Electronics Guide

Component Value Drift

Electronic components change their electrical characteristics with temperature, directly affecting EMC performance:

Capacitors: Ceramic capacitors, widely used in EMI filters, exhibit significant capacitance variation with temperature. Class II ceramics (X7R, X5R) can lose 15-30% of their capacitance at temperature extremes, reducing filter effectiveness. Class I ceramics (C0G/NP0) offer better stability but are limited in value and size. Electrolytic capacitors show even greater variation, with ESR increasing dramatically at low temperatures, which can actually improve high-frequency filtering but may cause stability issues in power supplies.

Inductors: Ferrite core inductors experience permeability changes with temperature. Many ferrites show a peak in permeability near the Curie temperature, which can cause filters to behave differently than expected at elevated temperatures. Powdered iron cores offer better temperature stability but may have lower initial permeability.

Resistors: Temperature coefficient of resistance (TCR) affects the impedance matching and damping characteristics of EMC circuits. While precision resistors minimize this effect, high-value resistors used in some filter designs may drift enough to affect performance.

Semiconductor Behavior Changes

Active electronic circuits exhibit temperature-dependent EMC characteristics through several mechanisms:

Switching speed variation: CMOS logic speeds up at low temperatures due to increased carrier mobility, generating faster edge rates with higher harmonic content. At high temperatures, reduced speeds may affect system timing but generally reduce emissions. This variation can shift the frequency content of emissions significantly.

Threshold voltage shifts: MOSFETs and digital logic exhibit threshold voltage changes with temperature, affecting switching points and potentially the waveform symmetry of clock signals. Asymmetric waveforms generate different harmonic spectra than symmetric ones.

Leakage current increases: Semiconductor leakage doubles approximately every 10 degrees Celsius of temperature increase. At elevated temperatures, increased leakage can affect the effectiveness of ESD protection circuits and change the DC bias conditions of some filter configurations.

Thermal-EMC Test Methods

Conducting EMC tests at temperature requires specialized facilities and procedures:

Temperature chambers with RF access: Purpose-built chambers combine temperature control with RF shielding and provisions for antenna placement. Cable penetrations must maintain shielding integrity while allowing thermal isolation. Some facilities use waveguide-beyond-cutoff techniques for cable entry to maintain shielding at high frequencies.

Equipment under test stabilization: Sufficient time must be allowed for the equipment under test (EUT) to reach thermal equilibrium. Internal temperature gradients can cause different subsystems to operate at different temperatures during transitions, potentially revealing transient EMC issues not present at steady state.

Measurement equipment considerations: Test receivers, spectrum analyzers, and antennas used for EMC measurements typically require operation at room temperature. Long cables between the temperature chamber and measurement equipment introduce losses that must be characterized and compensated, and cable electrical characteristics may change with temperature where they pass through the chamber wall.

Test sequence planning: Temperature transitions stress the EUT in ways that can affect subsequent measurements. Tests should consider whether to measure at temperature extremes only, during transitions, or both. Thermal cycling during EMC testing can reveal intermittent failures that occur only during temperature changes.

Hot and Cold Temperature Extremes

Different failure modes emerge at temperature extremes:

Cold temperature effects: At low temperatures, increased component brittleness can lead to solder joint cracking under mechanical stress. Lubricants in connectors may stiffen, increasing insertion and withdrawal forces and potentially affecting contact resistance. Battery capacity decreases, potentially affecting equipment operation and any voltage-dependent EMC behavior. Some materials contract at rates that differ from PCB substrates, stressing component terminations.

Hot temperature effects: At elevated temperatures, thermal expansion can cause mechanical stress on shielding gaskets, potentially opening gaps that compromise shielding effectiveness. Plastic enclosures may soften and deform. Conformal coatings and adhesives may soften, potentially allowing moisture ingress. Thermal interface materials may pump out from between components and heatsinks, affecting both thermal and electrical contact.

Moisture Absorption in Materials

Many materials used in electronic assemblies absorb moisture, changing their electrical properties:

PCB substrates: Standard FR-4 absorbs moisture that increases the dielectric constant and dissipation factor, affecting the characteristic impedance of transmission lines and the behavior of distributed filters. High-frequency circuits are particularly sensitive to these changes. For critical applications, low-moisture-absorption substrates like Rogers materials may be specified.

