Ever hear about the 45° heat spreading angle? Most of us have. However, no one I've talked to seems to know where it came from. Is it an urban legend, such as alligators in the sewers of Manhattan, or is it an eternal truth deposited into the collective subconscious of the thermal engineering community? However fascinating it might be to explore the historical origins of this … [Read more...]
The Thermal Conductivity of Moist Air
In the Tech Data series on thermal conductivity, gases - especially air - have been the subject of a few contributions. So far, the influence of two important parameters has been covered: temperature and pressure. However, people regularly ask: "What exactly is the influence of humidity?" And my standard answer has always been: "Nothing to worry about." Some time ago, one of … [Read more...]
Prediction of Thermal Contact Resistance
All engineering surfaces exhibit some level of microscopic roughness. The resistance to heat flow through a contact interface occurs because only a small portion (usually 1-2%) [1] of the nominal surface area is actually in contact. Heat may pass through the interface via three paths: conduction through the contact spots, conduction through the gas present in the gap between … [Read more...]
Problems with Thermal Interface Material Measurements: Suggestions for Improvement
Preface Increasingly it is apparent that measuring the thermal properties of Thermal Interface Materials (TIMs) causes much irritation for vendors and users alike. This article focuses on various measurement and standardization aspects and offers a strategy to overcome the problems. It is essentially an abbreviated version of a paper presented at SEMITHERM '03 … [Read more...]
E missivity in Practical Temperature Measurements
When making temperature measurements with infrared cameras and point detectors, we are always facing the question of the emissivity of our sample. In practice we would like to have a single number set in the IR device. To approach the question, let's first recall some fundamental definitions. Planck's radiation law gives the energy density emitted by a blackbody as a function … [Read more...]
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