Electronics Guide

Conductor Materials

The choice of bonding strap material affects conductivity, corrosion resistance, flexibility, and compatibility with mating surfaces:

Copper: Offers excellent electrical conductivity (second only to silver among practical materials) and good corrosion resistance in many environments. Tinned copper provides improved corrosion protection and better compatibility with aluminum surfaces. Bare copper is suitable for bonding to copper or brass structures but requires protection when used with dissimilar metals.

Aluminum: Provides good conductivity at lower weight than copper, making it attractive for aerospace applications. Aluminum straps work well when bonding aluminum structures but require careful attention to corrosion control. The native oxide layer on aluminum, while providing corrosion protection, must be penetrated during installation to achieve low-resistance contact.

Stainless steel: Offers excellent corrosion resistance but lower conductivity than copper or aluminum. Used where corrosion resistance is paramount and higher resistance can be tolerated, such as some structural bonds. Austenitic stainless steels (300 series) are non-magnetic, which may be important in some applications.

Specialty alloys: In demanding environments, materials such as Monel, Inconel, or cadmium-plated copper may be specified. These provide specific combinations of conductivity, corrosion resistance, and compatibility that standard materials cannot match.

Strap Construction Types

Bonding straps are manufactured in several forms, each suited to different requirements:

Flat braid: Woven from multiple fine wires, flat braid straps offer excellent flexibility while maintaining low DC resistance and acceptable high-frequency performance. The flexibility accommodates relative motion between bonded structures due to thermal expansion or vibration. Common widths range from 6 mm to 50 mm, with thicknesses typically between 1 mm and 3 mm.

Round braid: Tubular braided construction provides flexibility in all directions and is often used where the strap must accommodate complex three-dimensional movement. Round braid is common in aerospace applications where equipment is mounted on vibration-isolated platforms.

Solid strap: Stamped or machined from sheet metal, solid straps offer the lowest impedance but no flexibility. They are used for permanent bonds between structures that will not move relative to each other, such as between chassis sections or between equipment and building steel.

Laminated strap: Multiple thin layers of conductor bonded together provide flexibility while maintaining a larger cross-sectional area than braid of similar dimensions. Laminated straps offer a good compromise between the low impedance of solid straps and the flexibility of braid.

Sizing for Current Capacity

Bonding straps must be sized to carry the expected fault currents without excessive heating or damage. The required cross-sectional area depends on the magnitude and duration of fault currents the strap must survive:

For lightning protection bonds, industry standards typically require that straps can carry the full lightning current (up to 200 kA for severe lightning environments) for the duration of the stroke without fusing. This often requires straps with equivalent cross-sections of 25 mm squared or larger for copper.

For EMC bonding where only small currents flow, strap sizing is driven by impedance requirements rather than current capacity. A strap that provides adequately low impedance will typically have more than sufficient current capacity for normal EMC applications.

Safety ground bonds in industrial facilities must meet electrical code requirements, which typically specify minimum conductor sizes based on the rating of the overcurrent protection device. These requirements ensure the bond can carry fault current long enough for protective devices to operate without creating a fire hazard.

High-Frequency Performance

At radio frequencies and above, the impedance of a bonding strap is dominated by inductance rather than resistance. The inductance of a strap depends primarily on its geometry:

Short, wide straps have lower inductance than long, narrow straps. A useful approximation for a flat strap is:

L (nH) = 5.08 * length * [ln(2 * length / width) + 0.25]

where length and width are in centimeters. This shows that doubling the width reduces inductance by only about 30%, while doubling the length nearly doubles the inductance. Thus, keeping bonding straps as short as possible is critical for high-frequency performance.

The length-to-width ratio should ideally be less than 5:1 for RF bonding applications. Some standards specify a maximum ratio of 3:1 for critical bonds. Where long runs are unavoidable, multiple parallel straps can reduce total inductance.

At frequencies above about 100 kHz, skin effect concentrates current flow near the conductor surface. For flat braided straps, this means the individual wire strands act as parallel conductors rather than a solid mass, which can actually improve high-frequency performance compared to solid straps of equal cross-section.

DC Resistance Specifications

Many bonding standards specify maximum DC resistance values measured across the complete bond, including the strap, connection hardware, and interface resistances at both ends:

2.5 milliohms: This is a common requirement for high-quality RF bonds and lightning protection connections. Achieving this level requires excellent surface preparation, properly torqued hardware, and compatible materials. This value is often specified in aerospace and military applications for EMC-critical bonds.