Conformal coatings: Some conformal coatings absorb moisture over time, which can then penetrate to component surfaces and PCB conductors. While conformal coatings provide protection, they are not impermeable barriers. Moisture absorption can also change the dielectric properties of the coating itself.

Encapsulants and potting compounds: Materials used to encapsulate sensitive circuits or pot entire assemblies may absorb moisture at rates depending on their chemistry. Silicones generally absorb less moisture than epoxies, but provide less mechanical protection. Moisture at the interface between potting compound and component surfaces can create unexpected current paths.

Surface Conductivity Changes

Moisture on surfaces creates conductive paths that can affect EMC performance:

Surface leakage currents: Thin moisture films on PCB surfaces create leakage paths between conductors. At high humidity levels, these paths can provide low-impedance connections that bypass intended isolation or filtering. Ionic contamination from flux residues, handling, or the environment dramatically increases moisture conductivity.

Electrochemical migration: Under bias, in the presence of moisture and ionic contamination, metal ions can migrate between conductors, eventually forming conductive dendrites that short adjacent traces. This process is accelerated by high humidity and can create intermittent failures as dendrites form and dissolve.

Connector degradation: Moisture promotes oxidation and corrosion of connector contacts, increasing contact resistance. The resulting impedance variations affect both signal integrity and shielding effectiveness. Gold-plated contacts resist this degradation but may still suffer from fretting corrosion that exposes base metals.

Humidity Testing Procedures

Humidity testing for EMC follows established standards with specific considerations:

Steady-state humidity testing: The EUT is exposed to elevated humidity (typically 85% to 95% RH) at elevated temperature for extended periods, often with EMC measurements taken periodically. This approach evaluates the cumulative effect of moisture absorption.

Cyclic humidity testing: Temperature and humidity are varied cyclically, causing condensation to form on and within the EUT. Condensation accelerates moisture ingress and reveals failures related to water pooling in enclosures. EMC testing during or immediately after cycling captures worst-case moisture conditions.

Dew point control: Rather than controlling temperature and humidity independently, some test specifications define conditions in terms of dew point. When EUT temperature drops below the dew point, condensation forms. This approach more realistically simulates field conditions where equipment moves between environments.

Combined temperature-humidity profiles: Standards like MIL-STD-810 define specific temperature-humidity profiles that simulate real-world environmental transitions. Aggravated profiles accelerate degradation while maintaining failure mode relevance.

Condensation Effects

Condensation represents a particularly severe moisture condition:

When warm, humid air contacts a cooler surface, water condenses on that surface. In electronic equipment, condensation can form on PCBs, connectors, shielding surfaces, and within cable assemblies. The liquid water creates much more significant electrical effects than humidity alone.

Condensation on shielding surfaces can create conductive paths across gasket interfaces or through ventilation openings. Water droplets bridging connector pins can cause shorts or signal degradation. Condensation within cable shields can affect the cable's transfer impedance.

Equipment that experiences condensation should be designed with drainage paths that prevent water accumulation. Conformal coatings protect against condensation on PCB surfaces but must be applied completely to be effective. Heated enclosures or desiccant systems may be necessary in severe environments.

Vibration-Induced Failures

Vibration causes several types of EMC-relevant failures:

Connector contact intermittence: Vibration causes relative motion between mating connector contacts, momentarily breaking the electrical connection. In shielded connectors, this creates gaps in the shield continuity. In filtered connectors, intermittent contact can bypass filter elements. The resulting transient impedance variations can cause both emissions spikes and susceptibility failures.

Solder joint fatigue: Repeated mechanical stress from vibration can fatigue solder joints, eventually causing cracks and intermittent opens. Joints connecting heavy components like transformers, large capacitors, and heatsink-mounted semiconductors are particularly vulnerable. Surface mount components experience different stress patterns than through-hole components.

Shield deformation: Sheet metal enclosures and shields can vibrate at their resonant frequencies, causing temporary gaps at seams and gasket interfaces. At resonance, even small driving forces can cause significant deflection, compromising shielding effectiveness.

Cable abrasion: Cables that vibrate against sharp edges or rough surfaces can experience insulation damage over time. In shielded cables, this can expose the shield or inner conductors, creating opportunities for both emissions and susceptibility problems.

