Connectors in Rugged Design

Connectors in Rugged Design

Posted by Tom Muday on Mar 2nd 2018

High-quality rugged connectors are complex and expensive. Connectors can be 1/3rd or more of the total BOM cost on some rugged products. Selecting a connector wisely is a crucial step in rugged design. Even when the connector type is predetermined by legacy or mating equipment, there are usually trade-offs. MIL type connectors often are available from many suppliers. Even for proprietary connectors, there are trade-offs to consider in terms of mounting style, retention and electrical connection and so on. We work extensively with Glenair for rugged connectors so I reached out to them for input with this article. Their comments are intermixed and noted. Thank you Glenair team for your help.

"Precision machined, tight-tolerance connector designs with Mil-Spec pedigree are preferred for I/O interfaces on rugged systems. End-user customers in mil-aero environments have little patience for cheaper, lesser caliber interconnects that, while they may reduce the initial purchase price of the system, can adversely impact system life-spans and performance". - Glenair

Sealing

Connectors that aren’t sealed are almost never an option for rugged design. If a connector is not sealed while UNMATED then expect trouble if the product is required to meet IP67 or IP68. It’s naive to expect users to always replace dust caps or keep cables plugged in. Water can travel through long cables in the spaces between wire strands like a capillary. An unsealed connector, even on the end of a cable is trouble even if over-molded. When a user inevitably leaves a dust cap off and gets water into the connector, they will blow it out, maybe even with compressed air. That will drive the water deep into the connector causing immediate failure now from shorting or failure later due to corrosion.

"Sealing is complex and important element in connector design. Properly dimensioned o-rings, interfacial seals and wire grommet seals, with appropriate shore hardness and chemical resistance for the given application, all play a part in preventing dust and moisture ingress. Sealing plugs, used to fill unused contact cavities are also required in many rugged applications". - Glenair

Manufacturers

There are lots of great connector manufacturers out there. Our favorite rugged connector manufacturer is Glenair as we have a lot of experience with their team and Mighty Mouse products. ODU, Amphenol, Omnetics, Souriau, Fischer, and Lemo also have great reputations but we have less experience with their products. There are new offerings all the time, and we regularly meet with connector reps showing off new product. Some of the great stuff we’ve seen recently are fine pitch (thus smaller) high-speed connectors, tiny wire-to-wire internal connectors and wipe-clean connectors. Regardless of the manufacturer, the number of options available to the rugged product designer is staggering.

"For many application environments, the decision on which connector (or manufacturer) is driven by the mating specifications of the other equipment and cables in the system. Most airframe applications, for example, have standardized on D38999 series connectors. Soldier radio communications universally utilize M55116 spring-pin audio connectors and M26482 bayonet connectors for power. In the US and NATO, soldier power and data interconnects for rugged C4ISR applications are standardized around the Glenair Series 804 Mighty Mouse. - Glenair

Contact Size

Selecting the right contact size is mostly a matter of picking one that can handle the amperage and voltage. Usually, the amperage is the limiting factor unless we are working on a high voltage application. When considering high voltage, high amperage, or high-speed data, we consult electrical experts - those aren’t rugged mechanical considerations. Also, there are a lot of available contact technologies. Durability and low resistance are the goals all manufacturers are chasing. Some have highly proprietary designs so they need to be considered, but again that’s mostly an electrical issue. We are concerned here about form factor and physical use trade-offs.

"Contact size, wire size, and contact-to-contact spacing (insert density) are all inter-related and impact both the electrical performance of the interconnect as well as its package size and weight. For applications where size and weight are an issue (dismounted soldier systems, aircraft, etc.) the current trend is to select the smallest form-factor possible that will meet all requirements especially DWV. The evolution of connector designs to ever smaller packages (micro, nano, pico) always begins with the contact". - Glenair

Plug or Receptacle

Safety first, durability second. Almost by definition a rugged connector is exposed to the user. That means we need to think about safety first. If there is a potentially dangerous amount of power supplied by a connector, the contacts need to be protected from shocking the user. This usually means power sources are female receptacles with female sockets connected to a plug with male pins. Some manufacturers offer male plugs with female sockets which could be an option but this is less common. If safety is not a concern, the question of which part should have the plug and which the receptacle is one of form factor. It’s most common to put receptacles on housings and have plugs on cables. The biggest reason for this is drop protection. Plugs stick out and can be the first point of impact in a drop. This can be mitigated with some protective features like ribs or a recess in the housing design. Small shell sizes of even rugged plugs can be rendered useless with one hard impact. Cables do get knocked around when unplugged, but the mass of the cable end is not nearly as great as a housing. Dropping a cable end with an exposed plug will usually not render the plug out of round whereas a chassis mounted plug that takes a direct hit will usually get damaged.

