EMC & Thermal Management
The importance of an integrated approach to EMI shielding
Although the complexity involved in the effective EMI shielding of electronics is well documented, the pain can be alleviated by adopting an integrated approach. Harish Rutti, regional sales manager, Parker Chomerics Europe explains
H
istorically, the EMI shielding of electronics using thermoplastic housings has been a task fraught with complexities and costs. Today, OEMs no longer simply have to meet form, fit and function requirements, but must reduce total cost of ownership, lower part weight, shorten time-to-market, simplify supply logistics and manage quality. For these reasons, taking an integrated
approach by using a full-service, single- source supplier can take the strain from this challenging responsibility. Such a vendor will not only take on the aforementioned tasks, but many more besides, including part design assistance, material selection, tooling design and manufacture, injection moulding for thermoplastics, secondary EMI coating of non-conductive thermoplastics, and secondary component assembly. All of these help alleviate the pressure on manufacturers in sectors that include telecommunications, IT, consumer electronics, military electronics, medical equipment and transportation.
Making choices
For instance, selecting a suitable plastic injection moulding shop is not as straightforward as it sounds when it comes to electronic enclosures. Here, part design assistance from a reputable expert in EMI shielding will reap significant rewards. This is because the optimisation of part geometry will lead to the highest performance and lowest possible material, processing and tooling costs. Furthermore, upon part design completion, mould flow analysis will help create a 3D representation of flow patterns for the injection moulding process. As a result, it is possible to visualise flow rates, pressures and temperature values across the entire part, thus helping adjust the moulding process by locating entry gates and compensating for variable pressures or cooling rates to avoid part warpage or uneven shrink rates. A specialist in this area will also be able to source the tooling, build the optimum mould and provide full PPAP (Production Part Approval Process) reporting back to the customer. As a matter of course, assistance should also be available for factors such as optical display assemblies/filters, EMI shielding gaskets, thermal management components such as heatsinks or active cooling devices, microwave absorber
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materials, and EMC and safety compliance testing.
Material selection One of the principal areas where expert assistance is likely to be hugely beneficial is that of material selection. With a multitude of polymers available for design engineers to ponder, advice can prove invaluable. Essentially, with regard to electrically conductive thermoplastics, the selection of a material that can deliver high shielding performance at lower volume filling will allow a high retention of base polymer mechanical characteristics. Moreover, choosing a material that does not require secondary EMI coating will offer the highest potential for part cost reduction. Depending on part size, cost reductions of up to 60 per cent are available. Such a material would typically be suitable for parts with low cosmetic requirements, small overall dimensions, or those requiring less than 85dB shielding effectiveness. Alternatively, non-conductive thermoplastics with secondary EMI coatings are generally chosen for parts requiring high cosmetic appeal, high temperature resistance, chemical resistance, low water absorption, thin walls (<1mm), weight in excess of 200g, or those requiring shielding effectiveness greater than 85dB. Ultimately, the choice of thermoplastic is influenced by environmental, mechanical, maximum temperature, flammability and part design needs.
Secondary coatings Sometimes, there is no escaping the fact that the design calls for secondary conductive coatings, in which case it definitely will add considerable value to select a supplier of EMI shielding solutions that can also provide processes such as conductive painting, sputtered vacuum metallisation, electroless plating and thermal spraying.
The primary design factors to consider when choosing a secondary EMI coating are shielding effectiveness, environmental resistance, component geometry and production volume. For example, sputtered aluminium is a good solution in low- shielding applications where volume supports a higher tooling cost. Alternatively, while Ni/Cu provides excellent shielding, this coating is not generally recommended for use in high-humidity uncontrolled
Parker Chomerics’ automotive electronic control unit (ECU) demonstrates benefits of integrated approach to shielding
environments, while thermal spraying of tin-zinc provides high shielding with excellent resistance to harsh environments. In short, there is no single solution and all solutions vary in cost. This is why applied material costs must balance with performance to find the best solution. Companies such as Parker Chomerics are able to provide advice on the trade-offs between various approaches as well as assisting with implementations. In a similar vein, OEMs requiring secondary component assembly will prosper from seeking out a solutions provider that can offer competency in processes such as heat staking, sonic welding, solvent bonding and mechanical assembly. Depending on the part, assembly can often be done at the injection moulding press, within the cycle time, thus saving money for the customer.
Other factors
For many devices, an EMI shielding gasket is needed to complete the design. Here, materials range from fabric-over-foam, through finger stock, to elastomeric options, in line with requirements for indoor or outdoor applications. The gaskets can become an integral part of the housing using over-moulding, bonding or mechanical retention processes, for
example. This simplifies final assembly, allowing OEMs to have only one part to order, inventory and handle. With assembly complete, there are still many more factors to consider. Take cosmetic features, for example. Ensuring a specialist in EMI shielding is able to provide part-decorating processes such as pad printing, silk screening and hot stamping will certainly pay dividends. Testing is a further prime consideration for OEMs seeking an integrated solution. This is where proven suppliers will be able to review EMI package and board level design to avoid compliance testing failures when the product is ready to launch. Here, OEMs should seek out vendors able to offer compliance testing for emissions, immunity, susceptibility and safety requirements. Ultimately, through the selection of a single-source supplier able to deliver an integrated solutions approach, it is possible to reduce costs for shielded plastic housings, reduce weight in metal-to- plastic conversions (up to 75% compared with aluminium die-castings), reduce assembly costs through part consolidation, eliminate re-design at EMI certification stage, shorten the design cycle and reduce the supplier base.
www.Parker.com/Chomerics Components in Electronics June 2018 27
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