[Engineering Essentials]
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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