"Size and weight plays a part here as well. A lightweight cable with a push-pull plug (no coupling nut) is less susceptible to impact damage from dropping due to its lighter mass. Likewise, rugged electronic enclosures designed around small-form factor interconnects always perform much better in shock and vibration testing than larger, heavier equipment". - Glenair

Mounting Style

Mounting of most types of rugged connectors, both plugs and receptacles come in these basic styles:

  • Inline - for use in cables
  • Chassis Mount with nut on exterior
  • Chassis Mount with nut on Interior

Of course within these, manufacturers usually offer even more options. For example, a chassis mount receptacle may be flush mounted or it may extend from the chassis. Most chassis mount connectors use a single locking nut, others use 2-4 screws around the perimeter. Here are the trade-offs to consider. Obviously, cables will need an inline connector. Chassis mounting is less obvious. Having the mounting features (nut or screw) on the outside of the product will allow it to be re-tightened if it were to ever come loose. But, this can be a security risk for very sensitive equipment handling encryption or financial transactions. Having the nut or screws on the inside keeps users from tampering with them, but then we need to keep in mind that whatever electrical connection we need to make will have to fit through the hole in the chassis from the outside. Finally, while designing rugged products, we need to consider the system. If a product will regularly be used with a cable attached, we need to consider what leverage the mated cable will impart to the chassis. Long strain relief on cables are great at preventing damage to the cable, but since they are long they can put extra strain on the chassis. There are no hard and fast rules, we just need to use good engineering judgment and consider the whole system when deciding how to mount connectors. Moving the connector off the chassis with a short pigtail or flying lead to an inline connector can be an option in some products. That’s usually only acceptable if the pigtail in question will always be mated.

"Weld and solder mount receptacles are also an option, especially in tightly-sealed systems. For general environmental applications, designers tend to select jam-nut mounted receptacles (single hole) over “box” or “wall” mount receptacles (multiple holes) due to ease of assembly and superior sealing. Recessed receptacles have obvious advantages, but space must be provided for the shell inside the enclosure. As to cable strain-relief and cable routing, there are many innovative solutions on the market to protect the wire-to-connector attachment and eliminate awkward routing angles. Ultra low-profile right-angle backshells for example, or integrated connector/backshell assemblies such as the Mighty Mouse Cobra, may be specified when space outside the box is limited or cable-routing is a concern". - Glenair

Retention

In rugged environments, cables get pulled on, snagged, tripped over, shot at, and accidentally sliced. As designers, we can’t do much about stray bullets or paratroopers removing zip ties with a knife and cutting through their cables. But, good cable management can reduce some of the snag hazards. Still, cables will be abused and we need to mitigate damage and unintentional disconnects. Our usual preference is for quick disconnect connectors which breakaway when a set amount of force is applied. Snagging on a cable that won’t breakaway can be dangerous to the user and less importantly it can damage the product. Breakaway or quick disconnect cables can often be specified with high or low force. Glenair, for example, offers their Mighty Mouse connectors in breakaway forces of 5, 10, and 15 pounds. For applications where there aren’t safety concerns and physical locking is appropriate, there are pull-to-disconnect, turn-to-lock, and push button latches to name a few.

"Many push-pull (or breakaway) connectors are also available with locking mechanisms such as a retractable collar. The Glenair Mighty Mouse 824 is an example. Designs of this type push to mate but then require manipulation of a mechanical feature to de-mate". - Glenair

Electrical Connection

Non-Rugged products often mount electrical connectors directly to a PCB and then just cut an access hole in the chassis for clearance. This may be fine for a small USB hub that sits on a desk, but in a rugged environment, it won’t be long before that connector breaks off the PCB. For rugged applications, it’s more common to use an external connector that is chassis mounted, and then some internal flying lead. In this way, external impacts on the connector won’t cause damage to the PCB. The two common options are discrete wires and flex circuits. In either case, the PCB will have a small, less rugged mating connector attaching it to the flying lead. Discrete wire rugged connectors make their electrical connection with crimp contacts or solder cups. Flex circuit flying leads are usually soldered directly to through-hole PCB pins on the rugged connector. The flex circuit then comes off the back of the connector at a right angle which can be a great space saver in some designs but makes it nearly essential that the connector installs to the chassis from the inside with the retaining nut or screws on the outside. The flex is usually too large and awkward to fit through the mounting hole. In very high vibration designs, it may be important to reinforce the internal connections with potting compound. That’s a good reason to keep internal connections as light as possible and secure any discrete wires.

