Select the Right Connectors to Meet Stringent Mil/Aero Electrical and Mechanical Requirements

The requirements that are placed on connectors and interconnects for military and aerospace (mil/aero) applications, including avionics, unmanned aerial vehicles (UAVs), aircraft, radar, and satellites, are far more stringent than those for consumer, medical, and industrial applications. These mil/aero connectors are subject to a wide range of electrical, mechanical, and environmental stressors that would degrade or even damage conventional devices, yet they must continue to meet their rated performance specifications.

A high-reliability interconnect for mil/aero applications is not just a contact or set of contacts housed in a rugged enclosure. The body, seals, contact forces, and contact materials must function as an integrated system to ensure performance under the specified conditions.

This article examines the challenges designers face when selecting and using interconnects for mil/aero applications. It then introduces three examples from Molex and shows how they address these challenges.

Requirements for rugged connectors

A rugged connector is one that consistently meets specifications under extreme mechanical, environmental, and thermal stressors. These stressors differ depending on the operating environment, but also have considerable overlap. For example:

• Connectors in land-based military systems must withstand severe vibration, thick accumulations of dirt (dust, sand, grit), and extreme heat and cold.
• Seaborne and deep-sea connectors must withstand prolonged exposure to corrosive saltwater and crushing pressure.
• Aerospace connectors must withstand repeated takeoffs and landings, in-flight vibrations, and wide temperature ranges.
• Connectors for space experience more extreme temperature swings, vacuum exposure, outgassing, and intense mechanical stress during launch and reentry.

Meeting these required specifications involves an understanding of multiple fundamental physical factors, including:

• Vibration: Connectors in military vehicles or fighter jets are tested to withstand up to 20 g.
• Shock: This high-impact force during rapid acceleration or deceleration is distinct from vibration. It can be as high as 50 g for standard connectors and 100 g for nano and micro designs; there is even a standard for pyroshock events (high-magnitude, high-frequency, short-duration structural vibration caused by the detonation of explosive devices, such as stage separation on rockets or missile payload deployment).
• Temperature extremes: A ground-based system may face temperatures ranging from -65 to 125°C, while space systems may reach 200°C. Thermal cycling causes materials to expand and contract, potentially weakening them and affecting conductivity. In addition, differences in the coefficient of thermal expansion (CTE) among materials within a connector can introduce mechanical stress at material interfaces, possibly leading to misalignment or failure over time.
• Exposure to contaminants: To ensure reliable long-term operation, connectors must be safeguarded against moisture, dust, and other contaminants with sealing solutions such as O-rings, gaskets, and grommets.
• Corrosion: This is an ongoing issue caused by factors such as salt spray and oxidation. Connector materials must be properly selected and applied to prevent these unavoidable phenomena from degrading connector integrity.

What is reliability?

In simple terms, long-term reliability means maintaining consistent performance despite repeated use, environmental exposure, and mechanical stress. This performance is determined not only by first-time connector use but also by the ability to withstand repeated mating cycles and function properly. Many connectors, especially input/output (I/O) connectors, undergo hundreds, or even thousands, of mating cycles.

There are two intertwined aspects to a successful rugged design: the contacts themselves and the housing (body) that holds them in place (Figure 1).

Figure 1: Contact materials, geometry, and platings are essential factors in rugged connector design. (Image source: Molex)

The design of the contact surface is crucial for ensuring that connectors maintain low insertion force while providing reliable connections. Precision machining of contact geometries reduces wear and tear at the connection, while gold (Au) plating on the contact surface prevents oxidation. The gold plating is usually 50 microinches (µin.) thick and is applied over a nickel (Ni) underplating, which enhances plating adhesion and further improves corrosion resistance. 

These platings are applied over the copper (Cu) alloy base material of the contact. The combination of gold plating and nickel underplating is essential for long-term reliability in aerospace, defense, and space applications. Beryllium copper (BeCu) is widely used as the base material due to its excellent strength-to-weight ratio and exceptional fatigue resistance. It is particularly well-suited for spring member contacts, where elasticity and long-term resilience under stress are essential.

Phosphor bronze (CuSnP) is a suitable alternative for non-spring contacts, offering a balance of strength and conductivity. It is corrosion-resistant, has moderate spring properties, and is frequently used for compact and fine-pitch connectors that require some elasticity but are not subjected to continuous flexing.

