Environmental Effects on EMC

Electromagnetic compatibility is not a static characteristic that remains constant throughout a product's life. Environmental factors continuously interact with the materials, components, and structures that determine EMC performance, causing progressive changes that may eventually compromise compliance. Understanding these environmental effects enables engineers to design products that maintain EMC margins throughout their intended service life and to predict when maintenance or replacement may be necessary.

Environmental effects on EMC operate through multiple physical mechanisms. Moisture changes electrical properties of insulators and promotes corrosion of conductors. Temperature variations alter component values and cause mechanical stress through differential expansion. Radiation degrades polymers and coatings. Chemical exposure attacks metals and plastics. Biological activity creates contamination and consumes materials. Each mechanism affects specific aspects of EMC performance, and multiple mechanisms often act simultaneously, creating complex degradation patterns.

Moisture Ingress Effects

Moisture is one of the most pervasive environmental factors affecting electronic equipment, with multiple mechanisms by which it degrades EMC performance.

Dielectric Property Changes

Water dramatically changes the electrical properties of materials:

Dielectric constant increase: Water has a dielectric constant of approximately 80, compared to 3-5 for most insulating materials. Even small amounts of absorbed water significantly increase the effective dielectric constant of insulating materials. This affects transmission line impedances, distributed filter characteristics, and parasitic capacitances.

Loss tangent increase: Water increases the dissipation factor (loss tangent) of insulating materials, increasing RF losses. High-frequency signals experience greater attenuation. Filters and transmission lines lose effectiveness. Heat generation in dielectrics increases.

Impedance effects: Changes in dielectric constant shift characteristic impedances of transmission lines. Impedance mismatches cause reflections and standing waves. Signal integrity degrades. EMC performance that depends on proper termination is affected.

Frequency-dependent effects: The effects of moisture absorption vary with frequency. At low frequencies, bulk absorption dominates; at high frequencies, surface moisture becomes more significant. Different EMC parameters may be affected differently across the frequency range.

Surface Conductivity

Moisture on surfaces creates conductive paths:

Surface leakage: Thin moisture films on PCB surfaces, component bodies, and other insulating surfaces create leakage paths between conductors. Surface resistivity drops from greater than 10^12 ohms per square in dry conditions to 10^6-10^9 ohms per square or lower when wet. This leakage can short across filter components, reducing their effectiveness.

Ionic enhancement: Ionic contamination (from flux residues, handling, or environmental sources) dramatically increases the conductivity of moisture films. Leakage currents increase by orders of magnitude. Even trace contamination becomes significant when wetted.

Voltage-dependent effects: Under bias, surface moisture can support electrochemical processes. Ion migration creates conductive paths that persist even when moisture dries. Dendrite growth can cause permanent shorts.

High-frequency effects: Surface moisture affects high-frequency behavior differently than bulk moisture. Surface currents flowing through moisture films can bypass intended current paths, affecting shielding effectiveness and filter performance.

Seal Degradation

Moisture challenges equipment sealing:

Gasket permeation: All gasket materials are somewhat permeable to water vapor. Over time, moisture permeates through gaskets even when sealed joints are maintained. The rate depends on material, temperature, and humidity differential.

Seal degradation: Gaskets and O-rings degrade over time from compression set, chemical attack, and UV exposure. Degraded seals allow faster moisture ingress. Shielding gaskets may lose both sealing and electrical contact.

Breathing: Temperature cycling causes equipment to "breathe" as internal air expands and contracts. Each breath exchanges some internal air with external air. In humid environments, this gradually increases internal humidity.

Cable and connector entries: Cables entering enclosures create moisture paths along cable jackets and through connector bodies. Potted entries eventually allow moisture penetration as potting compounds age.

Condensation Effects

Liquid water creates more severe effects than humidity:

Condensation formation: When surface temperature drops below dew point, water condenses on surfaces. This occurs during rapid temperature transitions or when equipment is moved between environments. Internal condensation can occur when equipment warms slowly relative to internal moisture evaporation.

