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high temperature load retention, low temperature flexibil-ity, thermal conductivity, electrical characteristics, andcoefficient of thermal expansion. Low outgassing andgood resistance to many chemicals and solvents are themost common considerations when qualifying material forgasketing and sealing applications.Most sealing or permeability tests use the same basictechnique. The purpose is to measure the total quantity of fluid lost over the face and through the body of thegasket. Leakage rate is the quantity of fluid passingthrough the body and/or over the face of a gasket over a specified time. Working conditions such as media,pressure, temperature, and surface pressure profoundlyinfluence leakage rate. For a given set of application circumstances, the lower the leakage rate, the betterthe gasket material retains the fluid.Gasket pressure under operating conditions is a crucialdesign criterion to prevent unacceptable creep relaxationor failure of the gasket material. The limit on gasket compression is generally determined by the required sealing performance. The smaller the quantity of fluidlost with time, the better the sealability of the gasketmaterial.A few common gasket applications for input devicesare as follows:• A back mounted panel where the gasket is placed aroundthe topside perimeter and seal is nestled between theenclosure and the unit. Used in a back mount design,the gasket can serve a dual purpose as well; one to seal and one to be an EMI barrier. EMI compatible materialcan be used when choosing the appropriate gasketmaterial for your particular application.•A top mounted panel set into a recess in a bezel so thatthe gasket is around the bottom to prevent leakage. As the gasket compresses with installation it seals the paneland prevents liquids from entering.• A gasket can be designed to be placed around themiddle of two mating surfaces such as plastic halveswhich when compressed in assembly seals the unit.

Sealed Input Device - 54730 Adhesives

Pressure Sensitive Adhesives (PSAs) are widely used forsealing input devices. PSAs are generally a layer of uncuredacrylic adhesive that is laminated onto the surface of the keypad. When installed into the bezel, pressure andtemperature combine to cure the adhesive. This adhesiveacts as a physical barrier to a wide variety of liquids, chemicals, and particles. It is suitable for use in a widerange of temperatures and pressures. Pressure Sensitive Adhesives Hot Melt Adhesives Hot melt is designed to protect electronic circuitry andassemblies from potentially damaging conditions such as moisture, various corrosive chemicals, excessive heat,vibration, mechanical impact, thermal shocks and abrasionwhich might occur while the device is in operation.Hot melt adhesives are applied in the molten state andthen harden. The adhesive substance is melted and applied to the surfaces, which are then joined together. Once the adhesive cools and solidifies, the joint and seal are complete.For seal enhancement, a thermal formed plastic sheet canact as an encapsulate over the hot melt which forces thenecessary encasement of components and achieves asubmergible level of sealing. Hot melt can act as anelectrically conductive adhesive, a sealant, and a coating — or a combination of all three.Given the right composite, a circuit board can be sub-mersible without concern for the functionality of thecomponents. This, of course, depends on temperatureand viscosity.

Other Sealing Technologies

Elastomeric Sealing Bump or Rib One commonly used technique in a mid travel, backmount silicone elastomer keypad is to groove the elastomer tool so as to create tapered sealing ribs or protrudingbumps along the outside perimeter of the keys. Once theelastomer keypad is back mounted into an enclosure, the ribs compress and thus prevent liquid from entering theconsole. Assembly of components joins the two to preventleakage. The seal is formed between the ribs and the torque load from the back mounted bezel to achieve thedesired level of sealing under the operating conditions.