Military and aerospace applications cover avionics, unmanned aerial vehicles (UAVs), aircraft, radars, and satellites, and require far more stringent connectors and interconnectors than consumer, medical, and industrial applications. This type of military/aviation connector is required to withstand various electrical, mechanical and environmental stresses, and must always meet the rated performance indicators, while the performance of conventional devices will be reduced or even damaged under the same conditions.
Highly reliable interconnect devices for military/aviation applications are by no means just one or a set of contacts encapsulated in a rugged enclosure. Interconnecting device bodies, seals, contact forces and contact materials must function as an integrated system to ensure satisfactory performance under specified conditions.
This paper discusses the challenges faced by designers in selecting and using interconnect devices for military/aviation applications. The three Molex products are then taken as examples to explain why these devices help overcome these challenges.
Requirements for rugged connectors
Ruggedized connectors consistently meet specifications under extreme mechanical, environmental, and thermal stresses. These stress sources vary from the operating environment, but there is also a large degree of overlap. For example:
Connectors in land-based military systems must be capable of withstanding severe vibration, thick deposits (dust, sand, grit), and extreme heat and cold.
The marine and deep-sea connectors must be capable of withstanding long term exposure to corrosive seawater environments and withstanding high crush pressures.
The aviation connector must be able to withstand repeated takeoff, landing and vibration of flight device, with extremely wide temperature range.
Space connectors experience more severe temperature fluctuations, vacuum exposure, venting, and strong mechanical stresses during launch and return to the atmosphere.
To meet the specifications for these requirements, a variety of basic physical factors need to be understood, including:
Vibration: connectors in military vehicles or fighters have been tested to withstand up to 20g acceleration.
Impact: this kind of high impact force generated during rapid acceleration or deceleration is different from vibration. Up to 50 g impact for standard connectors and up to 100 g impact for nano and micro designs; Even specialized standards for blast conditions cover the high magnitude, high frequency, and short term structural vibrations caused by explosive device explosions, commonly seen in rocket stage separation or missile payload release.
Extreme temperatures: land based systems may experience temperature fluctuations from - 65 ° C to 125 ° C, while space systems may experience temperatures up to 200 ° C. Alternation of heat and cold causes the material to expand and contract, potentially weakening the material and affecting electrical conductivity. In addition, differences in the coefficient of thermal expansion (CTE) between different materials within a connector can create mechanical stresses at the material interface, which can result in misalignment or failure over long periods of use.
Pollutant exposure: in order to ensure long-term reliable operation of the connector, measures such as O-shaped ring, sealing gasket and protective wire sleeve must be taken to prevent moisture, dust and other contaminants.
Corrosion: This is an ongoing problem caused by salt mist and oxidation. Connector materials must be properly selected and used to prevent these inevitable conditions from destroying the integrity of the connector.
What is Reliability?
In simple terms, long-term reliability refers to the ability to maintain stable performance under repeated use, environmental exposure and mechanical stress. This performance depends not only on the conditions under which the connector is first used, but also on whether it can withstand repeated mating and work properly. Many connectors, especially I/O connectors, experience hundreds or even thousands of mating operations.
A successful rugged design has two inextricably linked aspects: the contact itself and the housing (body) of the stationary contact (Fig. 1).
Contact material, geometry and plating are key factors (click to enlarge)
Figure 1. Contact material, geometry, and plating are key to the rugged connector design. Image source: Molex)
The design of the contact surface is essential to ensure that the connector maintains a low insertion force while achieving a reliable connection. Precision machining of the contact geometry reduces galling at the connection and the gold-plated (Au) layer on the contact surface prevents oxidation. Gold plating is typically 50 micro inches (µ in) thick and is applied over a nickel (Ni) base coating, which is used to enhance plating adhesion and further enhance corrosion resistance.