Water pooling: In enclosures, condensed water may pool in low spots. Pooled water in contact with electronics creates severe short-circuit risk. Enclosure design should provide drainage paths.

Contact corrosion: Liquid water accelerates corrosion much faster than humidity alone. Connector contacts, exposed copper, and solder joints are vulnerable. Corrosion products may persist after water evaporates.

EMC during condensation: Equipment experiencing condensation may show severe EMC degradation that recovers as moisture evaporates. Intermittent problems correlated with temperature transitions may indicate condensation issues.

Corrosion Impacts

Corrosion progressively degrades metal surfaces that are critical for EMC performance, including shields, grounds, and connector contacts.

Galvanic Corrosion

Dissimilar metals in electrical contact corrode in the presence of an electrolyte:

Electrochemical series: When dissimilar metals contact in the presence of moisture, the more anodic (active) metal corrodes preferentially while the more cathodic (noble) metal is protected. Common electronics materials span a wide range of electrochemical potentials.

EMC-relevant examples: Aluminum enclosures with steel fasteners, copper traces with tin solder, nickel-plated connectors with gold contacts, and zinc-plated hardware in aluminum housings all create galvanic couples. The corrosion affects electrical continuity and shielding.

Rate factors: Galvanic corrosion rate depends on the potential difference between metals, the electrolyte conductivity, the area ratio (small anode/large cathode is worst), and temperature. Marine environments with salt-laden moisture cause rapid corrosion.

Prevention: Selecting compatible metals, applying protective finishes, isolating dissimilar metals electrically, and keeping surfaces dry all reduce galvanic corrosion. EMC requirements for electrical continuity must be balanced against corrosion prevention.

Oxidation and Surface Films

Oxide films form on metal surfaces and affect electrical contact:

Oxide formation: Most metals form oxide films when exposed to air. Aluminum rapidly forms a stable, insulating oxide. Copper forms semiconducting oxides. Iron forms rust. These films affect contact resistance and current distribution.

Contact resistance effects: Oxide films on connector contacts increase contact resistance. Initially small increases may not affect function but degrade EMC margins. As oxidation progresses, intermittent or complete contact failure occurs.

Shield continuity: Oxide films on shield surfaces can break electrical continuity at joints and gasket interfaces. The shield becomes discontinuous at high frequencies while appearing continuous to DC measurements.

RF surface effects: High-frequency currents flow in the skin depth near metal surfaces. Oxide films affect this surface current flow. Tarnished or corroded surfaces may have increased RF resistance even when DC resistance appears normal.

Corrosion of EMC Components

Specific EMC components are vulnerable to corrosion:

Shielding gaskets: Many EMC gaskets use wire mesh, metal foil, or metal-filled elastomers. Corrosion of the metal component degrades both mechanical properties (compression, resilience) and electrical properties (conductivity, contact resistance).

Filter components: Inductor windings, capacitor leads, and ferrite core surfaces can corrode. Corroded leads increase resistance; corroded cores may lose permeability. Filter performance degrades progressively.

Ground connections: Ground screws, ground straps, and ground planes are vulnerable to corrosion. Corroded grounds increase ground impedance and may become open circuits. Ground degradation affects both emissions and immunity.

Cable shields: Cable shields, particularly where exposed at terminations, can corrode. Braided shields have more surface area for corrosion than solid shields. Corroded shields have increased transfer impedance.

Corrosion Acceleration Factors

Several factors accelerate corrosion:

Temperature: Chemical reaction rates approximately double for every 10 degrees Celsius temperature increase. Corrosion in hot environments proceeds faster than in cool environments.

Humidity: Corrosion requires an electrolyte, typically provided by atmospheric moisture. Corrosion rates increase sharply above about 60% relative humidity. Condensation causes even faster corrosion than humidity.

