Electronic Design

Shrewd Thermal Management Helps Defeat The Heat

Optimization of thermal controls will significantly enhance electronic system reliability.

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:

λ = 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 lower-temperature objects. There are three types of heat transfer: conduction, radiation, and convection (Table 1). Devices employed in transferring heat to cool individual components, primarily semiconductors, include heatsinks, thermal interface materials, heat pipes, and thermoelectric modules.

Heatsinks
The most widely used thermal-management 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 heat-transfer 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. Table 2 describes the various methods for producing heatsinks.

Heat passes from the case to semiconductor heatsinks before it's emitted into the air. Thus, 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. Designers should aim for a low thermal resistance value for a given amount of power, which allows the semiconductor's junction to operate at an optimum temperature and provide a longer useful life.

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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 on the type of semiconductor being employed. If it's a microprocessor with more than 100 pins, the heatsink will be placed on top of it. On the other hand, a power semiconductor with three pins is usually placed on top of the heatsink.

Usually, there are clamps or some type of fastener to supply pressure between the mating surfaces to ensure good heat transfer between the heatsink and semiconductor. Figure 3 shows a splayed-pin heatsink forged from pure aluminum and oxygen-free 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. Therefore, 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:

θja = θjc + θca

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:

θja = θjc + θci + θis + θsa

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 (Table 3).

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Heat pipes
Heat pipes are an ideal heat transport for relatively long 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, it must maintain the pressure differential across its walls. Moreover, it must enable the transfer of heat to take place from and to the working fluid. Finally, it must be nonporous to prevent the diffusion of vapor as well.

Working fluids have to be compatible with the wick and wall materials, offer good thermal stability, provide wettability of wick and wall materials, and feature a vapor pressure that's appropriate for the operating temperature range. Also, a high latent heat of vaporization is desirable to transfer large amounts of heat with minimum liquid flow and to maintain low pressure drops within the heat pipe.

The primary purpose of the wick is to generate capillary pressure to transport the working fluid from the condenser to the evaporator. The wick is a porous structure usually made from steel, aluminum, nickel, or copper in various ranges of power sizes. Typically, they're fabricated using metal foams or felts.

Thermoelectric modules
Thermoelectric modules (TEMs) employ the Peltier effect, which rapidly heats or cools electronic components. TEMs are solid-state devices with no moving parts, so they're reliable and don't require maintenance. They're often used to eliminate hotspots on a circuit board or to cool high-brightness LEDs.

In operation, a TEM "pumps" heat from one ceramic face to the other ceramic face. Applying a low-voltage dc to the module cools one side and heats the other. Cooling is proportional to the amount of current applied. Varying the current applied and the direction of current provides tight temperature control in cooling applications.

Peltier elements have very low efficiency because they consume more power than they transport. In fact, actual Peltier elements may consume twice as much electrical energy as they transport in the form of heat. A typical module possesses two wires for the application of power that must be the correct polarity for cooling. Applying the voltage with the wrong polarity will heat rather than cool the component. If the cooling of the Peltier element fails, the results can be disastrous. Also, don't apply power to a TEM without a heatsink, or the device may overheat and fail.

Condensation is another potential problem with a TEM, one that can ultimately damage an electronic circuit - components could be cooled below ambient temperature. The exact temperature at which condensation occurs depends on the ambient temperature and humidity.

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