10 milliohms: A more relaxed requirement appropriate for general structural bonding where EMC is not critical. This level is achievable with standard hardware installation practices and moderate surface preparation. Many industrial standards use this value for equipment grounding connections.

25 milliohms: Some standards accept this level for non-critical bonds or where environmental conditions make lower resistance impractical to maintain. Bonds at this level may be suitable for ESD protection but are generally inadequate for EMC or lightning protection.

100 milliohms: The upper limit for bonds that provide any meaningful electrical function. Connections with higher resistance are considered ineffective and require remediation. This level might be acceptable for crude static dissipation but fails to meet any rigorous bonding standard.

Impedance Requirements at Frequency

For EMC applications, the high-frequency impedance of the bond is more important than DC resistance. A bond that measures well at DC may still be inadequate at radio frequencies due to inductance:

At 1 MHz, a bond with 100 nH of inductance has an impedance of approximately 0.6 ohms, regardless of its DC resistance. At 100 MHz, the same bond presents 63 ohms of impedance, which is far too high for effective EMC bonding.

EMC bonds typically should present less than 1 ohm of impedance at the highest frequency of concern. For digital equipment operating at clock rates above 100 MHz, this requires bond inductances below about 1.5 microhenries, implying very short, wide bonding straps or direct metal-to-metal contact.

Some standards specify impedance requirements directly. For example, aerospace EMC requirements may specify maximum bond impedance at specific frequencies, such as 2.5 milliohms DC and less than 1 ohm at 10 MHz.

Measurement Techniques

Accurate bond resistance measurement requires proper technique to avoid errors from lead resistance and contact placement:

Four-wire (Kelvin) measurement: The standard technique for measuring low resistances uses separate current-injection and voltage-sensing leads. This eliminates lead resistance from the measurement, allowing accurate readings down to the microohm range. Dedicated milliohm meters and low-resistance ohmmeters typically implement this method automatically.

Test current: Bond resistance should be measured using a test current high enough to provide stable, repeatable readings but not so high as to heat the bond and affect the measurement. Test currents between 100 mA and 10 A are typical, with 1 A being a common choice. The measurement should comply with the applicable standard; some specify minimum test currents.

Probe placement: The voltage sense probes should contact the bonded structures on either side of the complete bond, not on the bonding strap itself. This ensures the measurement captures all interface resistances. Probes should make contact on clean metal surfaces, not on paint or corrosion.

Contact resistance effects: The contact resistance between test probes and the structure can affect readings, especially for very low resistance bonds. Using sharp probe tips and applying consistent pressure helps achieve repeatable measurements. Multiple readings should be taken to verify consistency.

Environmental Effects on Resistance

Bond resistance can change significantly with environmental conditions:

Temperature: The resistance of metals increases with temperature, typically by about 0.4% per degree Celsius for copper and aluminum. A bond that meets requirements at room temperature may exceed limits at elevated operating temperatures. Some standards specify that measurements be made at standard temperature or that results be corrected to a reference temperature.

Humidity: High humidity can affect surface films and corrosion, potentially changing bond resistance over time. Initial measurements after installation in a dry environment may not represent long-term performance in humid conditions.

Aging: Over time, oxidation, corrosion, and contamination can increase bond resistance. Initial acceptance measurements should be well below limits to allow margin for aging. Periodic re-verification catches bonds that have degraded before they fail completely.

Types of Corrosion Affecting Bonds

Several corrosion mechanisms can degrade electrical bonds:

Oxidation: Most metals form oxide layers when exposed to air. For some metals like aluminum, the oxide layer is self-limiting and provides protection against further corrosion. For others like iron, the oxide layer is porous and corrosion continues. At bonding surfaces, oxide layers increase contact resistance and can eventually prevent electrical continuity entirely.

Galvanic corrosion: When two dissimilar metals are in contact in the presence of an electrolyte (even atmospheric moisture), galvanic action causes the more active metal to corrode preferentially. This is discussed in detail in the following section.

Crevice corrosion: Narrow gaps between bonded surfaces can trap moisture and contaminants, creating localized corrosion cells. The confined geometry prevents the circulation of fresh electrolyte, causing localized depletion of oxygen and concentration of corrosion products, accelerating attack.

Fretting corrosion: Small relative movements between bonded surfaces (from vibration or thermal cycling) can wear away protective oxide layers, exposing fresh metal to corrosive attack. The debris from fretting can itself be corrosive, accelerating degradation.

