more even temperature distribution in the board.
Finally, DC power integrity simulations can be useful for spotting areas in the PDN with excessive current density. These areas, where current density exceeds the nominal maximum, will lead to a higher temperature in the PDN, and this will increase the overall board temperature. These problems are simple to correct as they require slightly widening certain areas of a polygon or plane to prevent excessive current density and temperature rise. In some cases, it may be good to add an additional layer pair to your stackup as this will provide another route for current, which then reduces the overall DC resistance of the PDN.
At NWES, we provide PCB design and layout services to SMBs, large enterprises, and aerospace and defense electronics companies. We have experience designing and developing rugged embedded systems, and always provide a thermal analysis of PCBs for rugged systems. We’ve also partnered directly with EDA companies and multiple ITAR PCB manufacturing firms, and we help our clients get through the PCB manufacturing process with ease. Contact NWES for a consultation.