Choosing the right Sil-Pad Starts with the
Mechanical and Electrical Properties
MECHANICAL PROPERTIES
Woven fiberglass and films are used in Sil-Pads 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.
• SPK4
®
, SPK6
®
and SPK10
®
are very good at resisting
cut-through from sharp burrs left on heat sinks after
machining operations such as drilling and tapping
• Fiberglass is good at resisting the type of cut-through
encountered when device mounting flanges are pulled
into oversized mounting holes. This occurs when fasteners
are torqued. (SP400
®
, SP1000
®
, SP2000
®
)
Cut-through resistance is very dependent on the application and
depends on several factors:
• A very sharp burr may cause cut-through with less than
100 pounds while a blunt burr may require several
hundred pounds to cause cut-through
• When two flat parallel surfaces are brought together on
a Sil-Pad, over 1000 pounds of force can be applied
without damaging the insulator
• The Poly-Pad insulators are the most mechanically durable
Sil-Pads overall. The polyester resin used has a higher
modulus than silicone rubber. (Poly-Pad 400
®
,
Poly-Pad 1000
®
, Poly-Pad K4
®
and Poly-Pad K10
®
)
MOUNTING TECHNIQUES AND
MOUNTING PRESSURE
Typical mounting techniques include:
• A Spring clip, which exerts a centralized clamping force
on the body of 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, the force on the transistor is determined by the
torque applied to the fastener
In extremely low pressure applications, an insulator with
pressure sensitive 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.
Devices with larger surface areas need more pressure to
get the insulator 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 insulator
being used and either a different insulator or a different
mounting scheme should be used.
Interfacial thermal resistance decreases as time under
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.
2
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 isolate but have excellent thermal
properties.
The most important electrical property in a typical assembly
where a Sil-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 to consider when selecting a Sil-Pad material;
• Q-Pad II and Q-Pad 3 are used when electrical isolation
is not required
• Dielectric breakdown voltage is the total voltage that a
dielectric material can withstand. When insulating
electrical components from each other and ground, it is
desirable to use an insulator with a high breakdown voltage
• 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
(SP1750
®
and SP1950
®
are 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 special electrical
performance please contact the factory for more detailed testing
information.
Breakdown Dielectric Dielectric Volume
Voltage Strength Constant Resistivity
Material (kV) (Volts/mil) ( kV/mm) (Ohm-Metre)
SP400®-.007 5 700 18 5.5 1011
SP400®-.009 7 800 20 5.5 1011
SP1000®7 700 18 4.5 1011
SP2000®12 800 20 4.0 1011
SPK-4®7 1200 30 5.0 1012
SPK-6®7 1200 30 4.0 1012
SPK-10®7 1200 30 3.7 1012
Test Method ASTM D 149* ASTM D 149* ASTM D 150 ASTM D 257
*Method A, Type 3 Electrodes
TYPICAL ELECTRICAL PROPERTIES OF SIL-PADS®
3
Choosing the right Sil-Pad Starts with the
Mechanical and Electrical Properties
