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Design considerations for EMI-resistant and blind-mate connectors


By Shaun Findley, European Director of Product and Purchasing, and Jakub Kosinski, product manager, PEI-Genesis

Our world is criss-crossed with RF signals, since all our electrical and electronic equipment emit electromagnetic waves. These waves then interfere with each other, to not so great results: we have all witnessed this first-hand when moving a radio under power lines. The deafening 50Hz buzz that completely overwhelms the radio signal is a direct consequence of the power lines emitting powerful electromagnetic waves as the current alternates within the wires.

While merely irritating for the casual passer-by, 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.

This is clearly not a desirable outcome, and so a lot of work goes into ensuring that electronics are EMI-compatible, i.e., not emitting substantial EMI flux, and being resistant to external EMI sources.

Safe housing

Perhaps the most important aspect of EMI resistance is the connector enclosure’s 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, that are transparent to EMI and allow the interference to pass through unimpeded.

Enclosure material is so important that even the slightest change can make a big difference. For instance, 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, a growing number of 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 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.

Filtering out the noise

So, with a topologically smooth, zinc-nickel-plated, stainless-steel enclosure, we’ve severely limited 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. 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 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 that 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.

Fitting into the right space

Equally, some spaces where connectors should fit are hard to see or difficult to reach.

Blind mate connectors achieve a connection through a sliding or snapping action or by using guide pins, without the use of special tools. Unlike other connector types, many blind mate connectors have self-aligning features that correct small misalignments when mating, and help guide the connectors into the correct mating position. This feature is quite different from the more rigid coupling mechanisms found on threaded or bayonet couplings, reducing the chance of connector damage and preventing problematic errors caused by incorrect mating.

Since users might not be able to see if the connectors are correctly mated, blind mate connectors can provide auditory confirmation to give confidence that the connection is properly achieved. Again, this is a significant improvement from merely hoping that a reliable connection was made, especially when the application depends heavily on a stable connection.

There are various connector types available with blind mating options, including power and/or signal connectors with either cable or board mount options, connectors with multi-pin arrangement between racks and panels, optical connectors, and RF connectors. Blind mate connectors are also available in various orientations, including wire-to-board, board-to-board and wire-to-wire.

When to use blind mate connectors?

Blind mate connectors are a popular choice in a variety of mission-critical applications. From medical equipment to electric power transmission systems and military radar equipment, they excel in any situation where the receptacle is hidden or recessed so that it is difficult to see or reach. This includes the back of an interface card plugged into a chassis and other “black box” installations.

Using blind mate connectors provides several key benefits, including reduced production, installation and maintenance costs, shorter assembly times and a minimal impact on the size and weight of designs. Furthermore, using blind mate connectors typically results in fewer mis-mated connections. This is invaluable in mission-critical applications where any unplanned downtime is disastrous.

It’s important to remember that blind mate connectors are also designed to be extremely durable. If mistreatment occurs, they remain reliable and continue to perform as expected.

PEI-Genesis understands the frustrations of trying to mate connectors in hard to reach, not easily visible spaces. It also knows that the ability to achieve a stable, reliable and correct connection is crucial to any design, whether engineering downhole test equipment for the oil and gas industry or power and data connections essential to an autonomous vehicle’s safety.

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