Resonance Identification

Mechanical resonances amplify vibration effects and require careful characterization:

Every mechanical structure has resonant frequencies determined by its mass distribution and stiffness. At resonance, motion amplification factors of 10 to 100 are common, meaning small driving forces create large deflections. Identifying and either avoiding or damping resonances is essential for vibration reliability.

Component resonances: Individual components can resonate, with their leads acting as springs and their bodies as masses. Crystal oscillators are particularly sensitive to vibration, which can modulate the crystal frequency and cause jitter in timing signals. Large electrolytic capacitors mounted vertically have resonant frequencies in the tens of hertz range.

PCB resonances: Printed circuit boards mounted at their edges vibrate as plates, with resonant frequencies depending on dimensions, thickness, material properties, and mounting. Populated boards have different resonances than bare boards due to the added mass of components.

Enclosure resonances: Equipment enclosures have panel resonances that can cause temporary shielding gaps and generate acoustic noise. Large, thin panels resonate at lower frequencies than small, thick panels.

Combined Vibration-EMC Test Methods

Testing EMC under vibration requires specialized equipment and procedures:

Vibration table mounting: The EUT must be mounted to the vibration table (shaker) using fixturing that transmits vibration to the equipment as it would be transmitted in actual installation. Fixtures should not introduce resonances of their own in the test frequency range.

EMC measurement during vibration: Continuous EMC monitoring during vibration can capture intermittent failures that might be missed with snapshot measurements. Spectrum analyzers can be configured to hold peak values, revealing brief emissions spikes. Immunity testing can be configured to detect momentary susceptibility failures.

Shielded shaker systems: For radiated emissions and immunity testing, the entire vibration system may need to be shielded. Purpose-built facilities place the shaker inside an anechoic chamber. Alternatively, the EUT can be tested in a shielded enclosure that is itself vibrated, though this adds complexity.

Vibration profiles: Test vibration can be sinusoidal (swept through a frequency range), random (simulating real-world vibration spectra), or a combination. Random vibration tests excite all frequencies simultaneously, potentially revealing resonances and failures more quickly than swept sine tests.

Mechanical Fatigue Effects

Extended vibration causes cumulative fatigue damage:

Mechanical fatigue is a progressive process where repeated stress cycles cause microscopic damage that accumulates until failure occurs. The S-N curve (stress versus number of cycles) characterizes material fatigue behavior. Most materials do not have an endurance limit below which infinite life is possible; all stress levels cause some damage.

Vibration fatigue testing applies extended vibration profiles to accumulate damage equivalent to the expected service life. EMC measurements taken periodically during this testing can reveal gradual degradation before catastrophic failure. This approach is particularly valuable for identifying margin erosion that might cause field failures in products that initially pass EMC testing.

Solder joint fatigue under vibration depends strongly on component type, PCB material, and joint geometry. Modern lead-free solders have different fatigue characteristics than traditional tin-lead solders, and the industry is still developing comprehensive understanding of their behavior.

Corona and Arc Discharge

The dielectric strength of air decreases with decreasing pressure:

At sea level, the breakdown voltage of air is approximately 3 kV/mm for uniform fields. At 10,000 meters altitude (typical commercial aircraft cruising altitude), this decreases to roughly 40% of sea level values. High-voltage circuits that operate reliably at sea level may experience corona discharge or arcing at altitude.

Corona discharge effects: Corona produces broadband RF noise that can affect EMC performance. The noise has a characteristic spectrum with significant energy extending into the hundreds of megahertz range. Corona also generates ozone, which can attack certain materials.

Arcing concerns: If voltages are high enough, reduced pressure can lead to full arc breakdown rather than just corona. Arcing can damage equipment and create severe EMI transients. Equipment intended for high-altitude operation must be designed with increased clearances or pressurized enclosures.

Paschen's law: The breakdown voltage is not a monotonic function of pressure; it reaches a minimum at a certain pressure-gap product. For some configurations, breakdown is more likely at intermediate altitudes than at either sea level or very high altitudes.

Cooling Effectiveness

Reduced air density at altitude affects convective cooling:

Convective heat transfer coefficient decreases approximately proportionally to air density. Equipment designed for adequate cooling at sea level may overheat at altitude. The resulting higher operating temperatures affect EMC performance through the temperature-dependent mechanisms discussed earlier.