Pollutants: Sulfur dioxide, hydrogen sulfide, chlorides, and other pollutants accelerate corrosion. Industrial environments near chemical sources, coastal environments with salt, and urban environments with vehicle emissions are more corrosive.

Stress: Mechanical stress can accelerate corrosion through stress corrosion cracking. Components under tension in corrosive environments may crack unexpectedly. Connector contacts under spring load are vulnerable.

Thermal Expansion

Temperature changes cause dimensional changes in all materials, creating mechanical stress when materials with different expansion coefficients are joined together.

Differential Expansion Mechanisms

Different materials expand at different rates:

Coefficient of thermal expansion (CTE): CTE quantifies dimensional change with temperature. Aluminum has CTE of about 23 ppm/degrees C; copper about 17 ppm/degrees C; FR-4 about 14-16 ppm/degrees C in-plane and 50-70 ppm/degrees C out-of-plane; ceramics typically 6-8 ppm/degrees C; silicon about 3 ppm/degrees C.

Stress generation: When materials with different CTEs are joined (soldered, bonded, fastened), temperature changes generate stress. A ceramic capacitor soldered to an FR-4 board experiences stress as the board expands more than the capacitor.

Cumulative damage: Repeated thermal cycling accumulates fatigue damage. Each cycle adds stress cycles that progress cracks and degradation. The number of cycles to failure depends on temperature range, ramp rate, and material properties.

Strain relief: Some joints incorporate strain relief to accommodate differential expansion. Gull-wing and J-lead component leads provide compliance. Flexible cables between rigidly mounted assemblies accommodate movement.

Solder Joint Effects

Solder joints are particularly vulnerable to thermal expansion stress:

Shear stress: CTE mismatch between component and board creates shear stress in solder joints. Joints at component corners experience the highest stress. Large components generate more stress than small components.

Crack propagation: Repeated stress cycles cause fatigue cracks to initiate and propagate through solder joints. Cracks typically begin at stress concentrators and grow with each thermal cycle.

Resistance increase: As cracks develop, joint resistance increases. Initially small increases may not affect function but degrade EMC margins. Progressive cracking eventually causes intermittent or complete failure.

Lead-free considerations: Lead-free solders are generally more brittle than tin-lead solders, potentially affecting thermal cycling resistance. SAC alloys (tin-silver-copper) have different fatigue characteristics than traditional solders.

Shield and Enclosure Effects

Thermal expansion affects shielding integrity:

Panel buckling: Large flat panels may buckle when constrained at edges and heated. Buckling creates gaps at gasket interfaces, reducing shielding effectiveness. Panel stiffening and thermal relief features prevent buckling.

Gasket compression: Thermal expansion can change gasket compression. Cooling may reduce compression if mating surfaces separate. Heating may over-compress gaskets, causing permanent set. Temperature-compensating designs maintain consistent compression.

Seam gaps: Enclosure seams may open at temperature extremes if thermal expansion is not accommodated. Spring-loaded seam designs maintain contact. Overlapping joints accommodate sliding motion.

Fastener loosening: Thermal cycling can loosen fasteners due to differential expansion between fastener and housing materials. Lock washers, thread-locking compounds, or controlled thermal expansion materials prevent loosening.

Connector Effects

Thermal expansion affects connector performance:

Contact force variation: Connector contact springs change force with temperature. Most spring materials lose force at elevated temperatures. Reduced force allows increased contact resistance or intermittent contact.

Mating interface movement: Mated connectors experience relative motion when cable and equipment temperatures differ. This motion can cause fretting wear at contacts. Gold plating resists fretting damage better than base metals.

Housing stress: Connector housings expand differently than the equipment they mount to. Strain on mounting points can crack housings or stress solder joints. Compliant mounting accommodates differential expansion.

Cable strain: Cables connected to equipment expand and contract with temperature. If restrained, cables exert force on connectors. Strain relief and service loops accommodate cable movement.

