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Sil-Pad® Design Guide

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Sil-Pad® Design Guide Sil-Pad® Design Guide All statements, technical information and recommendations herein are based on tests we believe to be reliable, and THE FOLLOWING IS MADE IN LIEU OF ALL WARRANTIES, EXPRESSED OR IMPLIED, INCLUDING THE IMPLIED WARRANTIES OF MARKETABILITY AND FITNESS FOR PURPOSE. Sellers' and manufacturers' only obligation shall be to replace such quantity of the product proved to be defective. Before using, user shall determine the suitability of the product for its intended use, and the user assumes all risks and liability whatsoever in connection therewith. NEITHER SELLER NOR MANUFACTURER SHALL BE LIABLE EITHER IN TORT OF IN CONTRACT FOR ANY LOSS OR DAMAGE, DIRECT, INCIDENTAL, OR CONSEQUENTIAL, INCLUDING LOSS OF PROFITS OR REVENUE ARISING OUT OF THE USE OR THE INABILITY TO USE A PRODUCT. No statement, purchase order or recommendations by seller or purchaser not contained herein shall have any force or effect unless in an agreement signed by the officers of the seller and manufacturer. Registered Firm ISO 9001:1994 Certificate QSR-219 QS-9000 Third Edition Certificate Number, QSR-QS-017 2000-2001 Rev. 060100 © Copyright The Bergquist Company The Bergquist Company established the standard for contamination of elastomeric thermally conductive insulation material with the the solder. development of Sil-Pads® over 20 years ago. Sil-Pads were Cleaning baths developed as a clean, grease-free alternative to mica and grease. must also be Now, a complete family of materials is available to meet the avoided to prevent diverse and changing requirements of today’s design engineer. wash-out of the interface grease, MICA AND GREASE causing a dry joint Mica insulators have been in use for over 30 years and are still and contamination commonly used as an insulator. Mica is inexpensive and has of the bath. excellent dielectric strength, but it is brittle and is easily cracked or Assembly, soldering broken. Because mica used by itself has high thermal impedance, and cleaning thermal grease is commonly applied to it. The grease flows easily processes must be performed in one process while the greased and excludes air from the interface to reduce the interfacial insulators are installed off-line in a secondary process. If the grease thermal resistance. If the mica is also thin (2-3 mils), (50-80 µm), is silicone based, migration of silicone molecules occurs over time, a low thermal impedance can be achieved. drying out the grease and contaminating the assembly. Silicone However, thermal grease introduces a number of problems to migration onto electrical contacts can result in pitting of the the assembly process. It is time-consuming to apply, messy, and contacts and loss of electrical conductance. For this reason, silicone difficult to clean. Once thermal grease has been applied to an based, thermal grease has not been used in telecommunications electronic assembly, solder processes must be avoided to prevent systems. Why Choose Sil-Pad Thermally Conductive POLYIMIDE FILMS Polyimide films can also be used as insulators and are often combined with wax or grease to achieve a low thermal impedance. These polyimide films are especially tough and have high dielectric strength. ® ® Sil-Pad K-4 , K-6 and K-10® incorporate polyimide film as the carrier material. CERAMIC INSULATORS Other insulation materials include ceramic wafer insulators which have higher thermal conductivity than mica. They are often used thicker (20-60 mils), (.5 to 1.5 mm) to reduce capacitive coupling while maintaining a low thermal impedance. Drawbacks to ceramic insulators are high cost and they are rigid like mica and crack easily. Also, ceramic beryllia use requires careful handling since inhalation of beryllia dust can cause lung inflammation At Bergquist's Cannon Falls, MN manufacturing facility, a Sil-Pad material runs through (berylliosis). a 3-story coating press. SIL-PAD® MATERIALS SIL-PAD® CONSTRUCTION Sil-Pad Thermally Conductive Insulators are designed to be Sil-Pads are constructed with a variety of different materials clean, grease-free and flexible. The combination of a tough carrier including fiberglass, silicone rubber, polyimide film, polyester film material such as fiberglass and silicone rubber which is conformable, and fillers used to enhance performance. Sil-Pads are typically provides the engineer with a more versatile material than mica or constructed with an elastomeric binder compounded with a ceramics and grease. Sil-Pads minimize the