Designing rugged connectors requires careful consideration of multiple factors (Figure 2):

• Sustaining normal force is critical for reliability. A high-performance spring material maintains contact pressure and durability.
• A stronger contact force reduces air gaps, lowering resistance and improving signal integrity. Optimized geometry distributes pressure for stable conductivity.
• Contact engagement is the axial overlap between the pin and socket, which balances force, continuity, and mechanical stability.

Figure 2: Sustaining normal force is critical for reliability (top), while stronger contact force reduces air gaps (bottom), thereby lowering resistance and improving signal integrity. (Image source: Molex)

At the microscopic level, the mating contact zone is not just a meeting of two smooth, flat surfaces. Instead, the interface contains microscopic roughness, peaks, and irregularities at the points where ohmic contact is made or broken. Applying more force flattens these asperities, thereby improving conduction, reducing resistance, and ensuring consistent performance. However, increased force also impacts mating and unmating forces as well as contact-surface wear.

A well-engineered contact system balances engagement length and normal force to prevent weak connections, excessive wear, and mechanical stress. If the contact force is too low, electrical resistance rises, causing signal instability. Conversely, too much force speeds up plating wear and leads to premature fatigue in the contact structure.

Unlike commercial connectors with one or possibly two points of contact, ruggedized connectors incorporate multi-point contact systems to distribute mechanical loads from vibration or shock (Figure 3). These contact systems prevent arcing or signal loss caused by micro-movements and provide redundant contact paths for critical systems.

Figure 3: Multi-point contact designs improve stability and signal integrity. (Image source: Molex)

The contact system may also include spring elements to maintain a consistent contact force over time. These spring-loaded contacts compensate for slight variations in contact alignment while ensuring reliable conductivity across repeated mating cycles. However, excessive force can cause too much wear on the contact plating.

Beyond contacts: connector housings and enclosures

While rugged connector performance starts with the contacts, the connector housings do more than just enclose internal electrical contacts: they also protect them from mechanical stress, extreme temperatures, corrosive elements, and moisture ingress, all while maintaining a balance between durability and weight. There are several housing material options for designers to choose from:

• Thermoplastic polymers such as polyether ether ketone (PEEK), polyphenylene sulfide (PPS), and polyetherimide (PEI) provide excellent mechanical strength, thermal resistance, and chemical stability. These materials effectively absorb vibration and shock in lightweight structures.
• Composite materials such as fiberglass-reinforced polymers and carbon fiber composites offer excellent strength-to-weight ratios. They can be designed to optimize specific properties such as tensile strength, impact resistance, or thermal stability.
• Stainless steel and aluminum alloys are preferred materials for connector housings due to the high shock, vibration, and electromagnetic interference (EMI) in aerospace and defense applications.

Stainless-steel connector housings offer exceptional corrosion resistance and mechanical strength, making them well-suited for marine, industrial, and aerospace applications exposed to moisture, chemicals, or salt spray. Aluminum alloys provide a good balance of strong EMI shielding, light weight, and ease of machining, making them the preferred material for connector housings in military vehicles, avionics, and space applications where reducing weight is essential.

Some rugged connectors use low-profile latching systems that provide stability and secure mating while reducing overall size. Spring-loaded locks or push-to-lock mechanisms, for example, make connectors both mechanically reliable and easy to operate under battlefield conditions.

Space rated: another frontier

Connectors used in satellites, deep-space probes, and high-altitude aerospace systems are constantly exposed to ionizing radiation, which can degrade materials, impair electrical performance, and weaken structural integrity. These connectors must be built to resist radiation-induced embrittlement, loss of conductivity, and atomic oxygen erosion while maintaining reliability in vacuum environments.

For these applications, radiation-hardened (rad-hard) thermoplastics such as PEEK and PPS provide superior radiation resistance while maintaining low outgassing. Metal shielding made from aerospace-grade aluminum alloys with an electroless nickel finish provides structural durability while protecting against radiation and atomic oxygen exposure. Finally, gold plating forms a protective barrier against radiation damage, preserving electrical integrity and contact reliability during extended space missions.

Connector families illustrate a variety of solutions

No single rugged connector type fits all needs, so companies such as Molex offer a wide variety of options. A look at D-subminiature (D-sub), RF termination, and RF plate connectors highlights capabilities versus applications, ratings, and locking and retention mechanisms, among other characteristics.

Well-established, the D-sub remains widely used for its range of contacts (9, 15, 25, 37, and 50), signal-handling capability, physical keying, and various mating and retention options. An example is the Molex 0732841811 (Figure 4), an EMI-filtered, 9-pin female-to-9-pin male (plug/socket) in a “free hanging” arrangement. Among other applications, it can be used to mate two disparate connector genders.

