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Thermal Conductivity

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New for July 2010! Thermal conductivity is an important material property in regards to packaging electronic components. A related property that you must consider is thermal expansion. Thermal conductivity is denoted by the lower case letter "k".

The S.I Units of thermal conductivity are Watt/meter-degree K (W/M-K). Higher conductivity means heat is moved farther with less temperature rise (Generally a good thing).

For one dimensional steady-state heat transfer, you can calculate temperature rise simply by multiplication:

Temperature rise=heat x distance /(thermal conductivity)

However, if you base a critical design on this simple calculation, failure is all but guaranteed!

Thermal conductivity is often a strong function of temperature, this is true in gallium arsenide.

Here's a short list of packaging materials, you can click on them to look up thermal conductivities (assumed at room temperature). If you are in need of exact data, please search elsewhere, we don't take any reponsibility for accuracy.

Diamond

Aluminum

Copper

Invar

Kovar

Titanium

Molybdenum

Silicon carbide

Silicon

Gallium arsenide

Silver

Anisotropic thermal conductivity

Generally, the thermal conductivity of a material is a bulk property, and is isotropic. In cases where thermal conductivity is an average value of a material that consists of several layers, and the layers have disparate thermal thermal properties, heat transfer is not always going to be equal in all directions. Examples of this include composite "sandwich" materials such as copper-invar-copper. Many coins are sandwiched layers of materials, an American quarter is cupro-nickel (an alloy of 75 Cu/25Ni, and remarkably silver-colored), copper, cupro-nickel. The penny is copper, zinc, copper (only 2.5% copper today!)

In the early 1960s the U.S. government was stamping out pure silver quarters, dimes and half-dollars, which were imminently about to be worth more than the fiat value. Once a coin is worth more than it costs at the local bank, they start to disappear, even though is is a federal offense to export coins or melt them.

The U.S. treasury had some tricky materials engineering to do in order to replace the dime and the quarter, it was a requirement that the new coins would pass the coin validator in all vending machines. So the weight, hardness, magnetic and eddy current properties all had to be closely matched. Otherwise, there would be trouble down at the local ice cream parlor. In the immortal words of Chuck Berry:

Drop the coin right into the slot, you've gotta hear something that's really hot!

A September 1965 Popular Science article explains the solution, we borrowed the image below. The image serves as a good example of a coin metal sandwich. As far as the thermal conductivity of a quarter, this property was certainly degraded when the switch was made from pure silver. Alloys are notorious for being poor heat conductors.

 

One way to tell a silver quarter from the modern version is to drop them and compare the tones, silver quarters ring like a bell!

 

Author : Unknown Editor

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