Squirting on the shielding

Thanks to the availability of new materials and application techniques developed by shielding and thermal management expert Chomerics, a new philosophy can be applied to the often overlooked area of enclosure and equipment shielding.

Traditionally, EMI shielding gaskets take the form of a moulded or extruded section of polymer loaded with various particles to promote good shielding characteristics.

These individual gaskets are then fitted between mating enclosure parts, and located and retained in either pre-machined grooves or some other recess.

The problem with this approach is that to get tooling machined and parts produced in time for production, engineers must sign-off the gasket design some time before the production start date. Any design changes after sign-off can therefore lead to expensive and time-consuming modifications to tooling. Later changes to the gasket profile may have costly ramifications for the OEM.

Now, in a development said to have significant benefits over moulded and extruded parts, Chomerics has formulated a range of uncured materials – dubbed Cho-Form – that are dispensed via automated equipment directly onto metal or plastic housings.

The materials, which usually consist of a one or two-part silicone elastomer loaded with evenly dispersed particles of either silver, silver/copper, silver/aluminium or nickel graphite, cure in place at either room, or an elevated temperature. The materials don’t require mechanical retention as they adhere well to the target surface, and typical shear adhesion strengths are between 8 and 12 N/cm. The materials are also very compliant, having a cured hardness of between 35 and 65 shore A, meaning that good environmental and EMI shielding can be achieved with relatively low closure forces.

Application

The equipment used to dispense the gasket material is a standard three-axis robotic machine that can be configured with either single or multiple dispensing units depending on production volume requirements and machine frame size.

The dispensed gasket material can be applied with high accuracy and repeatability, with the application pattern being largely software defined. The route that the dispensing nozzle follows can either be ‘taught’ to the machine by manually guiding the head around the required path or by entering co-ordinates into a programming module. This process enables rapid prototyping and also means that design changes can be easily implemented.

Careful development of both material and dispensing equipment has meant that the tolerances and detail achievable with a dispensed gasket are extremely good.

Chomerics says that a placement accuracy of 0.025mm is standard with a cross-sectional height tolerance of 0.10mm being normal. The viscosity of the material is such that it can be applied to slopes of up to 45° without any running or change in bead profile.

Cost and Volume

For manufacturers of low and medium volume equipment the cost issues surrounding the selection of moulded gaskets have always been difficult.

From an assembly and quality point of view, a custom, one piece-moulded gasket was preferable as its design could be optimised for ease of assembly and its size and shape would always be the same, thus guaranteeing repeatability. However, from a procurement point of view the initial tooling cost, the lead-time for such tooling and lack of post design flexibility were major disadvantages.

An alternative that would be more acceptable to the procurement team, but less so to production and quality, would be the use of extruded lengths of a standard cross section of material. This would certainly negate the tooling burden, but at the cost of increased assembly time and poor repeatability. With shielding effectiveness being so critical, poor repeatability could manifest itself in the electrical malfunction of the equipment itself.

Dispensed EMC gaskets overcome many of the problems with both moulded and extruded EMC gaskets, as well as bringing some other important logistics benefits. The absence of the need for expensive tooling saves money, whilst the automated application of the material overcomes the repeatability issues associated with extrusions. Assembly costs are also reduced, as the gasket is already adhered in place when the enclosure arrives on the production floor.

Chomerics says that Dispensed EMC gaskets are suited to most manufacturing quantities, from low volume batches to continual 24:7 models. Highly representative small prototype quantities can be produced quickly and easily by programming the dispensing equipment. This program can subsequently be modified to suit each design modification until the same program ultimately evolves into the production version.

Supply Chain Options

As well as developing and manufacturing the dispensed gasket material, some companies are also able to manage the supply chain for their customers. By equipping their own premises and those of distributors, with the automated dispensing equipment, various supply chain options can be set up to enable the OEM to choose the one that best suits their needs.

To illustrate, Chomerics was recently approached by a major electronics manufacturer with a plastic enclosure for a telecomms application that required an EMI shielding solution.

The technical requirements were met with Chomerics’ Cho-Form dispensed gasket material. The solution involved the robotic application of a Cho-Form bead onto each of the mating enclosure halves. When assembled these halves sandwiched the main PCB in the unit and compressed the bead. This then made contact with etched copper traces on either side of the PCB, resulting in an effective EMI shield.

In this application, Chomerics assumed the role of lead supplier and managed all of the logistics and processing for the shielded housing. This included tasks such as ordering, scheduling and inspection as well as the application of the dispensed gasket. The benefits of this arrangement to the OEM were significant; on previous projects the procurement team had to deal with up to five suppliers that typically included the aluminium caster, a sand blasting service, plating vendor and a painter. With the new set up the OEM only had to deal with one vendor who was tasked with getting the processed casting delivered ready for final assembly.

Additional Benefits

A number of ‘spin-off’ benefits further add to the appeal of the single vendor option. Firstly, in the development phase, the manufacturer of the shielding material can work with the enclosure supplier and the OEM to ensure that the design of the housing meets all requirements in full. Design features that may hinder shielding performance of the enclosure or incur unnecessary cost can also be avoided at this stage. A further benefit is accountability; with a single supplier, the resolution of design, supply and quality issues is quick and easy compared to the complications that can occur with multiple suppliers.

In some cases, enclosure manufacturers may themselves choose to purchase both the dispensing equipment and uncured gasket material so that they can offer a ‘value added’ service to their customers. Training in the handling and application methods required can be given by Chomerics engineers to help the supplier get up and running with the process.

However, while OEM’s with large production numbers and an ongoing need for EMI shielded enclosures may also consider investing in the equipment, Chomerics warns that it could take the focus away from core activities.

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