Mechanical Stress

Mechanical stress from handling, installation, operation, and environmental loading affects the integrity of EMC-critical structures and connections.

Vibration Fatigue

Repeated vibration causes progressive fatigue damage:

Stress cycling: Vibration creates cyclic stress in structures and connections. The stress amplitude and number of cycles determine fatigue life. Even stress levels below the yield strength cause fatigue failure after sufficient cycles.

Resonance effects: At resonant frequencies, motion amplitude and stress are amplified. Components and structures operating near resonance accumulate fatigue damage faster than at non-resonant frequencies.

Random vibration: Real-world vibration contains energy at many frequencies simultaneously. Random vibration excites multiple resonances at once. Fatigue under random vibration differs from single-frequency fatigue.

EMC-relevant failures: Vibration fatigue can crack solder joints, break wire bonds, fracture PCB traces, and loosen fasteners. Each of these affects EMC performance through changed impedances, degraded grounds, or reduced shielding integrity.

Shock Damage

High-acceleration events cause immediate damage:

Component fracture: Ceramic components (capacitors, crystals, substrates) are brittle and can crack under shock loading. Cracked capacitors may remain functional initially but degrade over time. Cracked crystals cause frequency instability.

Connector damage: Shock can cause connectors to unmate if locking mechanisms are inadequate. Even momentary contact interruption can cause data errors. Remated connectors may have degraded contact quality.

Structural deformation: Severe shock can permanently deform enclosures, chassis, and mounting structures. Deformation may cause misalignment of mating surfaces, degrading shielding. Stressed gaskets may not return to proper compression.

Cumulative damage: Repeated minor shocks accumulate damage that eventually causes failure. Drop testing should consider the number of drops expected in service, not just survival of a single drop.

Static Loading

Constant mechanical loads cause long-term effects:

Creep: Materials under constant stress slowly deform over time. Creep is accelerated at elevated temperatures. Gasket compression set, connector contact relaxation, and cable deformation result from creep.

Stress relaxation: In constrained structures, initial stress gradually reduces over time. Spring contacts lose force; fasteners lose preload; gaskets lose compression. Functions depending on maintained force degrade.

Dead weight effects: Heavy components create constant stress on their mounting. Over time, this stress can cause progressive failure of solder joints or mounting points. Vertical mounting of heavy components may fail sooner than horizontal mounting.

Installation stress: Improperly installed equipment may be under constant stress from misalignment, over-torqued fasteners, or restrained cables. This stress accelerates failures that would not occur with proper installation.

Wear Mechanisms

Moving parts and contact interfaces wear over time:

Fretting: Small relative motion between contacting surfaces (as from vibration or thermal expansion) causes fretting wear. Oxide debris accumulates at the interface. Contact resistance increases progressively.

Abrasive wear: Surfaces sliding against each other wear away material. Connector contacts cycled repeatedly wear through plating. Cable shields rubbing against sharp edges lose strands.

Adhesive wear: Under high contact pressure, surfaces may momentarily bond and then tear apart. This creates wear particles and surface damage. High-force connector contacts may experience adhesive wear.

Erosive wear: Moving fluids (air, cooling liquid) can erode surfaces over time. Dust-laden airflow abrades surfaces in its path. This can affect antenna radomes, cooling fins, and exposed surfaces.

Chemical Degradation

Chemical attack from environmental exposure degrades materials used in EMC-critical components and structures.

Polymer Degradation

Plastics and elastomers degrade through various mechanisms:

Chain scission: Polymer chains break into shorter fragments, reducing molecular weight. This causes embrittlement, reduced strength, and changed electrical properties. Chain scission is accelerated by heat, UV radiation, and chemical attack.

Cross-linking: Polymer chains form additional bonds, making the material stiffer and more brittle. Excessive cross-linking degrades elastomer flexibility. Gaskets and seals lose their sealing ability.

