Thermal Interface Selection Guide - Bergquist Company - #51

• Breakdown voltage decreases as the area of the electrodes increases.This area effect is more pronounced as the thickness of the insulator decreases.

• Breakdown voltage decreases as temperature increases.

• Breakdown voltage decreases as humidity increases (Sil-Pad 1750 is less sensitive to moisture).

• Breakdown voltage decreases in the presence of partial discharge.

• Breakdown voltage decreases as the size of the voltage source (kVA rating) increases.

• Breakdown voltage can be decreased by excessive mechanical stress on the insulator

Dielectric strength, dielectric constant and volume resistivity should all be taken into consideration when selecting a Sil-Pad material. If your application requires specific electrical performance, please contact a Bergquist Sales Representative for more detailed testing information.

The thermal properties of a Sil-Pad material and your requirements for thermal performance probably have more to do with your selection of a Sil-Pad than any other factor

Discrete semiconductors, under normal operating conditions, dissipate waste power which raises the junction temperature of the device. Unless sufficient heat is conducted out of the device, its electrical performance and parameters are changed. A 10°C rise in junction temperature can reduce the mean-time-to-failure of a device by a factor of two. Also,

above 25°C, the semiconductor's total power handling capability will be reduced by a derating factor inherent to the device.

The thermal properties of Sil-Pad products are thermal impedance, thermal conductivity and thermal resistance. The thermal resistance and conductivity of Sil-Pad products are inherent to the material and do not change.Thermal resistance and thermal conductivity are measured per ASTM D5470 and do not include the interfacial thermal resistance effects.Thermal impedance applies to the thermal transfer in an application and includes the effects of interfacial thermal resistance. As the material is applied in different ways, the thermal impedance values will vary from application to application.

• The original Sil-Pad material, Sil-Pad 400, continues to be Bergquist's most popular material for many applications.

• Sil-Pad AI500 is chosen when greater thermal performance is required. Sil-Pad A2000 is ideal for high performance, high reliability applications.

Beyond these standard materials, many things can contribute to the selection of the correct material for a particular application. Questions regarding the amount of torque and clamping pressure are often asked when selecting a Sil-Pad material. Here are some guidelines:

• Interfacial thermal resistance decreases as clamping pressure increases.

• The clamping pressure required to minimize interfacial thermal resistance can vary with each type of insulator

• Sil-Pad products with smooth surface finishes (Sil-Pad AI500, Sil-Pad A2000, Sil-Pad K-4, Sil-Pad K-6 and Sil-Pad K-10) are less sensitive to clamping pressure than Sil-Pads with rough surface finishes (Sil-Pad 400) or smooth and tacky finishes (Sil-Pad I500ST).

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