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How important is Coefficient of Thermal Expansion (CTE) in selecting Epoxy or Urethane Compounds?
How important is Coefficient of Thermal Expansion (CTE) in selecting Epoxy or Urethane Compounds?

Coefficient of Thermal Expansion (CTE) is a measure of the expansion or contraction of a material as a result of changes in temperature. Most frequently it is provided as linear CTE but some times given as volumetric CTE.

 

When comparing materials it is crucial to note the type of CTE (linear or volumetric) and the units of temperature used to obtain the data. This is important because the volumetric Coefficient of Expansion will be much higher than the linear CTE for the same product under the same test conditions. In addition, a figure based on degrees Centigrade will be greater than those expressed in degrees Fahrenheit.

 

CTE must also be considered in conjunction with the Tg (glass transition temperature) property of the material. This is because, for most materials, the CTE will be different above and below the Tg. The CTE can be 2 to 3 times higher above the Tg than it would be below it. As a rule many other properties such as adhesive strength, moisture resistance, volume resistance and dielectric strength also degrade above the Tg.

 

Matching the CTE as closely as possible to that of the substrate to be potted is crucial in potting electronic circuits. A significant mismatch can result in component damage due to the forces exerted as the encapsulant expands or contracts with changes in temperature, possibly resulting in circuit failure.

 

This property is also extremely important in casting electrical components such as transformer bushings and transformer coils. Significantly mismatched coefficient of thermal expansion can result in cracking or insufficient adhesion to conductors resulting in corona or oil leaks.

 

In addition to the chemistry employed, thermal expansion is controlled by the fillers incorporated in a given compound. Selecting the correct filler types and particle size combination is critical to achieve the desired results. The addition of fillers also affects other mechanical properties (i.e. tensile strength, percent elongation, heat dissipation capabilities etc.) therefore it is important to also take those requirements into consideration when selecting materials.

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