Heat spreaders, heat sinks and thermal interface materials

Thermal considerations

From basic physics it is known that currents circulating through materials dissipate some energy as heat; this is referred as leakage. This leakage is proportional to the cross sectional area; so as that area gets smaller and smaller, like it's the case nowadays in ICs and PCBs, more heat is dissipated. Every device that uses electricity ends up generating heat as an undesirable byproduct.

Heat is bad for electronic circuits. There are several reasons: heat can damage the small circuitry of silicon; heat causes different rates of thermal expansion that stresses the materials and mightcause early fails, etc. So, heat has to be removed from the device to prevent damage and allow optimal performance. For low power ICs rated 5W or lower, normally air cooling is sufficient to dissipate heat generated.

For ICs rated 5 to 10W, the designer has to add a medium to help drive the heat out of the device due to non-homogeneous heat generation patterns. That device is normally known as a heat spreader (most commonly named lid as it seems to be a cover for the IC device). A picture of an Intel Pentium processor below shows a typical such configuration.

Simpler lid designs are available are commonly called heat slugs where the copper is thinner or cheaper.

For high power devices, a heatsink is added along with forced convection. For this applications there are obvious requirements of space and energy to allow the convection of heat and the powering of the fan, respectively. However it has been one of the most successful thermal disippations strategies.

For extremelly high power applications like supercomputers or multigaming consoles other means of cooling like compressed gases or liquid cooling are being considered. In laptops, heat tubes have been a mainstay for at least the last 5 generations.

This tower has two cooling fans and recirculation enabled to allow extra heat dissipation.

This Laptop has a liquid cooling solution enabled

Depending on some design considerations like rated or nominal wattage, location where the device is going to be placed in the board in relation to overall space, amount of free space for air to circulate, added features like fans, heat sinks, heat-tubes, the variation in dissipation can be extreme; for example, in 2006 it was claimed that the hot spot in the Pentium M processor (used on the Centrino platform) was reaching about the same heat density as in the surface of the Sun.

In order to understand dissipation patterns and help in designing the best cooling solution that accounts for all engineering aspects as well as costs there are many techniques used. Below a typical wind tunnel setup is used to determine thermal parameters.

In summary, the designer has to understand the environment, thermal patterns and material properties as it relates to heat resistance and dissipation. It might need to include systems like heat spreaders and sinks for some or all of the devices to be mounted in the board.

Materials

Heatsinks and heatspreaders are constructed of highly thermally conductive materials, typically metals. Normally copper is used due to its good balance between conductivity and price. However to prevent oxidation, it has to be passivated with a layer of nickel. These ICs operate under air cooled environments only because the heat sink removes the heat very efficiently. A parameter called Thetaca is used to qualify the ability of these materials to dissipate: in 2000 this parameter was around 1 W/K h, nowadays is around 0.18 and getting lower.

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