Forced air cooling: Fans move the same volume of air at altitude but less mass, reducing cooling effectiveness. Fan curves shift as air density decreases. Equipment may need to be derated for altitude or designed with additional thermal margin.

Component temperature rise: Higher component temperatures at altitude can push semiconductors, capacitors, and other components into temperature ranges where EMC behavior differs from sea-level conditions. This combination of altitude and temperature effects requires testing under realistic combined conditions.

Hermetic sealing: Equipment can be sealed at sea-level pressure to maintain sea-level electrical and thermal conditions at altitude. However, sealed enclosures must withstand the pressure differential, and sealing eliminates the pressure-equalized designs often used for shielding gaskets.

Altitude Testing Facilities

Altitude-EMC testing requires specialized chambers:

Altitude chambers: These vacuum chambers reduce pressure to simulate altitudes up to 30,000 meters or higher. The chamber must be large enough to accommodate the EUT with appropriate spacing for EMC measurements. RF-transparent windows or waveguide penetrations allow antenna placement outside the chamber when needed.

Combined altitude-temperature chambers: Many facilities combine altitude simulation with temperature control, since high-altitude environments are typically cold. This allows testing under realistic combinations of altitude and temperature.

In-chamber EMC testing: For some tests, antennas and measurement equipment can be placed inside the altitude chamber. This simplifies some measurements but requires equipment rated for low-pressure operation. Cable routing and feed-through design must maintain both pressure integrity and EMC measurement accuracy.

Rapid Decompression

Rapid pressure changes create additional stress:

Rapid decompression, as might occur during aircraft emergency descent or spacecraft anomalies, subjects equipment to sudden pressure changes that can cause mechanical damage. Sealed enclosures experience pressure differentials that stress seals and may cause structural deformation. Rapid pressure changes can also cause outgassing from materials, potentially contaminating optical or sensitive surfaces.

Equipment design for rapid decompression environments may include pressure relief valves, reinforced enclosure structures, and materials selected for low outgassing. EMC performance should be verified after simulated rapid decompression to ensure that pressure-induced mechanical stress has not degraded shielding or filtering effectiveness.

Corrosion Mechanisms

Salt accelerates multiple corrosion processes:

Galvanic corrosion: When dissimilar metals are electrically connected in the presence of an electrolyte (salt water), the more anodic metal corrodes preferentially. Many electronic assemblies contain multiple metals (aluminum enclosures, copper traces, tin-lead or tin solder, gold plating, nickel underplate) that can form galvanic couples.

Crevice corrosion: Salt solution trapped in crevices (under gaskets, in connector cavities, in shield seams) creates conditions for concentrated attack. The crevice geometry limits oxygen access, creating differential aeration cells that drive corrosion.

Pitting corrosion: Chloride ions in salt solutions can penetrate passive oxide films on aluminum and stainless steel, initiating pits that propagate into the base metal. Pits in shielding surfaces compromise shielding effectiveness; pits in connector contacts increase contact resistance.

Stress corrosion cracking: Some alloys can crack under the combined action of tensile stress and corrosive environment. Spring contacts in connectors are particularly vulnerable if made from susceptible alloys.

EMC Degradation from Corrosion

Corrosion affects EMC performance through several mechanisms:

Shield degradation: Corrosion of shield surfaces increases surface resistance, reducing shielding effectiveness for magnetic fields. Perforation of thin shields by pitting or crevice corrosion creates apertures that leak RF energy. Corrosion products can insulate overlapping shield sections, breaking electrical continuity.

Connector deterioration: Corrosion of connector contacts increases contact resistance and can cause intermittent connections. In filtered connectors, corrosion can bypass filter elements. Ground connections through corroded surfaces may develop impedance that compromises grounding effectiveness.

Component degradation: Components with exposed metal, particularly those on or near enclosure surfaces, can corrode. Inductor windings, capacitor leads, and exposed solder can all be affected. Corrosion changes component values and can cause opens or shorts.

Salt Fog Test Standards

Salt fog testing follows established standards with EMC considerations:

MIL-STD-810: This military standard includes salt fog procedures for equipment intended for marine or coastal deployment. Test duration and salt concentration are specified based on severity levels.

ASTM B117: This industrial standard defines a neutral salt spray test widely used for comparing corrosion resistance of materials and coatings. While primarily a comparative test, it is often referenced in specifications.