Oxidation: Oxygen attacks polymer chains, causing degradation. Antioxidants in polymers provide initial protection but are consumed over time. Once antioxidants are depleted, degradation accelerates.

Hydrolysis: Some polymers react with water, breaking chemical bonds. Polyester and nylon are susceptible to hydrolysis. High humidity and temperature accelerate hydrolytic degradation.

Environmental Chemical Attack

Chemicals in the operating environment cause specific degradation:

Ozone: Ozone attacks rubber and some plastics, causing cracking particularly under stress. Urban environments and areas near electrical equipment (which generates ozone) are ozone-rich. Gaskets and cable jackets are vulnerable.

Solvents: Organic solvents dissolve or swell many polymers. Fuel vapors, cleaning solvents, and industrial chemicals can damage plastics. Swelling changes dimensions; dissolution destroys parts.

Acids and bases: Strong acids and bases attack many materials. Battery acid, cleaning chemicals, and some industrial atmospheres are corrosive. Exposed metals and many plastics are affected.

Industrial atmospheres: Industrial environments may contain specific aggressive chemicals. Chemical plants, metal finishing facilities, and pulp mills have characteristic pollutants. Equipment design must consider the specific chemicals present.

Outgassing and Contamination

Materials can release chemicals that contaminate other parts:

Volatile release: Plastics, adhesives, and other materials release volatile compounds, particularly when heated. These compounds can condense on cooler surfaces, creating contamination.

Plasticizer migration: Flexible plastics contain plasticizers that can migrate out over time. Migration causes embrittlement of the source material and contamination of adjacent surfaces. Plasticizers on contact surfaces increase contact resistance.

Silicone contamination: Silicone compounds (from greases, sealants, and some molded parts) can migrate and contaminate contact surfaces. Silicone contamination is difficult to remove and causes high contact resistance.

Optical contamination: Outgassed materials can fog optical surfaces (displays, sensors, lenses). For equipment with both optical and EMC requirements, material selection must consider both effects.

Chemical Compatibility

Material selection must consider chemical compatibility:

Environmental compatibility: Materials must withstand the chemicals in their operating environment. Compatibility charts guide material selection for specific chemical exposures.

Material-to-material compatibility: Adjacent materials must be compatible with each other. Plasticizer migration, galvanic corrosion, and direct chemical reaction can occur between incompatible materials.

Processing chemical compatibility: Materials must withstand manufacturing and service chemicals. Cleaning solvents, flux residues, and conformal coatings must be compatible with all materials present.

Time and temperature effects: Chemical compatibility may change with time and temperature. Materials compatible at room temperature may react at elevated temperatures. Long-term contact may cause effects not seen in short-term testing.

UV Degradation

Ultraviolet radiation causes photochemical degradation of many materials used in electronic equipment.

Photochemical Mechanisms

UV radiation initiates chemical reactions:

Bond breaking: UV photons have enough energy to break chemical bonds in polymers and other materials. This initiates chain reactions that progressively degrade the material.

Free radical formation: UV absorption creates reactive free radicals that attack polymer chains. Oxidation proceeds through free radical mechanisms. Stabilizers scavenge radicals to slow degradation.

Color change: Photodegradation often causes color change before significant mechanical degradation. Yellowing of white plastics indicates UV damage. Color change may affect optical properties before structural failure.

Surface effects: UV penetration is limited, so degradation is concentrated at surfaces. Surface crazing, chalking, and embrittlement occur while bulk material remains sound. Surface degradation can affect electrical properties.

Material Vulnerability

Different materials have different UV sensitivity:

Unstabilized polymers: Many common plastics degrade rapidly under UV exposure. Polyethylene, polypropylene, and ABS are particularly sensitive. Outdoor exposure without protection causes rapid degradation.

UV-stabilized materials: UV stabilizers absorb UV radiation or scavenge free radicals, greatly extending UV life. Stabilized versions of common plastics are available for outdoor use. Stabilizer effectiveness eventually diminishes.

