FEATURE EMC & THERMAL MANAGEMENT FORM VERSUS STYLE
Gerard Young, applications engineering team leader at Parker Hannifin, Chomerics Division Europe explores the trade off between form-in-place technology versus conventional pre-formed gaskets in shielding and sealing applications and the pros and cons of selecting either option
W
hen it comes to the provision of high quality yet cost effective EMI
sealing and shielding, a growing number of modern electronics manufacturers are choosing form-in-place (FIP) gaskets in preference to conventional pre-formed components. This is because an extensive range of materials are now commercially available, giving a broader choice of properties and performance to meet various application requirements. FIP seals can be used in areas where there is limited space or attachment constraints. The small form factor provided by these dispensed gaskets, and their relatively high deflection capabilities allow them to be used where traditional gasket in groove designs are not feasible. To ensure the reliability and EMC
compliance of the latest electronics, especially RF devices, engineers require both environmental sealing and electromagnetic shielding. Everything from consumer mobile phones through to high-performance equipment for industrial, military or medical applications need to be protected. Typically, sealing or shielding gaskets are required where different parts of the enclosure are joined, and when space is limited FIP gasketing should be considered. One of the major benefits is that
manufacturers are able to avoid the lead-times associated with traditional pre-formed gaskets, as form-in-place alternatives are created at the time of production, mostly using a standard, software-controlled, inline dispenser. This eliminates any requirement for costly bespoke tooling or human input to place the gasket before final assembly. Lower material costs and greater flexibility to cope with small design changes and accelerated prototyping, are among further advantages. However, if damaged, the gasket and cover may have to be totally replaced whereas with a traditional gasket solution only the seal needs to be changed. Therefore in applications where the enclosure is likely to be opened repeatedly FIP gasketing may not be the correct design choice. The high-performance elastomers used by the latest form-in-place gaskets are engineered for precise and controlled dispensing, whilst also providing the
28 JULY/AUGUST 2017 | ELECTRONICS Figure 1:
Inline dispensing ensures high accuracy, efficiency and repeatability
required shielding and/or sealing attributes. These elastomers exhibit good shear adhesion to numerous typical housing substrates, including stainless steel, cast alloys and plated plastics. The materials permit close dimensional
control resulting in cross-sectional height tolerance of 0.10mm and positional accuracy within 0.05mm when deployed within a correctly optimised dispensing process. A further advantage is that small cross-sections and application to complex geometries are achievable. Gaskets can be applied to walls or flanges as thin as 0.76mm without any mechanical retention features such as a groove or ‘friction-fit’ design. However, relying entirely on the gasket
for the shielding performance will not give the best results and efforts to introduce as much metal-metal contact between the housing should be made as in normal gasket in groove designs. Modern processes are able to deposit
gaskets with minimal ‘tail’ characteristics and clean bead ends. In fact, almost any gasket bead path is possible, including those on sloping or uneven surfaces, thanks to the ability to dispense in three axes. Furthermore, in applications where the enclosure has several compartments, an effective seal at the T-joint between the sub-paths and perimeter bead is achievable, thus maintaining high levels of EMI shielding. This sort of solution approaches the levels that can be achieved by custom moulded gasket solutions. Where both environmental sealing and
Figure 2: Perfect joins can be achieved when sealing multi-compartment enclosures
Figure 3: Comprehensive help is available to optimise the deposition process for a given enclosure design
Figure 3: using it to deactivate
EMI shielding is required, the FIP gasket solution is preferable to the moulded or extruded solution, which invariably takes up more room. For shielding effectiveness of greater than 70dB, materials featuring silver- copper filler particles are ideal, although lower cost nickel/graphite fillers can still deliver EMI shielding greater than 65dB. Materials with nickel/aluminium fillers also result in reduced galvanic corrosion when mated with chromate converted aluminium, making them suitable for many outdoor applications.
Parker Chomerics Europe
www.parker.com/chomerics T: 01494 455400
/ ELECTRONICS
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