IEC 60068-2-11: This international standard covers salt mist testing for electronic equipment, with test severity selected based on intended environment.

For EMC evaluation, salt fog testing is typically followed by EMC testing rather than conducted simultaneously. The purpose is to evaluate whether salt exposure has degraded EMC performance. Visual inspection and electrical continuity checks before and after exposure can help identify specific degradation mechanisms.

Protective Measures

Protecting equipment from salt-induced EMC degradation requires multiple approaches:

Sealing: Sealed enclosures prevent salt-laden air from reaching internal components. Seals must be effective against both liquid water and humid salt air. Connector seals, cable glands, and pressure equalization devices must also resist salt ingress.

Protective coatings: Conformal coatings on PCBs protect against salt exposure. Enclosure coatings and plating protect external surfaces. Chromate conversion coatings on aluminum improve corrosion resistance, as does anodizing, though care must be taken to maintain electrical conductivity where needed for EMC.

Material selection: Selecting compatible materials minimizes galvanic corrosion potential. Stainless steel fasteners in aluminum enclosures should be electrically isolated or selected for compatibility. Connector plating should be chosen for both corrosion resistance and electrical performance.

Maintenance provisions: Equipment may need provisions for regular washing to remove salt deposits before they cause damage. Drain holes, access panels, and corrosion-resistant fasteners facilitate maintenance. Inspection windows or test points allow monitoring of corrosion-prone areas.

Coefficient of Thermal Expansion Effects

Different materials expand at different rates with temperature:

The coefficient of thermal expansion (CTE) varies widely among materials used in electronics. Aluminum has a CTE of about 23 ppm/C, while FR-4 epoxy-glass laminate has about 14-16 ppm/C in-plane and much higher out-of-plane. Ceramic components may have CTE values below 10 ppm/C, while some plastics exceed 50 ppm/C.

When materials with different CTEs are joined, temperature changes create mechanical stress. A component soldered to a PCB experiences stress as the board expands and contracts with temperature. Repeated thermal cycles accumulate fatigue damage that eventually causes failure.

Solder joint stress: CTE mismatch between components and PCBs stresses solder joints. The stress is highest for large components with stiff leads and for components near PCB edges where board expansion accumulates. Lead-free solders are generally more brittle than tin-lead, making solder joints more vulnerable to thermal cycling.

Connector stress: Connectors joining assemblies with different CTEs experience stress during thermal cycling. Board-mounted connectors must accommodate both board expansion and any differential expansion with mating connectors. Strain relief and compliant mounting can reduce these stresses.

Thermal Fatigue Mechanisms

Thermal cycling induces fatigue through repeated stress cycles:

Low-cycle thermal fatigue: Large temperature excursions (as in some industrial or military applications) can cause significant plastic deformation in each cycle, leading to failure in hundreds to thousands of cycles. Solder joints, particularly in ball grid array (BGA) packages, are vulnerable to this failure mode.

High-cycle thermal fatigue: Smaller temperature variations, repeated many times over product life, cause primarily elastic stress with gradual accumulation of fatigue damage. This is the predominant mechanism in consumer electronics that experience daily on-off cycles.

Creep and stress relaxation: At elevated temperatures, materials can deform slowly under constant stress (creep) or the stress in a constrained structure can decrease (relaxation). These effects interact with cyclic stress to create complex damage accumulation patterns.

Thermal Cycling Test Profiles

Thermal cycling tests use various profiles depending on intended application:

Temperature range: The temperature range should represent the extremes expected in service, often with some margin. Consumer electronics might be cycled from 0 degrees C to 70 degrees C, while military equipment might see -55 degrees C to +125 degrees C.

Ramp rate: Rapid temperature transitions create additional thermal shock stress. Very fast rates (greater than 20 degrees C per minute) are used for accelerated testing but may induce failure modes not seen in service. Slower rates more accurately simulate real-world conditions but require longer test times.

Dwell time: Time at temperature extremes allows the EUT to reach thermal equilibrium. Longer dwells ensure that internal components reach extreme temperatures; shorter dwells may leave internal assemblies at more moderate temperatures than external surfaces.

Number of cycles: The number of cycles needed to simulate product life depends on the application and acceleration model. Consumer electronics might require 500-1000 cycles, while aerospace applications might specify 2000 or more cycles.