Inherently stable materials: Some materials are inherently resistant to UV. Fluoropolymers (PTFE), silicones, and some acrylics have good UV stability. These materials are preferred for long-term outdoor exposure.

Paints and coatings: Coatings protect underlying materials from UV exposure. Paint pigments absorb UV radiation. Coating degradation (chalking, cracking) may expose substrate to UV.

EMC Effects of UV Degradation

UV degradation affects specific EMC-relevant components:

Cable jackets: UV-degraded cable jackets crack, exposing insulation and shields. Water and contaminants enter through cracks. Shield corrosion and changed impedance result.

Enclosure materials: Plastic enclosures exposed to sunlight may crack or become brittle. Cracks compromise sealing and potentially shielding. Embrittlement leads to impact failures.

Gaskets and seals: Elastomer gaskets exposed to UV become stiff and may crack. Seal function degrades, allowing moisture and contamination entry. Shielding gaskets may lose contact pressure.

Conformal coatings: Some conformal coatings are UV-sensitive. Degradation may change electrical properties or cause delamination. UV-stable coatings are available for exposed applications.

UV Protection Strategies

Several approaches protect against UV degradation:

Material selection: Choosing UV-stable or UV-stabilized materials for exposed components provides inherent protection. The additional cost is offset by extended service life.

Protective coatings: UV-absorbing coatings protect underlying materials. Coatings must be maintained; damaged coatings expose the substrate. Multi-layer coating systems may provide both UV protection and other functions.

Physical shielding: Shading, covers, and enclosures block UV exposure. This approach is effective but may affect other functions (ventilation, access, visibility).

Design for replacement: When UV exposure is unavoidable, designing for easy replacement of degraded components may be practical. Service intervals can be established based on expected degradation rates.

Biological Effects

Living organisms interact with electronic equipment in various ways that can affect EMC performance.

Fungal Growth

Fungi can grow on electronic equipment under favorable conditions:

Growth requirements: Fungi require moisture (typically above 60-70% RH), moderate temperature, and a food source. Many materials used in electronics (paper, cotton, some plastics) can serve as food sources. Contamination and dust provide nutrients.

Electrical effects: Fungal growth can create conductive paths between circuit traces. Metabolic products may be corrosive or conductive. Physical growth can displace components or obstruct openings.

EMC impact: Conductive fungal growth can short filter elements, create leakage paths that bypass isolation, and change impedances. Growth on connector contacts increases contact resistance.

Prevention: Controlling humidity prevents fungal growth. Materials treated with fungicides resist growth. Conformal coatings protect vulnerable surfaces. Proper enclosure sealing limits moisture access.

Insect and Rodent Damage

Animals can damage electronic equipment:

Wire gnawing: Rodents gnaw on wires, damaging insulation and potentially conductors. Damaged cables lose shielding effectiveness and may short-circuit. Fire hazard exists from damaged wiring.

Nesting: Insects and rodents may build nests in equipment enclosures. Nest materials obstruct ventilation and create fire hazards. Insects may be attracted to warm electronics.

Contamination: Animal droppings and debris contaminate surfaces, potentially creating conductive paths or damaging materials through chemical action.

Prevention: Proper enclosure sealing prevents entry. Wire and cable protection (conduit, armored cable) resists gnawing. Pest control measures address infestations. Design for environments where animals are present requires specific protective measures.

Marine Biological Fouling

Marine organisms attach to submerged equipment:

Attachment and growth: Barnacles, algae, and other organisms attach to underwater surfaces. Growth covers surfaces, adds weight, and changes flow patterns around equipment.

Effects on antennas: Biological fouling on underwater antennas changes electrical properties. Impedance mismatch increases. Efficiency decreases. Fouling must be removed periodically.

Cable effects: Fouling on cables adds weight and drag. Movement in currents causes fatigue. Attachment points may concentrate stress.