thermal resistance from thermally conductive filler coated on a carrier. the case of a power semiconductor to the heat sink. Sil-Pads electrically isolate the semiconductor from the heat sink and have The characteristics of your application often determine which sufficient dielectric strength to withstand high voltage. They are Sil-Pad construction will produce the best performance. Sil-Pads also tough enough to resist puncture by the facing metal surface. are typically constructed with an elastomeric binder compounded With more than 30 different Sil-Pad materials available there is a with a thermally conductive filler coated on a carrier. Sil-Pad matched to almost any application. Insulators? BINDERS FILLERS Most Sil-Pad products use silicone rubber as the binder. Silicone The thermal conductivity of Sil-Pad products is improved by rubber has a low dielectric constant, high dielectric strength, good filling them with ingredients of high thermal conductivity. The chemical resistance and high thermal stability. fillers change the characteristics of the silicone rubber to enhance thermal and/or physical characteristics. Silicone rubber also exhibits cold flow, which excludes air from the interface as it conforms to the mating surfaces. This flow eliminates the For instance, some fillers make the silicone rubber hard and need for thermal grease. A rough surface textured insulator needs to tough while still retaining the ability to flow under pressure. A flow more to exclude air than a smooth one. The smoother pads harder silicone helps the material resist cut-through. In other also need less pressure to wet out the surfaces and obtain optimum applications a filler is used to make the silicone rubber softer and thermal contact. more conformable to rough surfaces. While the range in thermal resistance of greased mica is quite large, the average is comparable THE CARRIER to elastomeric insulators filled with a blend of the appropriate The carrier provides physical reinforcement and contributes to ingredients. dielectric strength. High dielectric and physical strength is obtained by using a heavy, tight mesh, but thermal resistance will • Fiberglass based insulators (Sil-Pad 400®, Sil-Pad 1000® and suffer. A light, open mesh reduces thermal resistance, dielectric Sil-Pad 1500®) have a rough surface texture and will show a 15-20% strength and cut-through resistance. The carrier materials used in decrease in thermal resistance over a 24 hour period. Film based Sil-Pad materials include fiberglass, dielectric film and polyester Sil-Pads (Sil-Pad K-4®, Sil-Pad K-6® and Sil-Pad K-10®) are smoother film which is used in Poly-Pad® materials. initially and show a 5% decrease over the same period of time. 1 Choosing the right Sil-Pad Starts with the 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. MECHANICAL PROPERTIES Devices with larger surface areas need more pressure to Woven fiberglass and films are used in Sil-Pads to provide get the insulator to conform to the interface than smaller mechanical reinforcement. The most important mechanical property devices. In most screw mount applications, the torque in Sil-Pad applications is resistance to cut-through to avoid electrical required to tighten the fastener is sufficient to generate the shorting from the device to the heat sink. pressure needed for optimum thermal resistance. There are exceptions where the specified torque on the fastener does • SPK4®, SPK6® and SPK10® are very good at resisting not yield the optimum thermal resistance for the insulator cut-through from sharp burrs left on heat sinks after being used and either a different insulator or a different machining operations such as drilling and tapping mounting scheme should be used. • Fiberglass is good at resisting the type of cut-through Interfacial thermal resistance decreases as time under encountered when device mounting flanges are pulled pressure increases. In applications where high clamping forces into oversized mounting holes. This occurs when fasteners cannot be used, time can be substituted for pressure to are torqued. (SP400®, SP1000®, SP2000®) achieve lower thermal resistance. The only way to know Cut-through resistance is very dependent on the application and precisely what the thermal resistance of an insulator will be in an depends on several factors: application is to measure it in that application. • 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®) 2 Mechanical and Electrical Properties ELECTRICAL PROPERTIES • Breakdown voltage decreases as the area of the electrodes increases.
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