"One of the challenges in effecting the I/O connector-to-board termination is maintaining the electrical “goodness” of the signal. In high-speed systems that have little or no budget for signal degradation, designers sometimes opt for the shortest and most direct board termination. This juncture may be ruggedized by selecting PCB mount connectors that incorporate auxiliary mounting posts (threaded attachments) or extremely rugged eye-of-the-needle (compliant pin) PC tail terminations. When opting for ribbon wire or flex for final termination to the board, care must be given to insure impedance-matching and shielding is maintained from the I/O all the way to the board". - Glenair

Shielding

Many rugged products have difficult EMI requirements. Full 360-degree metal shielding might not alone be enough to pass MIL-STD-461 or Tempest, but it’s usually a minimum requirement. For chassis mount connectors, this means getting good all around electrical contact between the connector shell and the metal or shielded plastic housing wall. For inline connectors on cables, a combination of high-quality braid and foil needs to be electrically connected around the connector shell. This is accomplished with a metal backshell, a metal band like a BAND-IT, or a crimp sleeve.

"Shielding requirements are usually defined as a measure of allowable resistance (dBs) within a certain frequency range. Effective shielding in electrical wire interconnect systems is a multidisciplinary approach that depends not just on component selection, but also conductive plating, wire/cable shielding, conductive gaskets/fittings and more. Poorly shielded connectors (bad plating, out-of-tolerance mating interface) can rarely be overcome by augmenting shielding in some other part of the system. Intermateability (or lack of) between manufacturers is a concern in this regard as resistance qualification testing can never be performed on all the many combinations of connectors, sizes, platings and so on between manufacturers". - Glenair

Corrosion Resistance

Once we know the environmental requirements for a rugged product, we can decide on a connector material and finish. Plastic connector shells are not common in rugged applications but could be useful in extremely corrosive environments where shielding is not needed. More often, stainless steel and aluminum are the material of choice. With each of these, there is a host of finish options which will be selected based on appearance, corrosion resistance, and RoHS compliance. Common colors are chrome, black and olive drab and there are usually a few variations within each of these colors. Here is a list of 23 options Glenair offers. Note the different substrate materials, colors, RoHS compliance and salt spray values. Salt spray tests are a standardized measure of corrosion resistance.

"A critical aspect of material and finish selection is “sacrificial” versus “barrier” plating in conductive (shielded) applications. In the former, the conductive material in the connector (say nickel plating) is protected from corrosion by bonding it to a sacrificial material (typically zinc or cadmium) that gets eaten first by electrolytic corrosives in the environment. In barrier plating the conductive material (again nickel) is sealed from the environment by an electrically pervious material (such as PTFE plastic). While there are 23 options in the linked table, practically speaking engineers limit their choices to a small list of proven combinations that meet their durability and conductivity (resistance) benchmarks". - Glenair

Cycles

A designer must consider the conops for each connection and determine how many mate / demate cycles each connector will endure in its lifetime. Manufacturers will rate their connectors for a number of cycles. I’ve personally seen test cables exceed those ratings by 10x, but those were in a controlled environment and we should not assume the manufacturer has been overly conservative.

"Mating cycle benchmarks are born out of military specification norms and are typically de-rated from the mating pair’s actual capabilities. Practically speaking, two things are of concern to the end-user: (1) the performance of the coupling nut (or other mechanical mating technology) over time (obviously less of a concern in push-pull connectors) and (2) changes in contact electrical performance due to fretting or other forms of contact wear. For this reason rugged systems almost always use mil-qualified contacts (M39029) with regulated dimensional and plating specifications. Properly engineered into the connector, these contacts can go many thousands of cycles without issues". - Glenair

Dust Caps

Connectors are getting finer pitched to carry more data on smaller connectors. Dust and mud are a big problem with any connector, but especially fine pitch connectors. Dust caps are usually a must in rugged environments. Most manufacturers offer a few styles of dust cap, but we’ve occasionally had to make our own to fit a unique application. Users will inevitably forget or ignore dust caps which is why it’s critical to have sealed connectors that can be cleaned out. If mud gets into an unsealed connector, good luck cleaning that out without contaminating the product.

"Military applications require the use of dust caps on virtually all electronic systems. Resilient material (rubber) caps are often preferred on rugged soldier systems because they do not make noise". - Glenair

Keying

Lastly, if a product is part of a system with many cables, we need to consider keying. Keying keeps similar cables from being accidentally plugged into the wrong receptacle. Incorrect connections cause malfunction or electrical damage so unique keying is important. Small fine pitch connectors can be hard to mate if the user can’t tell which orientation fits. Commonly little color-coded dots or lines will help the user find the right receptacle and orientation. For plugs that need to be mated without looking, large tactile indicators help the user find the receptacle even with gloves on.

"Rugged system connector designers are told to picture a soldier on a pitch black night, in the middle of an intense firefight, crouched in an awkward position (taking shelter). Now, design a connector mating interface that can be reliably mated and de-mated, with audible, visual and tactile indicators of mating. Keying is absolutely a big part of this solution, both for reliably mating of isolated connector pairs, but also as noted to prevent mis-mating of plugs and receptacles". - Glenair