Your Thermal Design Guide for High Power PCBs
If you’re designing your next PCB on a beach in Cancun, you’ll need to consider some thermal management techniques, both for yourself and your board. The right thermal management practices can help keep your board and yourself cool as you design the next revolutionary electronic product.
Whether you’re working with power electronics, embedded systems, industrial equipment, or just designing a new motherboard, you’ll have to contend with temperature rise in your system. Constantly running at high temperature reduces board lifetime and can even lead to failure at some critical points in your system. Taking account of temperature rise early in the design process can help you extend your board’s rated service life and that of your components.
Thermal Design Begins with Estimating Operating Temperature
Before beginning your new design, you’ll need to consider how hot your board will be allowed to run, the environment in which it will operate, and the power dissipated by components. These factors will all work together to determine the operating temperature of your board and your components. This will also help you determine how aggressive you need to be with your cooling strategy.
Placing your board in an environment with higher ambient temperature will cause it to hold on to more heat, thus it will run at a higher temperature. Components that dissipate more power will need more aggressive cooling methods in order to maintain the temperature at a set level. Important industry standards may dictate the maximum temperature of your components and substrate during operation.
Be sure to check your allowed operating temperature of your components in datasheets as well as the allowed temperature in important industry standards before designing a thermal management strategy. You’ll need to combine active and passive cooling with proper board layout in order to prevent damage to your board.
Active vs. Passive Cooling: Which is Right for Your Board?
This is a significant question that any designer should consider. Generally, passive cooling works best when the ambient temperature is much lower than the operating temperature. The thermal gradient between the system and the environment will be large, forcing larger heat flux away from your component and the board itself. With active cooling, you may be able to provide greater temperature reduction, depending on the active cooling system, even if the ambient temperature is higher.
You should try to incorporate some minimal level of passive cooling for active components in order to dissipate heat into the ground plane. Many active components include a thermal pad on the bottom of the package to allow heat to be dissipated to a nearby ground plane through stitching vias. These stitching vias are then run up to a copper pad beneath the component. There are some useful PCB calculators that can be used to estimate the size of the copper pad you need beneath your component.
Obviously, a copper pad beneath a component cannot extend beyond the edge of the actual component as this will interfere with surface-mount pads or through-hole pins. If a single pad does not bring the temperature down to the required level, then you may need to add a heat sink on the top of the device to dissipate more heat. You can also increase heat flux into a heat sink with a thermal pad or thermal paste.
Evaporative cooling is another option for cooling active components that generate a significant amount of heat. However, evaporative cooling components are very bulky and are not ideal for many systems. If the system ever leaks or ruptures, you’ll be left with fluid leaking all over your board. At this point, you might as well implement an active cooling method as this will provide the same or better heat dissipation.
Industrial-scale evaporative cooling at a coal power plant
If you need to go even further to reduce the temperature of active components like FPGAs, CPUs, or other devices with high switching speed, active cooling with a fan may be required when passive cooling does not do the trick. Fans do not always run at full speed and they may not even be on at some times. Higher temperature components and those that generate more heat require a fan that runs at a faster speed.
Fans are noisy as the PWM signal generates some noise due to switching. Your board will need to include a circuit to generate a PWM signal to control the fan speed, as well as a sensor to measure the temperature of the component in question. AC-driven fans with an electronic switching controller also produce radiated EMI at the fundamental switching frequency and at each higher order harmonic. Nearby traces components will need to have sufficient noise rejection/immunity or even shieling in extreme cases if a fan is used.
You can also provide massive levels of cooling with an active cooling system that uses a coolant liquid or refrigerant. This is an uncommon solution as it requires a pump or compressor to move the cooling liquid or refrigerant through the system. As an example, water-cooled systems are used in high performance gaming computers to cool GPUs, and other systems are available for high speed CPUs.
Some Simple Thermal Design Guidelines
While using a ground plane below your signal traces improves signal integrity and noise rejection, it also acts as a heat sink. Components with thermal pads that run stitching vias down to a ground plane will allow the ground plane to more easily dissipate heat from the surface layer. Heat generated in traces on the surface layer then can easily dissipate into the ground layer.
Traces that carry high current, especially traces in DC circuits, will need to have higher copper weight in order to dissipate the appropriate amount of heat in your board. This may require having wider traces than are typically used in high speed or high frequency devices. The geometry will affect the trace impedance seen by AC signals, meaning that you may need to change your stackup in order to keep impedance matched to the value defined in your signalling standard or your source/load components.
Beware of thermal cycling in your board as repeated temperature cycling between high and low values causes stress to accumulate in vias and traces. This can lead to barrel cracking in vias with high aspect ratio. Cycling over long periods can also cause trace delamination on the surface layer, effectively ruining your board.
Working through EMI with particularly power sensitive boards is a challenge
If you want to keep temperature in check in your PCB, you’ll need more than a thermal design guide. You need the right PCB layout and design software with a full suite of design tools. Allegro PCB Designer and Cadence’s full suite of design tools can help you implement the cooling strategy you need in your design to help dissipate heat and keep temperatures low.
If you’re looking to learn more about how Cadence has the solution for you, talk to us and our team of experts.