Shrewd Thermal Management Helps Defeat the Heat

Despite great strides made by electronic system designers in developing products that perform sophisticated tasks, engineers may encounter performance-limiting factors beyond electronic circuitry—like thermal management. Even if good design practices and reliable components are in place, system reliability can suffer if appropriate temperature controls are not implemented. That’s why circuit designers should have a basic understanding of how to manage operating temperature.

The relationship between operating temperature and reliability is defined in a system’s failure rate (useful system life in failures per 106 hours), as expressed in the Arrhenius Model:

where:

  • l = failure rate
  • A = constant
  • Ea = activation energy for the particular failure mechanism
  • k = Boltzmann’s constant
  • T = Kelvin temperature

Equation 1 shows that failure rate is a function of the temperature stress: the higher the stress, the higher the failure rate (more failures per 106 hours). Typically, according to Equation 1, each 10°C rise in temperature increases the failure rate by 50%. Conversely, cutting the operating temperature by 10°C reduces the failure rate.

Failure rate and its inverse, mean time between failures (MTBF), are measures of the effectiveness of thermal management in electronic systems. Understanding thermal management involves the electronic system designer’s entry into the domain of the packaging or thermal design engineer. The first concept to understand is heat transfer.

Heat Transfer
Heat is typically transferred from high-temperature objects to lowertemperature objects. There are three types of heat transfer: conduction, radiation, and convection. Among the devices employed in transferring heat to cool individual components, primarily semiconductors, are heatsinks, thermal interface materials, heat pipes, and thermoelectric modules.

Heatsinks
The most widely used thermalmanagement device, the heatsink, transfers heat by conduction from a semiconductor to a specially constructed metal plate. The most common heatsinks include many metal fins. Combined with a large surface area, the metal’s high thermal conductivity transfers the heat from the semiconductor to the heatsink and then to the surrounding air. The heatsink’s ability to transfer heat depends on its material, geometry, and overall surface heattransfer coefficient.

Heatsinks usually consist of aluminum or copper, although the latter is heavier and more expensive. Another advantage of aluminum is that it’s easily shaped into different geometries.

Heat passes from the case to semiconductor heatsinks before it’s emitted into the air. Consequently, the heatsink increases the effective heat dissipation area and removes heat from the semiconductor, permitting it to operate at higher power levels. In other words, the heatsink provides a low thermal resistance path from the semiconductor’s case to the ambient air.

A key parameter when using a heatsink concerns the thermal resistance of the associated semiconductor package. That refers to its ability to conduct heat away into the surrounding environment. Overall, designers should aim for a low thermal resistance value for a given amount of power. Thus, the semiconductor’s junction can operate at an optimum temperature and provide a longer useful life.

Figure 1 illustrates the flow of heat through a semiconductor without a heatsink, and Figure 2 shows heat flow with a heatsink. Schematically, thermal resistances are represented as resistors, though they’re really the equivalent thermal values. Mathematically, thermal resistance is the rise in the drain (or collector) junction temperature above the case temperature per unit of power dissipated in the device (Equations 2 and 3):

where:

  • θjc = thermal resistance from junction- to-case in °C/W, which is a function of the semiconductor and its package
  • θja = thermal resistance from junction- to-ambient in °C/W
  • Tc = semiconductor case temperature in °C
  • Ta = ambient air temperature in °C
  • Tj = semiconductor junction temperature in °C
  • Pd = semiconductor power dissipation in watts

Physical placement of the heatsink depends mostly on the type of semiconductor being employed. If it’s a microprocessor that integrates more than 100 pins, the heatsink will be placed on top of it. On the other hand, a power semiconductor that contains three pins is usually placed on top of the heatsink.

Usually, there are clamps or some type of fastener that supply pressure between the mating surfaces. This will ensure there’s good heat transfer between the heatsink and the semiconductor. Figure 3 illustrates a splayed-pin heatsink forged from pure aluminum and oxygenfree copper.

Thermal Interface Materials
Ideally, heatsinks require intimate surface-to-surface contact with the semiconductor to be cooled. In the real world, irregular surface areas on the semiconductor and heatsink prevent this intimate physical contact from occurring.

As a result, some type of thermally conductive interface material is necessary to fill any gaps between the mating surfaces. In many cases, this interface material must also act as an electrical insulator and thermal conductor.

Figure 2 shows the thermal resistances between the semiconductor junction and ambient air temperature. Without a heatsink, it’s:

Here, θca (case-to-ambient) thermal resistance is a relatively high value. So θja is also high, and the semiconductor may operate at a relatively high, unreliable temperature. With a heatsink, it’s:

where:

  • θci = thermal resistance from caseto- thermal interface in °C/W
  • θis = thermal resistance from thermal interface-to-sink in °C/W
  • θsa = thermal resistance from sinkto- ambient air in °C/W

Here, the heatsink and thermal interface material reduce the overall thermal resistance, and the semiconductor operates at a reliable temperature. For best cooling, select a heatsink and thermal interface material with a low thermal resistance.

Heat Pipes
Heat pipes are an ideal heat transport for longer distances. One example would be if a sensitive heat source becomes separated from a remote heat exchanger or heatsink. Heat pipes also spread heat well. Combined with folded aluminum fins, they can be used as efficient and lightweight heatsinks.A heat pipe consists of a sealed aluminum or copper container whose inner surfaces have a capillary wicking material (Fig. 4).

A liquid under its own pressure in the container enters the pores of the capillary material, wetting all internal surfaces. Applying heat at any point along the surface of the heat pipe causes the liquid at that point to boil and enter a vapor state.

When that occurs, the liquid picks up the latent heat of evaporation. The gas, which then has a higher pressure, moves inside the container to a colder location where it condenses.Thus, the gas gives up the latent heat of vaporization and moves heat from the input to the output end of the heat pipe. Heat pipes have an effective thermal conductivity many times that of copper.

The container isolates the working fluid from the outside environment. Therefore, it must be leak-proof. And the container must maintain the pressure differential across its walls.

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