“HEATSINKS – What are they, why do we need them”

There seemed to be a lot of talk this week on temperatures over heating of chassis, modules, devices like the Rasp Pi etc.

This subject has many aspects to it and if I left something out, please speak out so we can all benefit from the information, please bear with me about the order that it is presented.

Heat sinks are mechanical devices that are used in electronic products as heat exchangers. Below are Rasp Pi 3B+ heatsinks as received with the Pi.

Heat energy caused by the dissipation of power in components is directed away from the circuit components by the use of heat sinks.

Doing this reduces the associated temperature rise of the components.

Shape and size also varies between heatsinks depending on their individual application.

Many materials are used for heat sinks including aluminum, aluminum alloys, beryllium oxides, brass, ceramic and copper.

#1

Heatsink compound is the interfacing part that completes the maximum amount of heat to transfer from the device’s surface over to the heatsink to dissapate the heat that it collects.

Beryllium oxide is a paste used in high power RF devices. The white paste was the standard compound used for decades between the device and the insulating mica insulator and the surface of the heat sink. The powder form is more dangerous to inhale than asbestos.

#2

Beryllium Oxide (White paste) is a CARCINOGEN–HANDLE WITH EXTREME CAUTION. * Contact can cause eye irritation, redness, itching and burning. * Beryllium Oxide can irritate and burn the skin. Higher exposure may cause skin ulcers to develop.

Heat sinks have several important specifications associated with their suitability for use in a particular application.

Power dissipation at temperature rise indicates the maximum amount of heat that can be dissipated by the heat sink when sitting in an environment with the specified temperature.

Thermal resistance at force airflow is a measure of the temperature rise per watt of power dissipated that would be expected based upon airflow through the heat sink fins. This is usually specified in liters per minute (LPM).

Thermal resistance at natural or still air is the fan-less, temperature rise expected per watt dissipated.

Heat sinks have many attachment methods including adhesives, bolts, clips, board locks, press-fits, thermal tapes and soldering.

There are no mounting brackets for a heat sink on any Raspberry Pi model, which makes it necessary to use a thermally conductive heat sink. These finned heatsinks are attached with a conductive adhesive tape attached to the Raspberry Pi case surface, making installation simple and fast.

Silicon thermal grease or thermal rubberized pads may be used to provide a thermal coupling between the heat sink and the device being cooled. Note – rubber pads have been shown to be not as efficient as thermal grease or paste between the two surfaces.

What Does Thermal Grease Do?

Air, which happens to be a great thermal insulator, inhibits heat transfer from a heat source to the thermal management device ( Heatsink). Thermal grease’s semi-liquid properties allow it to penetrate the microchasms in both the surface of the heat source and the surface of the heatsink.

Without the use of thermal grease, a heat source in direct contact with a thermal accessory would develop an array of air pockets between the two devices. These air pockets would directly reduce the thermal management system’s thermal conductivity. Using thermal grease eliminates these air pockets, thus improving the subsequent thermal management accessories’ effectiveness. But it’s not just about improving efficiency. Without thermal grease, operating the heat source normally risks damaging the internal semiconductor die and integrated circuit.

#3

Above is the heatsink in a Fusion repeater transmitter, note the painted surface.

A way to check temperatures.

#4

To check the calibration of your measuring tool at home.

#5 Calibration

Obviously we need to know what the device that we testing can handle temperature wise.

#6

Using our temperature equation and the specifications from the datasheet we see that:

Thus the MINIMUM thermal resistance we need in a heatsink will be 9.29 C/W; however in engineering we always want to give ourselves a margin of error, in this case lets go with 20%.

This means we actually need to find a heatsink with a thermal resistance of not more than 7.43 C/W. At this point we can go to an electronics part distributor website such as Mouser or Digikey and search for heatsink that will fit a TO-220 case and have the right thermal resistance.

I quickly found part No. 532-504222B00 from Mouser which has a thermal resistance of 6.4 C/W; this will work perfectly for our application. When attaching the heatsink it is important to use thermal compound between the case and heatsink. This is because it will fill up any tiny pockets of air that may be between the two and cause an increase in thermal resistance- we definitely don’t want that.

As shown the 7805 voltage regulator specifications indicate the Maximum operating temperature is +125 degrees Celsius which is about +257 degrees F.

So it is very easy to think that an electronic device is overheating by touching and feeling the heat of the device. You should always use a measuring device to be able to accurately know wht the device is operating at.

#7 DYI Modified Temperature probe

Above a thermal couple has been placed in between the fins and filled with thermal epoxy.

#8 Thermal Couple bead & Liquid probe

#8

Using a plastic spring clamp doesn’t exert too much pressure on the device under test and securely hold the modified thermal couple to the cpu’s case, without any electrical issues contacting the circuit board. ( also a drop of thermal compound is used between the two surfaces)

There are several Heatsink Calculators online  that can assist you in the selection of the proper size and type of heatsink that you may need for a particular application.

The heatsinks found inside your computers are called Vapor Chambers because of the spaces between the fins.

When Heat Pipes are used the heat is moved to another location and can have additional cooling applied from fans or by running liquid coolant through the heat pipes fins at the remote location.

#9

This is an instance where it is very much ok to have more than needed surface area than not enough heat transferring area, allowing a device to reach a critical operating temperature.

I welcome comments and questions about this topic.

73

WB6AMT

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