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An initial tendency when choosing a material for heat dissipation is to
select the one with the highest bulk conductivity value listed on its
datasheet. For a few reasons, this may not always be the most
appropriate choice.
The overall ability of a system to remove heat from electronic devices is
controlled not just by the conductivity through the bulk of the material but by
other factors also, for example its ability to make intimate contact between
the material and adjoining surfaces without voids or air gaps or by its ability
to form thin bond lines.
Thus, it is the summation of all the factors across interfaces and through
the materials that govern the overall abilities to remove heat from electronic
devices. This is measured by:
 | Thermal Resistance = the property of a
material that describes this total of all the factors resisting the flow of
heat. Thermal resistance for a given interface can then be defined as the
temperature difference across that interface per watt of energy flowing across
the interface. The lower the thermal resistance (usually written in unit of
°C/W), the better the flow of heat across that interface. |
| Thermal Impedance = Thermal impedance is the
thermal resistance normalized over a unit area. The conventionally used units
to describe thermal impedance are cm² °C/W or in² °C/W. |
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The overall thermal resistance or impedance is composed primarily of two
factors -
Bulk Conductivity = the property of a material that describes the
material’s ability to conduct heat. This property is independent of the
material’s shape and size.
Contact Resistance = a measure of how well the material makes contact
with the substrate. This value can increase due to surface roughness, the
presence of air voids and/or the interface material not conforming completely
to the substrate(s). The applied pressure will affect the results.
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Bulk conductivity becomes more important for thicker bond lines, while the
contact resistance can become more critical for very thin bonds.
Even when comparing bulk conductivity values between materials, you need to
measure these values by similar methods for a valid comparison. There are
many different ASTM methods, using different types of equipment for checking
thermal conductivity. Each test method will typically lead to different
values, due to the technique it employs. Even using the same ASTM test,
values can be different due to measurement conditions. Conductivity values will
vary depending on the pressure and temperature when the measurement is
made.
What you should understand is that a reported conductivity number is
specific to its test method and test conditions. You need to take this
into account when comparing conductivity values between competitive materials
supplied by different vendors.
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