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Avoid Thermal Issues with PCB Design

With more demanding and power hungry devices these days, avoiding thermal issues with your product is as important as it has ever been. One of the first places you can look to help you is the way your PCB is designed.

A characteristic that the PCB designer must understand is the continuous operating temperature of materials, and the coefficients of thermal expansion (CTE). Some of these properties are provided by the reinforcement of the PCB (The copper clad laminates used to produce PCBs consist of 3 parts; the resin, the reinforcement, and the copper foil). There are many different types ranging from polyimide-woven aramid, which has a CTE range of 5 – 6 PPM/°C, to aluminium, which has a CTE range of 20 – 24 PPM/°C. The values can be referenced in IPC-2221. One of the best is copper, which is good because it is a good conductor as well! Therefore, large planes of copper can be used on your PCB to perform the function of heat sinking to keep your board cool.

The resin part of your PCB also responds to heat, and the more there is, the more it expands. This expansion needs to be considered because it will put strain on your plated through-holes, and barrels. Therefore hole size is very important as bigger holes provide more copper, and therefore better heat sinking.

The combination of resin and reinforcement gives an expansion model for the thickness, or Z axis of the PCB, which is normally measured by parts per million (ppm / °C). All laminates have a uniform rate of Z axis expansion until they reach a particular point. Once this point is reached, the expansion rate increases dramatically, and with no surprise, this is when most damage can occur. This point is known as the glass transition temperature, or Tg, and most laminates are sold based on their Tg capability. For instance, a glass epoxy laminate has a Tg rating of around 125°C, whereas glass polyimide laminate has a Tg rating of 270°C. You can ask you PCB fabricator about this. Not surprising though, a higher Tg rating means higher cost.

If the current draw is high going through the tracks on your PCB, then they can produce heat too. It is important to check the proper track width is sufficient to cope with the wattage being passed through it, and therefore keeping the copper at a safe temperature. There are PCB trace width calculators available, just double check they are based on the IPC-2221 standard.

Another way of removing heat is by using a special plane in the PCB purely for heat sinking. Use thermal vias to pull heat away from areas and transfer it to the cooler plane. This plane can then be connected to the frame, which could then be connected to a larger, purpose built heat sink. More than one vias can be used to perform this function, stitching patterns can be made to efficiently pull the heat away. This can also help with grounding.

Datasheets these days often included cooling notes for devices, and how a particular footprint for a device should be designed on your PCB. This combined with the information above should give you a sound foundation for understanding, and progressing your PCB design.

A final thought, if you are designing a complex PCB, it is a good idea to get your PCB fabricator involved from as early as possible. At the end of the day, there is no point designing an amazing, super-efficient PCB if it can’t be made! They will be able to offer guidance about design for manufacture, and hopefully make your design a reality.

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