These coatings cover the copper (Cu) alloy base material of the contact. The combination of gold and nickel plating is essential for long-term reliability in aerospace, defense, and space applications. Beryllium copper (BeCu) is widely used as a base material due to its excellent strength to weight ratio and excellent fatigue resistance. This alloy is particularly suitable for contacts of spring members where elasticity and resilience after long-term stress are indispensable.
Phosphor bronze (CuSnP) is a suitable alternative to non-spring contacts, providing a balance between strength and conductivity. This material is corrosion resistant and has moderate spring properties and is commonly used in compact and fine pitch connectors that require some flexibility but do not require continuous bending.
Designing a rugged connector requires careful consideration of many factors (Figure 2):
Maintaining the normal force is the key to ensuring reliability. High performance spring material maintains contact pressure and durability.
Better contact force reduces air gap, reduces resistance and improves signal integrity. Optimized geometry distributes pressure to ensure stable conductivity.
Contact engagement is the axial overlap between the pin and receptacle, which combines force, continuity, and mechanical stability.
Maintaining normal forces is critical to reliability
Figure 2: Continuous normal force is the key factor to ensure reliability (top), while larger contact forces reduce the air gap (bottom), thereby reducing resistance and improving signal integrity. Image source: Molex)
At the microscopic level, the mating contact area is not simply a simple fit between two smooth flat surfaces. On the contrary, where ohmic contact is formed or disconnected, the contact interface has microscopic roughness, surface peak and irregular shape. Higher contact forces flatten these tiny protrusions, improving electrical conductivity, reducing contact resistance, and ensuring consistent performance, but increased contact forces also affect insertion and withdrawal forces, increasing contact surface wear.
Well-designed contact system balances engagement length and normal force to prevent loose connections, excessive wear and mechanical stresses. If the contact force is too small, the contact resistance will increase and the signal will be unstable. Conversely, excessive contact forces accelerate the abrasion of the plating and lead to premature fatigue of the contact structure.
Unlike commercially available connectors with only one or two contacts, rugged connectors employ a multi-contact system to distribute mechanical loads resulting from vibration or shock (Figure 3). These contact systems prevent arcing or loss of signal due to jogging and provide redundant contact paths for critical systems.
Multicontact design for improved stability and signal integrity
Figure 3: Multi-contact design for improved stability and signal integrity. Image source: Molex)
The contact system may also include spring elements to maintain a consistent contact force over time. Spring-loaded contacts compensate for small changes during contact alignment while ensuring reliable conductivity through repeated plugging and unplugging. However, excessive forces can cause excessive wear of the contact plating.
More than contacts: connector housing and protective housing
The core performance of a rugged connector starts with the contact, but the connector housing serves much more than the electrical contact that surrounds the interior: it resists mechanical stresses, extreme temperatures, aggressive media, and moisture while maintaining a balance between durability and weight.g. A wide range of enclosure materials are available for the designer:
Thermoplastic polymers such as polyether-ether ketone (PEEK), polyphenylene sulfide (PPS), and polyetherimide imide (PEI) provide excellent mechanical strength, heat resistance, and chemical stability. These materials effectively absorb vibration and impact from lightweight structures.
Composites such as glass fibre reinforced polymers and carbon fibre composites have excellent strength-to-weight ratios. The design allows optimisation of specific properties of such materials, including tensile strength, impact resistance or thermal stability.
Stainless steel and aluminum alloys are the preferred materials for connector housings due to the high impact, high vibration, and strong electromagnetic interference (EMI) in aerospace and defense applications.
Stainless steel connector housings offer excellent corrosion resistance and mechanical strength, making them ideal for marine, industrial, and aerospace applications exposed to moisture, chemicals, or salt mist. Aluminum alloys offer not only strong EMI shielding, but also light weight and ease of processing, making them the preferred material for connector housings in military vehicles, avionics, and space applications.
Some rugged connectors employ flat latching systems that provide stability and secure mating while reducing overall dimensions. For example, a spring lock or hold-down device provides both mechanical reliability and ease of operation of the connector under battlefield conditions.