How to prevent your PCB from overheating
PCB Materials for Engineers Explained
Printed circuit board (PCB) materials are formulated to withstand a certain amount of heat. What happens when the temperatures rise beyond certain limits? Performance takes a nose dive, especially at higher frequencies. This is why cost-effective heat management is perhaps an engineer’s number-one priority.
Of course, heat-tolerant PCB materials and carefully designed circuits can tolerate a certain amount of heat. First, the circuit designer should understand the different parameters of the materials’ behaviour when temperatures rise.
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Heat comes from various sources
Circuit boards are assembled in increasing density to make smaller, lighter designs. A component mounted to the circuit board can produce heat – so can an external source, such as found in automotive electronic systems.
Heat causes most materials to expand
Because of the smaller wavelengths at higher-frequencies, microwave and especially millimetre-wave (30 GHz and higher) circuits have small features that can become distorted as a circuit board expands due to heat.
Adding to this problem is the demand for more compact electronic designs. It’s common to see circuits designed with materials that have higher dielectric constants with smaller circuit features for a particular frequency and wavelength.
When the temperature rises, the circuit materials expand, changing the form of transmission lines and altering the impedance of conductors from a desired value. The results? Loss of linearity, distortion and shifts in frequency because of changes in transmission-line dimensions.
Materials expand at different rates
Circuit boards are made of composite materials, including dielectric layers and conductive metal layers. These composite materials tend to expand at different rates and to different extremes.
The coefficient of thermal expansion (CTE) describes the amount of expansion a material experiences. In an ideal world, the CTE of your board’s dielectric layers are close in value to the copper or other conductive metals laminated to the dielectric materials. Then both materials expand together at high temperatures.
What are your options?
You have several. First, a few points to consider:
- The resistance of copper traces and vias account for significant power loss and heat generation
- Electrical connections with a larger cross-sectional area have lower resistance, which reduces the amount of power lost to heat.
Most PCBs use copper to the equivalent of about 1 ounce per square foot. If using fans or heat sinks are out of the question, a PCB with high current should use at least double this amount of copper. If your circuits operate at over 10 amps, then go as high as 3 or 4 ounces per square foot.
What does heavier copper mean for the width of traces?
You’ll have to increase them. This doesn’t mean losing usable area, however. Place the traces deeper into the board. The heat will dissipate into the board itself and into nearby thermal vias.
Yes, this will probably mean use a thicker board, but that’s a good thing in high-current devices. With thin boards, everything can heat up to a high temperature.
A thicker board requires more thermal energy to reach a high temperature. This helps keep the temperature low at the top of the board.
Consider heat pipe integration
This is especially helpful when your application is a compact electronic device, such as a mobile. This cost-effective, reliable method of passive heat transfer gives you effective thermal conductivity, vibration-free operation and no moving parts.
The pipe contains a small amount of fluid, which can be water, nitrogen, acetone, sodium or ammonia. The fluid absorbs the heat. The resulting vapor then travels along the heat pipe to a condenser region and condenses back into a liquid.
Use thermal interface materials (TIM)
Thermal interface material fills the gaps between surfaces with a substance that offers better thermal conductivity than the air that would otherwise fill those microscopic gaps. In turn, this improve heat transfer between the two surfaces.
Typical TIMs conduct heat around 100 times better than the air they displace. Thermal greases are the favoured material in the electronics sector for improving the performance of the heat sink.
Best of all: active cooling integration
Don’t think just because your application is compact that you can’t use a fan. Micro fans are the most effective way to cool any mobile device. In fact, cooling fans are always your best option, no matter what the size or voltage for your application.
If you’re concerned about the noise fans can produce, you needn’t be. The amount of noise generated is determined by the fan’s size and how fast it spins. The faster, the noisier. But fans can be altered, such as insulating the duct, installing sound-dampening material or simply mounting the fan with soft materials, such as rubber.
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