Circuit Components I Thermal Managment


power at the steady-state condition. For determination of the overall thermal resistance of an individual component, standard test PCBs were used. The component in the centre position was measured. Since equation (1) can be rewritten to


detecting the maximum film temperatures by infrared thermal imaging as a function of dissipated power. The standard PCB was replaced by two electrically isolated copper blocks (60 mm x 60 mm x 10 mm). In Figure 4, values are given for passive components including chip resistors, chip resistor arrays, and MELF resistors.


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a simple approximation leads directly to the thermal resistance RthFA = 250 K/W for a 0603 chip resistor.


Integration Level In Figure 3, a single 1206 chip resistor mounted on the PCB leads to RthFA = 157 K/W. Additional resistors on the PCB (same load each) lead to an enhanced temperature rise (204 K/W for 5 resistors and 265 K/W for 10 resistors, respectively).


Figure 4. Internal thermal resistances derived from experimentally determined maximum film temperatures as a function of dissipated power


Table 1 demonstrates that thermal


Figure 3. Temperature rise and thermal resistances derives from experimentally determined maximum film temperatures as a function of dissipated power


Internal thermal resistance Replacing the PCB by an ideal body with a high thermal conductivity and heat capacity tending to infinity (in the real world a bulk copper block is suitable) leads to


resistance decreases with contact width. The best ratio of thermal resistance and chip size is provided by wide terminal resistors. The internal thermal resistance of a 0406 wide terminal chip resistor (30 K/W) is nearly the same as the thermal resistance of a 1206 chip resistor (32 K/W). The PCB design and environmental conditions of the entire assembly mainly determine the overall thermal resistance. As demonstrated, a reduced integration level of heat-dissipating components also leads to lower temperatures of individual components. This is contradictory to the ongoing trend for miniaturization, but might be considered in certain partial board areas. Besides changes in the PCB design, heat dissipation can be enhanced significantly on the component level by the choice of optimised components, such as wide-terminal resistors.


Again, the internal thermal resistance was determined experimentally by


Vishay Intertechnology | www.vishay.com Dr. Kevin Raiber is a member of the research & development department for the Vishay Draloric/Beyschlag Resistors Division


EXPERIMENTALLY DETERMINED INTERNAL THERMAL RESISTANCES FOR SURFACE-MOUNTED RESISTORS


RESISTOR COMPONENT SIZE 0406


1206 0805 0603 0402


ACAS 0612 ACAS 0606 MELF 0207 MELF 0204


Table 1 www.cieonline.co.uk Components in Electronics September 2011 37 RthFC


30 32 38 63 90 20 39 26 46


[K/W] SAVE UP TO 45%


ON ENCLOSURE FLANGE TO PCB DEPTH


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