Text version of the page

Mechanical, Electrical and Thermal Properties

Mechanical Properties

Woven fiberglass and films are used in Sil-Pad products to provide mechanical reinforcement.The most important mechanical property in Sil-Pad applications is resistance to cut-through to avoid electrical shorting from the device to the heat sink. Devices with larger surface areas need more pressure to get theinsulator to conform to the interface than smaller devices.In most screw-mount applications,the torque required to tighten the fastener is sufficient to generate the pressure needed for optimum thermal resistance.There are exceptions where the specified torque on the fastener does not yield the optimum thermal resistance for the insula-tor being used and either a different insulator or a different mounting scheme should be used.Interfacial thermal resistance decreases as timeunder pressure increases.In applications where high clamping forces cannot be used,time can be substituted for pressure to achieve lower thermal resistance.The only way to know precisely what the thermal resistance of an insulator will be in an application is to measure it in that application.

Electrical Properties

If your application does not require electrical insulation,Q-Pad II or Q-Pad 3 are ideal grease replacement materials.These materials do not provide electrical isolation but have excellent thermal properties. Hi-Flow phase change materials should also be considered for these applications.(Reference pages 32-44 of this guide.)The most important electrical property in a typical assembly where aSil-Pad insulator is used is dielectric strength.In many cases the dielectric strength of a Sil-Pad will be the determining factor in the design of the apparatus in which it is to be used.Here are some general guidelines regarding electrical properties toconsider when selecting a Sil-Pad material:• Q-Pad II and Q-Pad 3 are used when electrical isolation is notrequired.• Dielectric breakdown voltage is the total voltage that a dielectricmaterial can withstand.When insulating electrical components from each other and ground,it is desirable to use an insulator with a high breakdown voltage.

Cut-Through Resistance - Bergquist introduced its TO-220 cut-through testto help customers better understand typical application performance.

Mounting Techniques and Mounting Pressure

Typical mounting techniques include:• A spring clip,which exerts a centralized clamping force on the bodyof the transistor.The greater the mounting force of the spring,the lower the thermal resistance of the insulator.• A screw in the mounting tab.With a screw-mounted TO-220,theforce on the transistor is determined by the torque applied to the fastener.In extremely low-pressure applications,aninsulator with pressuresensitive adhesive on each side may give the lowest thermal resistance since the adhesive wets-out the interface easier than the dry rubber. This decreases the interfacial thermal resistance.

SIL-PAD

SIL-PAD TYPICAL ELECTRICAL PROPERTIES

BREAKDOWN VOLTAGE DIELECTRIC STRENGTH DIELECTRIC CONSTANT VOLUME RESISTIVITY Material (kV) (Volts/mil) (kV/mm) (1000 Hz) (Ohm-Meter) Sil-Pad 400 - 0.007 3.5 500 20 5.5 10
11 Sil-Pad 400 - 0.009 4.5 500 20 5.5 10
11 Sil-Pad 900S 5.5 600 24 6.0 10
10 Sil-Pad A1500 6.0 600 24 7.0 10
11 Sil-Pad 2000 4.0 400 16 4.0 10
11 Sil-Pad K-4 6.0 1000 39 5.0 10
12 Sil-Pad K-6 6.0 1000 39 4.0 10
12 Sil-Pad K-10 6.0 1000 39 3.7 10
12 Test Method ASTM D149* ASTM D149* ASTM D150 ASTM D257
* Method A,Type 3 Electrodes * Method A,Type 3 Electrodes

50