EMC & Circuit Protection
‘Form-in-place’ starts to attract attention
Christoph Loret explains why form-in-place technology is able to outperform traditional pre-formed gaskets when it comes to equipment shielding and sealing
A
s today’s electronic manufacturers strive to achieve more cost- effective, flexible and responsive manufacturing solutions, form-in-place gaskets are increasingly preferred over traditional pre-formed components to provide sealing and EMI shielding. A wide range of materials is now available in the market, giving designers a broad choice of properties and performance to meet the requirements of various applications. Sealing and electromagnetic shielding
are needed to ensure reliability and EMC compliance in today’s electronics, particularly RF devices, ranging from consumer mobiles to high-performance equipment for industrial, medical or military applications. Where a sealing or shielding gasket is needed, such as where separate parts of the enclosure are joined, product designers are increasingly likely to specify a form-in-place gasket in preference to a conventional pre-formed type.
As the form-in-place gasket is created at the time of manufacture, typically using a software-controlled inline dispenser (Figure 1), this enables manufacturers to avoid the lead-times associated with pre- formed gaskets. Further benefits of this approach include simpler logistics, reduced materials costs and extra flexibility to accommodate small design changes and speed up prototyping.
Supply chain advantages Using a form-in-place gasket can deliver up to 60% reduction in installed cost (total cost of ownership) compared to a custom pre-formed gasket. Logistics are also simplified, as there is no need to maintain multiple Stock-Keeping Units (SKUs) for individual gaskets, or to arrange delivery of the correct number and type of gaskets to the production
42 September 2014
line. Form-in-place gaskets can be produced inline within a short cycle time using standard dispensing equipment, saving any need for expensive custom tooling or human intervention to place the gasket prior to final assembly. Today’s most advanced form-in-place materials are high-performance elastomers that are engineered for accurate and controlled dispensing, in addition to displaying optimum shielding and/or sealing properties. They typically exhibit good shear adhesion to a variety of common housing substrates such as cast alloys, stainless steel and plated plastics.
When used within a properly optimised dispensing process, the materials allow tight dimensional control resulting in positional accuracy within 0.05mm and cross-sectional height tolerance of 0.10mm. Small cross sections and complex geometries can be achieved, and gaskets can be applied to walls or flanges as narrow as 0.76mm without requiring mechanical retention such as a groove or “friction-fit” design. This can free PCB space and allow product designers to achieve smaller overall dimensions. A variety of materials are available, including formulas with low deflection force that are ideal for use with enclosures that have low mechanical rigidity. Today’s processes can deposit gaskets with clean bead ends and minimal “tail” characteristic. Virtually any gasket bead path can be achieved, and dispensing can be programmed in three axes thereby allowing the gasket to be deposited onto sloping or uneven surfaces if required. In situations where the enclosure has multiple compartments, for example to separate processing and signal-generating circuitry (Figure 2), it is possible to achieve a perfect seal at the T joint between sub-
Components in Electronics
paths and the perimeter bead thereby maintaining optimum EMI shielding.
The Right Formula
Conductive form-in-place elastomers such as the CHO-FORM family are chosen where both EMI shielding and environmental sealing are required. Form- in-place elastomers are generally available in formulations that cure at either room or elevated temperature.
Silver-copper filler particles can help form-in-place materials achieve a shielding effectiveness of greater than 70dB. Lower- cost nickel/graphite fillers meanwhile deliver EMI shielding greater than 65dB, and provide good resistance to galvanic corrosion when mated with aluminium and are therefore suitable for outdoor use. For high-performance applications, materials such as CHOFORM 5560 from Chomerics are able to provide excellent
Figure 1: Inline dispensing ensures high accuracy, efficiency and repeatability
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