When working with power amps or processors, you’ve got a few parts that make up the majority of the thermal load. These will likely be MOSFETs or processor packages.
For MOSFETs, you can get rid of about 3-4W of heat load on the PCB copper, without a heat sink. Any more than that and you’ll need a dedicated heat sink and you’ll have to deal with TIMs (thermal interface materials) between the heat sink and FET. You’ll probably have to deal with electrically isolating TIMs, which tend to perform more poorly than non-electrically isolating TIMs.
For power amps, fans are often discouraged due to noise, but as long as you’ve got enough heat sinking area and airflow you won’t need them. Always put the fans blowing in (not out) to keep them in the cool air if you need to use them.
For processors there are many solutions that you’re probably aware of. Again, you’ll have to deal with TIMs (or grease), and if the heat sink is large enough, you may have to electrically isolate it since it will likely mount to something metal, which may cause you to use poorer performing TIMs (or more expensive TIMs).
Free standing heat sinks avoid having to use electrically isolating TIMs which will help you use a smaller heat sink because of the reduced thermal resistance.
The goal overall is to get the heat from the junctions on the silicon heat generating devices to the air with as little resistance as possible. Heat sink area is your friend.
Thanks for this question!
Heat sinks are what you have to spend money on when you’ve run out of cheaper solutions using PCB copper (my view).
To choose one, first figure out what TIM (thermal interface material) you’ll have to use to conduct the heat from the chip to the heat sink. If this is grease or other non-electrically isolating material, it will perform better for cheaper. If it has to be electrically isolating, it will usually perform worse and/or cost more.
If you can get away with a smaller heat sink that will not need electrical isolation, do that. If you can reduce the thermal resistance of the TIM, you can get away with a smaller heat sink.
To choose a heat sink, do the steady state heat equations (very simple) based on the thermal resistance of the heat sink, the TIM, and the silicon package. Add up all those thermal resistances, figure the input heat wattage, and you’ve got a good guess on what your final temps will be.
For heat sinks, they are clearly rated with a thermal resistance that’s proportional to the airflow speed, or natural convection. Choose one with a low enough thermal resistance at the airflow you have to keep your MOSFET junction cool enough to be reliable over the life of your product.
PCB material doesn’t matter much, but the amount of copper (in layers and weight/layer) is what makes a difference.
2oz copper, for example, is much more thermally conductive than normal thin copper, and if you’ve got 8 layers of it, that’s better than 4 to spread the heat out.
Thanks for your question!
I go over all this in my class on Jan. 24th if you want to know more.
We go over this in more detail (we do several estimations and talk about the implications) in my heat management class on Jan. 24th if you want to know more detail. Thanks!
Thanks to everyone who asked questions and thanks to @kevencoates for these amazing answers! Join Keven on November 8th for a webinar on thermal considerations for SMD PCBs.
My pleasure! It’s always fun to spread knowledge. I hope this session helped.
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