Monitoring During Thermal Cycling

Capturing EMC degradation during thermal cycling requires careful monitoring:

Continuous electrical monitoring: Simple electrical tests (continuity, insulation resistance, functional checks) can be monitored continuously during cycling to detect intermittent failures that might recover when cycling stops.

Periodic EMC testing: Full EMC testing at intervals during the thermal cycling program reveals progressive degradation. Testing at the same temperature each time eliminates temperature-dependent variation, isolating the effects of cumulative damage.

Post-cycling characterization: Detailed EMC testing after cycling completion provides final pass/fail determination. Comparison with pre-cycling baseline measurements reveals degradation. Physical inspection and failure analysis of degraded units identifies specific failure mechanisms.

Shock-Induced Failures

Shock causes several types of EMC-relevant failures:

Component fracture: Brittle components like ceramic capacitors and crystals can crack under shock loads. Even if not immediately fatal, cracks can create intermittent failures or paths for moisture ingress. Large, heavy components experience the highest shock forces and are most vulnerable.

Connector unmating: Shock can cause connectors to partially or fully unmate if locking mechanisms are inadequate. Even momentary contact interruption can cause data errors or system resets. Shielded connectors may lose shielding continuity during shock.

Solder joint failure: The combination of high acceleration and component mass creates forces that can crack solder joints. Through-hole joints are generally more shock-resistant than surface mount joints, but both can fail under severe shock.

PCB flexure: Shock can cause PCB bending that damages components mounted on the board. BGA components are particularly vulnerable to board flexure, which can crack solder balls or cause pad cratering.

Shock Test Parameters

Shock tests are characterized by their pulse shape, amplitude, and duration:

Classical shock pulses: Half-sine, sawtooth, and trapezoidal pulses are commonly specified. Each shape creates different stress patterns. Half-sine pulses are most common for general qualification. Sawtooth (terminal peak) pulses simulate more severe deceleration impacts.

Shock amplitude: Peak acceleration is specified in g (multiples of gravitational acceleration). Consumer electronics might be tested to 30-50g, while military and aerospace applications often require 75-100g or more. Specialized applications (munitions, spacecraft launch) may see peaks of several hundred g.

Pulse duration: Longer pulses transfer more energy to the EUT and are generally more damaging. Duration is typically specified as 3-18 ms for classical pulses. The natural period of equipment structures relative to pulse duration determines the response.

Shock response spectrum: Complex, real-world shocks can be characterized by their shock response spectrum (SRS), which shows the peak response of single-degree-of-freedom oscillators across a range of natural frequencies. SRS testing uses synthesized shocks matching a specified spectrum.

Drop Testing

Drop tests simulate accidental drops during handling, installation, or use:

Drop testing involves releasing the EUT from a specified height onto a specified surface. The height determines the impact velocity, while the surface hardness affects the shock pulse duration and peak acceleration. Harder surfaces produce shorter, higher-amplitude pulses.

Free fall drops: The EUT is dropped without guidance, impacting in an orientation determined by its release. This tests multiple orientations and highlights the most vulnerable impact configurations.

Guided drops: The EUT is guided to impact in a specific orientation, allowing testing of edges, corners, and faces individually. This provides more controlled, reproducible results but may not capture worst-case orientations.

Tumble testing: The EUT is repeatedly tumbled down an incline, experiencing multiple impacts at random orientations. This simulates rough handling during shipping.

EMC Testing After Shock

EMC performance should be verified after shock exposure:

Visual inspection for visible damage precedes electrical testing. Connectors should be checked for proper seating. Any loosened fasteners or displaced components should be noted, as they may indicate marginal design even if function is maintained.

Functional testing verifies basic operation. Full EMC testing then characterizes any degradation in electromagnetic performance. Comparison with pre-shock baseline measurements reveals shock-induced changes.

Some specifications require EMC testing during shock, particularly for military applications where equipment must function through weapons fire or nuclear weapon effects. This requires sophisticated test setups that can measure EMC performance during the extremely short shock duration.

Electromagnetic Effects

Solar radiation creates electromagnetic interference through several mechanisms:

Solar radio emissions: The sun is a strong source of radio frequency energy, with emissions varying throughout the solar cycle. Solar radio bursts during flares can be intense enough to affect some receivers. Satellite communications and GPS systems can experience degradation during major solar events.

