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As semiconductor densities increase, they are dissipating increasing amounts of power while at the same time decreasing in size. Because higher junction temperatures are directly associated with decreased reliability and increased failure rates, field repair costs could unexpectedly skyrocket unless a thermal management solution is designed in. Unfortunately, it’s not as simple as just using a bigger heat sink. The heat sink needed to adequately cool today’s power semiconductors can be larger than the device it cools.
In a typical case, a hot spot can develop directly over an IGBT, which is caused by “thermal spreading resistance.” A new type of heat sink that counters this is a vapor chamber heat sink that uses a two-phase heat transfer process to spread heat evenly across the device’s base. By alleviating thermal spreading resistance, these heat sinks keep devices cooler than traditional heat sinks, increasing system reliability and decreasing field repair costs.
The amount of heat that can be transferred from an IGBT module into the air depends on the surface area of the heat sink and the speed of air flowing over its surface. Most heat sinks consist of aluminum shaped into fins or pins in order to maximize its surface area. This increases the amount of heat that can be dumped into the air. Today, dedicated fans are often added to blow air directly across the heat sink. As the power increases, larger heat sinks are needed to provide more surface area, with faster fans to increase the airflow. Unfortunately, as a fan’s speed increases, its reliability decreases.
While IGBT power dissipation has been increasing, their size is decreasing, making it even more difficult to get rid of the heat. As the heat sink becomes larger than the IGBT module, a hot spot occurs over the module and the surface temperature gets cooler away from that spot (Figure 1). The cooler the surface, the less heat it is able to transfer into the air. Making a heat sink an inch longer may help, but making it yet another inch longer won’t help as much. Eventually, the end of the heat sink is so cool that it’s useless.
Vapor chamber heat sinks spread heat and eliminate hot spots, as shown in Figure 2. The cooling fins are more efficient because they are all the same temperature. Vapor chamber heat sinks are smaller and lighter than traditional heat sinks. They use a smaller, slower fan, and often don’t require a fan at all. Most importantly, they keep electronic devices cooler, extending life and reducing failure rates.
Vapor Chamber Operation
A vapor chamber is a vacuum vessel with a wick structure lining the inside walls that is saturated with a working fluid (Figure 3). As heat is applied, the fluid at that location immediately vaporizes and the vapor rushes to fill the vacuum. Wherever the vapor comes into contact with a cooler wall surface it condenses, releasing its latent heat of vaporization. The condensed fluid returns to the heat source via capillary action, ready to be vaporized again and repeat the cycle. The capillary action of the wick enables the vapor chamber to work in any orientation with respect to gravity. A vapor chamber heat sink consists of a vapor chamber integrated with cooling fins, pins, etc.
Due to the way the vapor chamber operates, the heat source can be placed anywhere on the base without affecting its thermal resistance. In addition, there can be multiple heat sources dissipating the same or different amounts of power. The rate of fluid vaporization at each source will stabilize and the vapor chamber will be nearly isothermal.
Vapor chamber heat sinks can use a variety of working fluids, depending on the operating temperature. The thermodynamic properties of water make it an order of magnitude better than any other fluid for the majority of electronics cooling applications. Water’s high latent heat of vaporization spreads more heat with less fluid flow. Its high surface tension, when presented to a copper powder wick, generates a large capillary force that allows operation in any orientation. Its high thermal conductivity minimizes the temperature drop associated with conduction through the wick. The amount of water in a vapor chamber is only enough to saturate the wick structure. If a unit were ever punctured, air would leak into the vacuum space but no water would leak out – the water being held in by the wick’s capillary force.
There is a newly developed vapor chamber product family called Therma-Base™. This family uses sintered copper powder as the wick structure. Therma-Base heat sinks have been freeze/thaw tested without adverse thermal or mechanical effects. The passive copper/water materials combination has been proven to be reliable in millions of heat pipes and has been life tested for over 17 years.
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