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Thermal Interface - Wet Dispensed

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Testing considerations

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. 

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.

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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  1. Thermal Interface Materials Tutorial

  2. Why is Heat Bad for Electronic Devices?

  3. Types of Wet Dispensed Thermal Interface Materials

  4. Types of Fabricated Films and Pad Thermal Interface Materials

  5. Key Characteristics – Wet Dispensed Thermal Interface Materials

  6. Key Characteristics – Fabricated Pad and Film Thermal Interface Materials

  7. Potential Applications for Thermal Interface Materials

  8. Testing considerations

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