Prevention: Anti-fouling coatings discourage biological attachment. Copper-based materials have natural anti-fouling properties. Regular cleaning removes accumulated growth.

Biodegradation

Some materials are subject to biological degradation:

Cellulose materials: Paper, cotton, and wood-based materials are food for various organisms. Insulation papers, labels, and packaging materials are vulnerable.

Natural rubbers: Natural rubber is susceptible to fungal attack. Synthetic rubbers generally have better biological resistance.

Biodegradable plastics: Increasing use of biodegradable plastics for environmental reasons introduces materials that may degrade in service. Application selection must consider intended versus premature degradation.

Resistance testing: Standards exist for testing biological resistance of materials. Equipment for humid tropical environments should use tested materials.

Pressure Effects

Atmospheric pressure variations affect electronics in various ways, with implications for EMC performance.

Reduced Pressure Effects

Low pressure at altitude affects electronic behavior:

Dielectric strength reduction: Air's dielectric strength decreases approximately linearly with pressure. Corona discharge and arcing occur at lower voltages. Creepage and clearance distances adequate at sea level may be inadequate at altitude.

EMI from discharge: Corona discharge produces broadband electromagnetic noise. Equipment that operates silently at sea level may generate significant EMI at altitude due to corona on high-voltage circuits.

Cooling changes: Reduced air density decreases convective cooling effectiveness. Equipment runs hotter at altitude. Temperature-related EMC effects combine with altitude effects.

Pressure cycling: Equipment that moves between altitudes experiences pressure cycling. This cycles stress on sealed enclosures and can pump air through imperfect seals.

Pressurized Enclosures

Some equipment uses pressurized enclosures:

Pressure maintenance: Sealed enclosures maintain internal pressure at altitude, preserving sea-level electrical properties. Seals must withstand pressure differential. Pressure relief may be needed for extreme altitude excursions.

Pressurization methods: Enclosures may be sealed at sea level, pressurized with dry gas, or connected to pressurized aircraft cabin air. Each method has implications for moisture control and seal design.

EMC considerations: Sealed enclosures must still meet EMC requirements. Cable penetrations must maintain both pressure integrity and EMC integrity. Pressure-rated connectors and feed-throughs are available.

Structural requirements: Enclosures must withstand pressure loads without deformation that would affect EMC performance. Gasket compression must be maintained despite pressure loading.

Pressure-Equalized Designs

Pressure equalization allows internal pressure to follow external pressure:

Breather devices: Controlled openings allow pressure equalization while blocking water and contaminants. Gore-Tex and similar materials pass gas but block liquid water.

Desiccant systems: Breathers may include desiccant to dry incoming air. Desiccant absorbs moisture as air enters during cooling. Desiccant capacity limits moisture protection; replacement or regeneration may be needed.

EMC implications: Breathing openings are potential EMC apertures. They must be designed to limit EMC coupling while allowing pressure equalization. Metalized filters can provide both functions.

Contamination risk: Breathing allows external contaminants to enter. Filters block particles but gases pass through. Chemical contamination risk must be assessed for the operating environment.

Rapid Pressure Changes

Rapid pressure changes create additional stress:

Explosive decompression: Rapid loss of pressure (as in aircraft cabin breach) creates large pressure differentials across sealed enclosures. Enclosures must either withstand this differential or provide rapid pressure equalization.

Outgassing: Rapid pressure reduction causes dissolved gases to come out of solution. Materials may outgas, creating contamination. Vacuum-compatible materials minimize this effect.

Structural stress: Pressure differentials stress enclosure structures. Fatigue from repeated pressurization contributes to long-term degradation. Design must consider cumulative pressure cycles.

Component effects: Some components are sensitive to rapid pressure changes. Electrolytic capacitors with vented seals may leak electrolyte. Crystals may shift frequency temporarily.