Ionospheric effects: Solar radiation ionizes the upper atmosphere, creating the ionosphere that affects radio wave propagation. Solar events cause ionospheric disturbances that can enhance or disrupt radio communications. Equipment designed for specific frequency bands may experience unexpected propagation during solar events.

Space radiation effects: In addition to electromagnetic radiation, the sun emits charged particles that can upset electronic circuits directly. Single-event effects in semiconductors can cause bit flips, latch-up, or permanent damage. Shielding against particle radiation requires mass, not just electromagnetic shielding.

Thermal Loading

Solar heating creates significant thermal stress:

Direct solar radiation deposits approximately 1000 W/m squared at Earth's surface on a clear day. Dark-colored equipment enclosures can reach temperatures 30-40 degrees C above ambient in direct sun. This additional thermal load must be considered in equipment thermal design.

The combination of high temperature and temperature cycling (from day-night cycles) creates stress conditions discussed in earlier sections. EMC performance at solar-heated temperatures may differ from laboratory temperature testing if the test chamber does not reach equivalent temperatures.

Solar heating is not uniform across equipment surfaces, creating temperature gradients that cause differential expansion stress. The sun-facing surface is hottest, while shaded surfaces may be near ambient or cooler if radiating to a cold sky.

UV Degradation

Ultraviolet radiation causes material degradation:

Polymer degradation: UV radiation breaks polymer chains in plastics, causing embrittlement, cracking, and chalking. Cable jackets, enclosure materials, and conformal coatings can all be affected. Degraded plastic enclosures may crack, compromising sealing and shielding.

Coating degradation: Some protective coatings are sensitive to UV exposure. Degraded coatings may crack, peel, or lose effectiveness. EMC-relevant coatings like conductive paints should be specified for UV resistance if outdoor exposure is anticipated.

Optical component effects: Optical devices (displays, sensors, fiber optic cables) can experience degradation from UV exposure. Changes in optical properties may affect equipment function even if EMC performance is maintained.

Solar Simulation Testing

Solar simulation tests replicate the effects of sun exposure:

Solar simulation lamps: Xenon arc lamps with appropriate filters closely match the solar spectrum from UV through infrared. The EUT is exposed to specified intensity levels for defined durations. Chamber temperature may be controlled independently or allowed to rise from solar heating.

Accelerated weathering: Higher-than-solar UV intensities accelerate degradation for long-term testing in shorter times. Care must be taken that acceleration does not change the degradation mechanism.

Combined exposure: Solar simulation may be combined with humidity or temperature cycling to simulate realistic outdoor environments. The combination of UV, moisture, and temperature changes accelerates many degradation mechanisms.

Dust Ingress Mechanisms

Dust enters equipment through various paths:

Ventilation openings: Cooling vents necessary for thermal management provide paths for dust entry. Larger openings admit more dust but also provide better airflow. Dust accumulates on intake surfaces and internal components downstream.

Seals and gaskets: Imperfect seals allow fine dust to enter over time. Pressure cycling from thermal changes or altitude variation pumps air (and entrained particles) through small gaps. Aging seals become less effective.

Cable entry points: Even sealed cable entries may admit dust along cables or through imperfect gland sealing. Dust accumulation in connector cavities can affect contact resistance.

EMC Effects of Dust

Dust contamination affects EMC through multiple mechanisms:

Conductive dust: Metal particles, carbon, and some mineral dusts are electrically conductive. Accumulation of conductive dust can bridge circuit traces, contaminate connector contacts, or short across filter elements. Even moderately conductive dust can affect high-impedance circuits.

Hygroscopic contamination: Some dusts absorb moisture, becoming conductive when humidity increases. Salt-laden dust from marine environments is particularly problematic. The combination of dust and humidity can create intermittent failures that depend on environmental conditions.

Thermal effects: Dust accumulation on heatsinks and in cooling passages reduces cooling effectiveness, raising component temperatures. The resulting higher operating temperatures affect EMC performance through temperature-dependent mechanisms.

Mechanical interference: Dust in moving parts (fans, relays, switches) can cause failures that indirectly affect EMC. Fan failure leads to overheating; contact contamination causes intermittent connections.

Dust Testing Methods

Dust testing evaluates susceptibility to particulate contamination:

Blowing dust: Fine dust (typically silica or talc) is blown against the EUT in a test chamber. Wind velocity and dust concentration are controlled. The EUT may be operated during exposure to detect immediate failures.

