PCBs in transportation, military, and aerospace applications encounter environmental variables such as shock, vibration, temperature, and humidity. Selecting printed circuit board connectors and adjacent components that can endure these rough conditions is crucial to operational success.
Selecting printed circuit board connectors (PCB connectors) that can perform reliably in harsh environments involves several important considerations. Whether connecting two or more PCBs together with a direct board-to-board connection or with a cable, PCBs in transportation, military, and aerospace applications encounter many environmental variables. Shock, vibration, temperature, humidity, and changing altitude/atmospheric conditions are just a few of the many factors that can compromise the integrity of contacts and disrupt the signal. That’s why the design of contact-to-board terminations, plating materials and thicknesses, surface treatments, contact beam forces, and redundancy all need to be considered along with the mechanical design.
Designers can simplify the connector specification process for these demanding applications by establishing a clear understanding of how different environmental factors affect PCB connectors and employing tools made to navigate various design features and benefits, and facilitate effective selections.
Vibration and Shock
Because many electronic commercial and military aviation devices are subject to extreme forces of motion, it’s important to understand how a connector handles vibration and shock. Typical problems include:
- Accidental connector disengagement during extreme acceleration or deceleration from excessive gravitational (g) forces.
- Connector or contact disengagement during torsion or flexure of a PCB undergoing roll, pitch, yaw, and other abrupt motions.
- Micro-interruptions of the signal due to wear of contacts by vibrations.
A connector uses mechanically deflected metallic beams arrayed within a shell or housing to form an interface that is separable, yet electrically “invisible.” Therefore, the first objective of connector design is to maintain the stability of the contact interface against all the factors that could cause signal or power loss.
At a macro level, vibration and shock can involve gross physical displacement or create a gap between the mating elements. At a micro level, fretting motion (or micro-motions) can gradually wear through the surface coating or plating of the mating surfaces, contributing to fretting corrosion. Solutions to these problems include:
- Designing contacts with sufficient contact forces so vibration doesn’t cause the beam to jump.
- Applying plating with enough thickness and quality to withstand the wear of fretting motion.
- Increasing the points of contact at the separable interface to provide redundancy through multiple contact points.
- Use of appropriate contact lubricants or surface treatments that inhibit fretting.
Beam design is another important element of vibration resistance. In a two-point connector, each contact spring beam makes a single point of contact with the mating wafer pads. In connectors with a four-point or quad-redundant contact system, the contacts are designed so each beam makes two points of contact. This feature roughly doubles the contact patch area and helps balance vibration loads by spreading the impact of fretting motion over a larger surface area.
Today, many manufacturers offer beam designs with four points of contact. In addition to mitigating the impact of fretting corrosion, they also significantly reduce the risk of accidental disengagement. For example, if the probability of contact disengagement for a given contact is 0.01, the probability that all four points of a four-point contact could be simultaneously affected is (0.01)4 or 10-8. This risk reduction illustrates the power of redundancy.
The shape of the beam also affects wear levels, because an optimum beam radius helps balance the load and surface pressures within the contact system. In a four-point contact design, contact beam length and areas are not symmetrical to one another. Different lengths give each beam a unique frequency mode in response to vibration, which helps eliminate the possibility of both beams simultaneously achieving resonance. In addition, even with increased contact points, these enhanced contact systems do not increase mating forces.
The wafers in the mating half of these designs can also be slightly modified to take full advantage of the four-beam structure. By making two of the four contact points on the split beams deeper, the signal pads on the wafer can be extended by 1.2mm to maintain at least 2mm of contact wipe during mating. This feature helps maintain quad redundancy throughout the entire mating range of the original design.
Guide posts can also be manufactured into the module to increase stability. The tolerances between mating guide posts and receptacles are more generous than for mating contacts. Even so, mated guides help restrict motion, as well as provide a keying function during mating. In addition, recent technological advances have tightened tolerances. Highly machined posts are now available to provide a precision fit that increases stability under extreme vibration to reduce fretting motion.
A circumferential grounding spring can also be employed in the guide module assembly to both further improve stability and provide electrostatic discharge (ESD) protection for the plug-in module. To stabilize the PCB, stiffeners can be applied as a composite resin in the PCB itself or integrated into the chassis to reduce board flexion.
At high altitudes, electronics can be exposed to temperature extremes. Aerospace applications generally require printed circuit board connectors that can function in operating temperatures extending from -55°C to 125°C, or even a wider range. The effects of temperature take various forms, including:
- Relaxation of normal forces due to metal expansion and stress relief at high temperatures, especially for copper-based metals.
- Loss of structural integrity due to differing coefficients of thermal expansion between different materials, such as an aluminum shell with brass pins.
- Thermal shock that degrades the signal when the contact is exposed to extremely cold temperatures.
How connectors behave under extreme temperatures can be evaluated by several thermal performance tests, such as thermal shock tests involving hundreds of hot-cold cycles, temperature life tests (e.g., exposing connectors to 125°C for 1,000 hours), and combustion and fire-safety material testing.
Space applications typically dictate very stringent temperature and radiation requirements to evaluate how materials hold up structurally. In the vacuum of space, material selections must also consider outgassing from depressurized metal alloys, polymers, and finishes.
Humidity can accelerate corrosion in a contact interface, especially when wear from thermal cycling or vibration exposes base contact metals to fretting corrosion. Temperature and humidity tests are typically performed in the laboratory following mate/unmate cycle testing. Plastic housings are also tested to evaluate moisture absorption that can reduce insulation resistance.
PCB termination areas can degrade significantly when exposed to man-made chemicals, sunlight, saltwater, and other harsh chemicals in the natural environment. In these cases, the durability of boards and contacts can be enhanced with conformal coatings using resins, vapor phase deposition, or fluorocarbon formulations.
Simplifying PCB Connector Selection
Evaluating all the factors that affect printed circuit board connectors in harsh environments is a complex task. A thorough understanding of the aspects of connector design addressed here can guide the specification process. Combining this knowledge with tools made to navigate options and achieve design objectives can make the process even easier. For example, TE Connectivity offers a High-Speed Products Developer Kit that provides designers with product specifications, test results, and catalogs for input/output (I/O) wired, board-to-board, and radio frequency (RF) connectors, as well as cabling and fiber optics. Tools like this can help designers narrow down product options and have more confidence when selecting PCB connectors for harsh environments.
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- Selecting Printed Circuit Board Connectors for Harsh Environments - February 4, 2020