Aging Acceleration

Environmental factors accelerate the intrinsic aging of materials and components, reducing service life.

Temperature Effects on Aging

Temperature is the primary accelerator of aging:

Arrhenius relationship: Many degradation reactions follow the Arrhenius equation, doubling in rate for every 10-15 degrees Celsius temperature increase. This relationship enables accelerated aging testing and life prediction.

Capacitor aging: Electrolytic capacitor wear-out is temperature-dependent. Every 10 degrees Celsius temperature reduction approximately doubles capacitor life. Derating for temperature extends life significantly.

Insulation aging: Electrical insulation ages faster at elevated temperature. Thermal class ratings (105, 130, 155 degrees C) define maximum continuous operating temperatures for specified life.

Semiconductor aging: Some semiconductor failure mechanisms are temperature-accelerated. Hot carrier injection, electromigration, and time-dependent dielectric breakdown all depend on temperature.

Combined Stress Acceleration

Multiple environmental factors combine to accelerate aging:

Temperature-humidity: Moisture and temperature together accelerate corrosion and polymer degradation more than either factor alone. The combination is particularly damaging.

Temperature-voltage: High temperature and high voltage together accelerate insulation degradation. Derating for both extends life.

Cycling fatigue: Repeated temperature and humidity cycling causes fatigue and stress accumulation. The number of cycles and the stress amplitude both affect life.

Cumulative damage: Different stresses contribute damage that accumulates over time. Linear damage accumulation models (Miner's rule) approximate the combined effect of different stress exposures.

Aging Effects on EMC

Aging degrades specific EMC characteristics:

Filter degradation: Capacitor value decrease and ESR increase degrade filter performance over time. The effect is greater at higher frequencies where capacitor properties are more critical.

Shield degradation: Gasket compression set, contact oxidation, and material degradation reduce shielding effectiveness. The effect may be gradual or may appear suddenly when a threshold is reached.

Grounding degradation: Connection loosening, corrosion, and mechanical wear increase ground impedance. High-frequency grounding is affected first as connection inductance becomes significant.

Margin erosion: Each degradation mechanism reduces margin from specification limits. Products with initially adequate margins may fail EMC requirements after aging.

Life Prediction and Management

Understanding aging enables life prediction and management:

Accelerated testing: Elevated stress testing accelerates aging, enabling life prediction in practical test times. Acceleration factors relate test results to expected field life.

Condition monitoring: Monitoring key parameters during service reveals aging trends. Intervention can occur before failure. EMC-relevant parameters can be included in monitoring programs.

Maintenance planning: Predicted aging informs maintenance intervals. Components with predictable wear-out (capacitors, gaskets) can be scheduled for replacement.

End-of-life determination: When EMC margins approach limits, products may need refurbishment or replacement. Life prediction enables planning for these actions.

Conclusion

Environmental factors continuously interact with electronic equipment, progressively affecting the materials, components, and structures that determine EMC performance. Moisture alters dielectric properties and creates conductive paths. Corrosion degrades metal surfaces essential for shielding and grounding. Thermal expansion creates mechanical stress that fatigues connections and distorts structures. Chemical exposure attacks polymers and metals. UV radiation degrades plastics and coatings. Biological organisms contaminate and consume materials. Pressure variations affect dielectric strength and create mechanical stress.

These environmental effects operate through specific physical and chemical mechanisms that can be understood, predicted, and mitigated. Understanding the mechanisms enables selection of appropriate materials, design of protective features, and prediction of service life. Testing under realistic environmental conditions reveals vulnerabilities before products reach the field.

Effective design for environmental EMC performance requires attention to material selection, protective treatments, structural design, and maintenance provisions. Products that maintain EMC compliance throughout their service life under anticipated environmental conditions represent successful integration of EMC engineering with environmental design. This integrated approach, recognizing that EMC performance is not static but evolves with environmental exposure, is essential for reliable electronic systems in demanding applications.