Settling dust: Dust is allowed to settle on the EUT over extended periods, simulating indoor accumulation. This test is relevant for equipment in manufacturing environments or uncontrolled spaces.

Sand and dust combination: For extreme environments (desert, construction sites), testing may include larger sand particles that can abrade surfaces in addition to fine dust that penetrates enclosures.

Post-exposure evaluation: EMC testing after dust exposure determines whether contamination has affected performance. Internal inspection reveals dust accumulation patterns and potential problem areas. Comparison with pre-exposure measurements quantifies degradation.

Dust Protection Strategies

Protecting equipment from dust requires attention to design details:

Filtration: Intake filters capture particles before they reach internal components. Filter effectiveness must be balanced against airflow restriction. Filters require regular cleaning or replacement to maintain effectiveness.

Sealed enclosures: Equipment can be sealed to prevent dust entry entirely. This requires alternative cooling strategies (conduction, radiation, or sealed internal circulation). IP ratings characterize dust protection levels.

Positive pressure: Maintaining slight positive pressure inside enclosures prevents dust from being drawn in through imperfect seals. This requires a supply of clean air, which may need its own filtration.

Smooth surfaces: Dust accumulates more readily on textured surfaces and in corners. Smooth internal surfaces with gentle contours facilitate cleaning and reduce accumulation.

Test Sequence Optimization

The order of tests affects both efficiency and validity:

Some environmental exposures cause permanent damage that affects subsequent tests. Generally, non-destructive tests should precede potentially destructive ones. EMC baseline testing should occur before environmental stress. Progressive stress levels (starting mild, increasing severity) reveal the onset of degradation.

Some combinations are synergistic and should be tested together. Temperature and humidity interact to produce condensation. Vibration failures may only appear at temperature extremes. These interactions argue for simultaneous application of related stresses.

Practical considerations often dictate sequence. If facilities for combined testing are not available, sequential testing is the alternative. EMC testing after each environmental exposure can reveal the effects of that specific stress, though this approach may miss combined-stress failure modes.

Facility Requirements

Combined environmental EMC testing has demanding facility requirements:

True combined testing requires facilities that can apply environmental stress and perform EMC measurements simultaneously. Few facilities have fully integrated capabilities. More commonly, modular approaches are used, with environmental chambers configured for EMC access or EMC test chambers equipped with environmental control.

Compromises may be necessary. Full anechoic chambers are impractical for many environmental tests; shielded rooms or enclosures may be adequate for conducted emissions but limit radiated testing. Temperature and vibration are more commonly combined than humidity and vibration, due to practical difficulties.

Test equipment must function in the environmental conditions or be isolated from them. Instrumentation operated at temperature extremes must be characterized for accuracy under those conditions. Remote equipment separated by long cables introduces losses and potential pickup that must be managed.

Data Correlation

Meaningful results require careful data management:

Baseline measurements before environmental exposure provide reference for detecting changes. Measurements during exposure capture dynamic effects. Measurements after exposure reveal permanent degradation. All measurements should be made under consistent conditions (temperature, humidity, operating mode) for valid comparison.

Statistical treatment may be needed when measurement variation approaches the degradation being detected. Multiple measurements at each condition improve confidence. Trend analysis across progressive stress levels reveals the onset of degradation before complete failure.

Documentation should include not just measurement results but also environmental conditions, equipment configuration, and any anomalies observed. This information is essential for interpreting results and for failure analysis if problems are found.

Standards and Specifications

Multiple standards address combined environmental EMC testing:

MIL-STD-810: This military standard provides comprehensive environmental test methods including temperature, humidity, altitude, vibration, shock, and many other conditions. While primarily an environmental standard, it recognizes the need to measure performance parameters including EMC during or after environmental exposure.

RTCA DO-160: This aerospace standard for airborne equipment includes environmental and EMC requirements with explicit recognition that tests should be performed in combination where relevant.

IEC 60068: This international standard series covers environmental testing. While EMC is addressed in separate IEC standards, the environmental methods can be adapted for combined testing.

Automotive standards: Vehicle manufacturers have developed combined environmental EMC requirements reflecting the harsh under-hood environment. Temperature, vibration, and EMC are commonly tested together.