Figure 4: The 0732841811 is a 9-pin male/female D-sub adapter. (Image source: Molex)

Its pins have a low 10 milliohm (mΩ) contact resistance, while the integral 1000 picofarad (pF) capacitors provide a 3 decibel (dB) cutoff frequency of 3.2 megahertz (MHz) for EMI and radio-frequency interference (RFI) filtering. The shell is approximately 0.304 in. wide × 0.64 in. long (7.72 × 16.26 mm) and is made of Ni-plated zinc, while the body insulator is glass-filled polyester.

For RF cable termination, the 0732870620 (Figure 5) is a 26.5 gigahertz (GHz), 50 Ω coaxial-connector plug (male pin) used to cap (terminate) an unused RF port. Doing so prevents signal energy from reflecting back down the cable, which could cause signal distortion, interference, and even damage to sensitive electronic components.

Figure 5: The 0732870620 is a 26.5 GHz, 50 Ω SMA terminator that caps an unused RF port to prevent signal reflections. (Image source: Molex)

The 0732870620 features a near-unity voltage standing wave ratio (VSWR) of 1.05:1 at DC, rising to just 1.35:1 at its maximum frequency. The body is passivated stainless steel, while the conductor is Au-plated BeCu. The device is rated for 1 watt power handling (continuous) at 25˚C, with a maximum rating of 1 kilowatt (kW) with a 5 microsecond (µs) pulse and a 0.05% duty cycle.

RF filter plate interconnects are less well known than standard connectors but serve an important role. These are specialized, high-density components designed to suppress EMI at the bulkhead or module level. Unlike signal pass-throughs, filter plates block or attenuate EMI within a specified frequency range, thereby maintaining signal integrity and reducing noise, while also preventing crosstalk and distortion in high-frequency applications.

A plate such as the 0732860030 (Figure 6, left) features multiple filtered signal lines to reduce installation labor and save space on circuit boards. It features two rows of six straight pins each within its 1.06 in. (26.92 mm) long plate, and uses a 100 pF feed-through capacitor (C-style) filter with a 3 dB maximum cutoff frequency of 50.3 MHz. It has an insertion loss of 0 dB at around 50 MHz, rising to 50 dB (typical) at 10 GHz (Figure 6, right).

Figure 6: The 0732860030 filter plate (left) features two rows of six pins, uses a 100 pF C-style filter with a 3 dB cutoff frequency of 50.3 MHz, and has an insertion loss of 0 dB at around 50 MHz, rising to 50 dB (typical) at 10 GHz (right, line B). (Image source: Molex)

The base brass plate is tin-plated, while the Au-plated pins can handle 100 volt signals at 3 amperes (A).

The requirements for connectors and interconnects in rugged applications, as well as their materials, are defined by standards from various organizations. Many of these are listed in Relevant Standards.¹

Conclusion

The requirements for ruggedized connectors and interconnects used in military, aerospace, near- and deep-space, and other harsh environments are stringent. They require an understanding and careful consideration of the tradeoffs involved in the materials, design, and fabrication of contacts and housings to produce suitable connectors for these conditions. Molex offers ruggedized solutions, each with a wide variety of options, so designers can choose an optimized solution and meet critical performance objectives.

Relevant Standards

• MIL-STD-202 Test Method Standard, Electronic and Electrical Component Parts
• MIL-STD-810 Environmental Engineering Considerations and Laboratory Tests
• MIL-STD-1344 Test Methods for Electrical Connectors
• EIA 364-27 Mechanical Shock Testing of Electrical Connectors and Sockets
• MIL-DTL-83513 General Specification for Connectors, Electrical, Rectangular, Microminiature, Polarized Shell
• MIL-STD-348 Department of Defense Interface Standard: Radio Frequency Connector Interfaces for Military Applications
• NASA ASTM-E595 Standard Outgassing Test
• NASA-STD-6012 Corrosion Protection for Space Flight Hardware
• NASA-STD-5019 Fracture Control Requirements for Spaceflight Hardware
• NASA-STD-7003 Pyroshock Test Criteria
• IP67 Ingress Protection Code
• UL94V-0 Standard for Safety of Flammability of Plastic Materials for Parts in Devices and Appliances

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1. Aerospace Technology Solutions from Molex
2. Meeting Critical Demand in the Aerospace and Defense Sectors
3. Defining and Advancing Rugged, Reliable Connectivity in Aerospace and Defense
4. Engineering Contact Engagement and Normal Force for Long-Term Connector Performance
5. EMI Filter Plates