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Sil-Pad Materials Carriers

Sil-Pad thermally conductive insulators are designed to be clean, grease-free and flexible.The combination of a tough carrier material such as fiberglass and silicone rubber which is conformable,provides the engineer with a more versatile material than mica or ceramics and grease.Sil-Pad products minimize the thermal resistance from the case of a power semiconductor to the heat sink.Sil-Pad materials electrically isolate the semiconductor from the heat sink and have suf- fiicient dielectric strength to withstand high voltage.They are also strong enough to resist puncture by the facing metal surface.With more than 30 different Sil-Pad materials available,there is a Sil-Pad matched to almost any application. The carrier provides physical reinforcement and contributes to dielectricstrength.High dielectric and physical strength are obtained by using a heavy,tight mesh,but thermal resistance will suffer.A light,open mesh reduces thermal resistance,dielectric strength and cut-through resistance. The carrier materials used in Sil-Pad materials include fiberglass and dielectricfilm.

Fillers

The thermal conductivity of Sil-Pad products is improved by fillingthem with ingredients of high thermal conductivity.The fillers change the characteristics of the silicone rubber to enhance thermal and/or physical characteristics.For instance,some fillers make the silicone rubber hard and toughwhile still retaining the ability to flow under pressure.A harder silicone helps the material resist cut-through.In other applications a filler is used to make the silicone rubber softer and more conformable to rough surfaces.While the range in thermal resistance of greased mica is quite large,the average is comparable to elastomeric insulators filled with a blend of the appropriate ingredients.Fiberglass-based insulators (Sil-Pad 400 and Sil-Pad 1500) have a roughsurface texture and will show a 15-20% decrease in thermal resistance over a 24-hour period.Film-based insulators (Sil-Pad K-4,Sil-Pad K-6 and Sil-Pad K-10) are smoother initially and show a 5% decrease over the same period of time.

Sil-Pad Construction

Sil-Pad products are constructed with a variety of different materials including fiberglass,silicone rubber,polyimide film,polyester film and fillers used to enhance performance.Sil-Pad materials are typically constructed with an elastomeric binder compounded with a thermally conductive filler coated on a carrier.The characteristics of your application often determine which Sil-Pad construction will produce the best performance.

Binders

Most Sil-Pad products use silicone rubber as the binder.Silicone rubber has a low dielectric constant,high dielectric strength,good chemical resistance and high thermal stability.Silicone rubber also exhibits cold flow,which excludes air from theinterface as it conforms to the mating surfaces.This flow eliminates the need for thermal grease.A rough-surface-textured insulator needs to flow more to exclude air than a smooth one.The smoother pads also need less pressure to wet-out the surfaces and obtain optimum thermal contact.
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