Heat Sink Design Software
Simple but powerful tool for designing and selecting heat sinks. Download a free trial now! We can provide customers with thermal design assistance. The service includes thermal analysis using simulation software, heat sink design and optimization. Simple but powerful tool for designing and selecting heat sinks. Download a free trial now!
Shares 18 With the increase in heat dissipation from microelectronics devices and the reduction in overall form factors, thermal management becomes a more a more important element of electronic product design. Both the performance reliability and life expectancy of electronic equipment are inversely related to the component temperature of the equipment. The relationship between the reliability and the operating temperature of a typical silicon semi-conductor device shows that a reduction in the temperature corresponds to an exponential increase in the reliability and life expectancy of the device. Therefore, long life and reliable performance of a component may be achieved by effectively controlling the device operating temperature within the limits set by the device design engineers. Heat sinks are devices that enhance heat dissipation from a hot surface,usually the case of a heat generating component, to a cooler ambient, usually air. For the following discussions, air is assumed to be the cooling fluid. Inmost situations, heat transfer across the interface between the solid surface and the coolant air is the least efficient within the system, and the solid-air interface represents the greatest barrier for heat dissipation.
A heat sink lowers this barrier mainly by increasing the surface area that is in direct contact with the coolant. This allows more heat to be dissipated and/or lowers the device operating temperature. The primary purpose of a heat sink is to maintain the device temperature below the maximum allowable temperature specified by the device manufacturers. Thermal Circuit Before discussing the heat sink selection process, it is necessary to define common terms and establish the concept of a thermal circuit.
The objective is to provide basic fundamentals of heat transfer for those readers who are not familiar with the subject. Notations and definitions of the terms are as follows: Q: total power or rate of heat dissipation in W, represent the rate of heat dissipated by the electronic component during operation. For the purpose of selecting a heat sink, the maximum operating power dissipation issued. T j: maximum junction temperature of the device in °C.Allowable T j values range from 115°C in typical microelectronics applications to as high as 180°C for some electronic control devices. In special and military applications, 65°C to 80°Care not uncommon. T c: case temperature of the device in °C.
Since the case temperature of a device depends on the location of measurement, it usually represent the maximum local temperature of the case. T s: sink temperature in °C. Again, this represents the maximum temperature of a heat sink at the location closest to the device. T a: ambient air temperature in °C. Using temperatures and the rate of heat dissipation, a quantitative measure of heat transfer efficiency across two locations of a thermal component can be expressed in terms of thermal resistance R, defined as R = T/Q Were T is the temperature difference between the two locations. The unit of thermal resistance is in °C/W, indicating the temperature rise per unit rate of heat dissipation.
This thermal resistance is analogous to the electrical resistance R e, given by Ohm’s law: R e = V/I With V being the voltage difference and I the current. Figure 1: Thermal resistance circuit Consider a simple case where a heat sink is mounted on a device package as shown in Fig 1.
Using the concept of thermal resistance, a simplified thermal circuit of this system can be drawn, as also shown in the figure. In this simplified model, heat flows serially from the junction to the case then across the interface into the heat sink and is finally dissipated from the heat sink to the air stream. The thermal resistance between the junction and the case of a device is defined as R jc = (T jc)/Q = (T j– T c)/Q This resistance is specified by the device manufacturer.
Although the R jc value of a give device depends on how and where the cooling mechanism is employed over the package, it is usually given as a constant value. It is also accepted that R jc is beyond the user’s ability to alter or control. Similarly, case-to-sink and sink-to-ambient resistance are defined as R cs = ( T cs)/Q = (T c– T s)/Q R sa = ( T sa)/Q = (T s– T a)/Q respectively. Here, R cs represents the thermal resistance across the interface between the case and the heat sink and is often called the interface resistance. This value can be improved substantially depending on the quality of mating surface finish and/or the choice of interface material. R sa is heat sink thermal resistance.
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Obviously, the total junction-to-ambient resistance is the sum of all three resistances: R ja = R jc + R cs + R sa= (T j – T a)/Q Required Heat-Sink Thermal Resistance To begin the heat sink selection, the first step is to determine the heat sink thermal resistance required to satisfy the thermal criteria of the component. By rearranging the previous equation, the heat sink resistance can be easily obtained as R sa = ((T s – T a)/Q) – R jc– R cs In this expression, T j, Q and R jc are provided by the device manufacturer, and T a and R cs are the user defined parameters. The ambient air temperature T a for cooling electronic equipment depends on the operating environment in which the component is expected to be used.
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Typically, it ranges from 35 to 45°C, if the external air is used, and from 50 to 60°C, if the component is enclosed or is placed in a wake of another heat generating equipment. The interface resistance R cs depends on the surface finish, flatness, applied mounting pressure, contact area and, of course, the type interface material and its thickness. Precise value of this resistance,even for a give type of material and thickness, is difficult to obtain, since it may vary widely with the mounting pressure and other case dependent parameters. However, more reliable data can be obtained directly from material manufacturers or from heat sink manufacturers. Typical values for common interface materials are tabulated in Table 1.