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How to deliver a reliable connector design

What makes for a good connector design, especially one that looks to mitigate electromagnetic interference? New Electronics talks to PEI-Genesis.

Few components are subjected to extreme conditions in the way that connectors are. From commercial aircraft or military vehicle to medical ventilators, connectors need to be able to endure extreme variation in conditions.

Whether it’s the rapid temperature fluctuations and changes in humidity, to persistent vibration, impacts and signal interference, connectors must be able to operate reliably to ensure their users can get the ‘job done’.

“The industry-standard D38999 is a military-specification connector that was originally designed in the 1970s and is now on its third-generation design,” explains Ammar Lokhandwalla, customer application engineer at connector specialist PEI-Genesis.

“Like other connectors of its kind, it’s made up of a few basic components: a hard outer shell, a neoprene rubber insert with holes to house the pins, or contacts, and sometimes a backshell on the outer housing that provides additional shielding and durability.”

When it comes to selecting a connector, engineers need to consider a wide variety of properties depending on their application. One of the primary considerations is the choice of materials, for both the electrical terminations and shell housing. For example, although copper offers better electrical and thermal conductivity, aluminium is cheaper and easier to form and plate.

“While copper may be chosen for high-voltage industrial applications where heat dissipation and conductivity are vital, aluminium may better serve aerospace and military applications where weight and corrosion-resistance are more important,” explains Lokhandwalla.

Ingress protection is another consideration and connectors that are designed for industrial food and beverage manufacturing must be sealed against water jets to allow equipment and machinery to be washed down between shifts.

“This protection extends to marine applications, such as those in the oil and gas sector where equipment may need to be fully submersible for prolonged periods of time. In these applications, it may be necessary to select a polycarbonate connector, with the right o-rings and grommets to provide a moisture seal,” suggests Lokhandwalla.

While aluminium is the preferred choice of connector material for many construction, rail, industrial and military applications, it may still need to undergo plating to improve its corrosion-resistance, to provide further electromagnetic shielding, and to meet camouflage and colour needs.

“Some military applications use olive-drab green, a colour that was historically achieved with a toxic cadmium coating,” explains Lokhandwalla. “In recent years, this has been replaced with a black zinc nickel plating that meets RoHS and REACH regulations. If engineered correctly, this black plating can deliver the same performance as cadmium coatings and withstand over 500 hours of salt-spray.”

Not all connector contacts can be solder-terminated, according to Lokhandwalla, “Under certain extreme conditions, the operating temperature of the application can exceed the melting point of the solder, causing connection failure. For applications where this is a risk, engineers may prefer to specify a crimped connector.”

Lokhandwalla makes the point that when it comes to crimping, contacts are joined to the wire by mechanically squeezing them together to ensure that they remain in contact no matter the temperature. Instead of a soldered connection where the wire is fed through an eyelet or hook and then soldered, crimping involves material being deformed to lock the termination together using a special crimping tool.

Electromagnetic interference

Electronic equipment is sensitive to electromagnetic interference (EMI) and connectors are no exception, according to Jakub Kosinski, a product manager at PEI Genesis.

“EMI is a serious concern for engineers in almost every application, but especially situations where signal integrity is vital — such as mission critical military communications, fly-by-wire avionics and medical applications. In those situations, EMI can cause orders, control adjustments and medical data respectively to be miscommunicated, with potentially fatal consequences.”

The most important aspect of EMI resistance is the enclosure, according to Kosinski, both in terms of it material and topology.

“The enclosure material is the first line of defence against EMI. Conductive metallic enclosures are ideal here, because any electromagnetic waves incoming or outgoing induce a current in the enclosure which saps the energy away from the waves. As a result, they act as an insulating shield, as opposed to other non-conductive enclosures, including plastic ones, which are transparent to EMI and allow the interference to pass through unimpeded.”

Enclosure material is critical, even the slightest change can make a big difference. Traditional EMI resistant enclosures have been plated with cadmium to reduce corrosion. This thin plated layer also works to increase the opacity of the material to EMI.

Unfortunately, cadmium has toxic effects on the kidneys as well as the skeletal and respiratory systems.

“Recently, however, more and more enclosures are being plated with zinc-nickel to make them Restriction of Hazardous Substances (RoHS) compliant - zinc-nickel offers similar EMI shielding and corrosion resistance but without using any cadmium, with its associated negative health impacts.”

The second line of defence is the topology, or shape, of the connector enclosure.

“Imagine a rectangular enclosure for example. Here, sharp edges act as weak points for EMI to leak in and out of the connector, and flat faces create impromptu waveguides where the EMI is trapped and interferes with itself, creating even more electromagnetic noise,” explains Kosinski.

“So, with a topologically smooth, zinc-nickel-plated, stainless-steel enclosure, it is possible to severely limit EMI flux either emitted or absorbed by the connector.

“Backshells like the Amphenol M85049, Polamco 35 Series and Sunbank M85049 are specifically designed to give a 360 degree connection with the cable braid, which offers the best EMI protection for the wire itself.”

This way there’s nowhere for EMI to leak out of the connection, but what about EMI generated by or already present in the wiring itself?

“This can be addressed from two angles,” suggests Kozinski. “The first is to use braided coaxial cabling. Like the conductive connector enclosures, coaxial cables include a conductive sheath to protect the signal wire from EMI. For the best protection, the coaxial sheath should be grounded to the backshell of the connector to allow an escape route for the EMI induced current.

“The second approach is to include filtering components in the connectors which are tuned to pass power and signal frequencies, but remove EMI frequencies. Using filters is quite convenient because they can easily be retroactively applied to typically noisy networks with little to no reworking or redesigns of equipment needed.”

Design early, design once

“One of the biggest mistakes I see manufacturers make is considering connector design too late in the design process of their product. This often means that a product’s time to market is delayed while the design is reworked,” points out Lokhandwalla.

“It’s important to remember that your connector may have physical design constraints like a minimum wire-gauge or number of contacts, so it’s vital to consult with your connector supplier early in the process. At PEI-Genesis, we are able to offer customers a 3D wire model of the connector that customers can drop into their design to see if it fits.

“If it doesn’t, our engineering team can help refine or redesign the existing design, or propose a different connector entirely, that meets the specification. This includes changing features like threaded, bayonet and friction fittings, or accessories like backshells, or something simple like a dust cap.”

Author
Neil Tyler

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