Environmental attack: Industrial atmospheres containing sulfur compounds, chlorides, or other aggressive chemicals can cause rapid corrosion of unprotected bonds. Marine environments are particularly severe due to salt spray and high humidity.

Protective Measures

Various techniques protect bonds from corrosion:

Conductive compounds: Joint compounds containing metallic particles (typically zinc or aluminum) suspended in grease or other carriers fill surface irregularities, exclude moisture, and maintain conductivity even if some corrosion occurs. These compounds are applied to prepared surfaces before assembly and are especially important for aluminum bonds.

Plating: Protective platings on bonding straps and connection points provide corrosion resistance. Tin plating on copper straps improves compatibility with aluminum. Cadmium plating (where still permitted) provides excellent protection in marine and aerospace environments. Silver or nickel plating provides both corrosion resistance and improved contact characteristics.

Sealants: After assembly, the bond perimeter should be sealed to prevent moisture intrusion. Flexible sealants that accommodate thermal movement without cracking are preferred. The sealant should not penetrate into the actual contact interface where it could increase resistance.

Encapsulation: For severe environments, the entire bond assembly can be encapsulated in potting compound or protected by heat-shrink tubing. This provides excellent protection but makes inspection and maintenance difficult.

Drainage: Bond assemblies should be designed to prevent water accumulation. Drainage holes, sloped surfaces, and avoiding upward-facing crevices help prevent moisture from pooling around bonds.

Corrosion Inspection

Regular inspection helps identify corrosion before it causes bond failure:

Visual inspection: Look for discoloration, white or colored deposits (corrosion products), swelling of finishes, and moisture accumulation. Even if a bond still measures acceptable resistance, visible corrosion indicates conditions that will lead to eventual failure.

Resistance trending: Tracking bond resistance measurements over time reveals degradation trends. A bond that increases from 1 milliohm to 5 milliohms, while still within limits, is clearly degrading and requires attention before it fails.

Environmental assessment: Evaluating the environment around bonds helps predict corrosion risk. Bonds in areas exposed to moisture, chemicals, or temperature extremes require more frequent inspection than those in controlled environments.

The Galvanic Series

Metals can be ranked by their electrochemical activity. The further apart two metals are in the galvanic series, the greater the driving force for galvanic corrosion when they are coupled. The series (listed from anodic/active to cathodic/noble in seawater) includes:

• Magnesium and magnesium alloys (most active)
• Zinc and zinc coatings
• Aluminum alloys
• Carbon steel and cast iron
• Stainless steel (active)
• Lead-tin solder
• Copper and copper alloys
• Stainless steel (passive)
• Silver
• Gold and platinum (most noble)

The position of some metals, particularly stainless steels, can vary depending on their condition and the specific environment. Stainless steel that has lost its passive oxide layer becomes much more active.

Factors Affecting Galvanic Corrosion Rate

The severity of galvanic corrosion depends on several factors beyond just the metal combination:

Potential difference: The voltage between the two metals (measured in the specific environment) determines the driving force for corrosion. Larger potential differences cause faster attack.

Area ratio: The ratio of cathodic to anodic area dramatically affects corrosion rate. A small anode (active metal) connected to a large cathode (noble metal) will corrode rapidly because the corrosion current is concentrated on a small area. The reverse situation (large anode, small cathode) produces slow, distributed attack.

Electrolyte conductivity: Higher conductivity electrolytes support higher corrosion currents. Seawater is much more aggressive than fresh water; humid air is less aggressive than liquid water but can still cause significant corrosion over time.

Temperature: Higher temperatures generally accelerate electrochemical reactions, increasing corrosion rates.

Oxygen availability: Many galvanic corrosion reactions consume oxygen. Abundant oxygen at the cathode surface promotes corrosion, while oxygen-depleted conditions slow it.

Compatible Material Combinations

For reliable bonds, materials should be selected for galvanic compatibility:

Copper to copper: All copper alloys (brass, bronze, copper) can be bonded together without galvanic concerns. Avoid if aluminum is nearby and might be exposed to runoff from the copper bond.

Aluminum to aluminum: Aluminum alloys can generally be bonded to each other, though some alloy combinations have different potentials. Use appropriate joint compound and hardware.

Aluminum to cadmium-plated steel: Cadmium plating provides a compatible surface for bonding to aluminum. However, cadmium use is restricted due to toxicity concerns.

Tinned copper to aluminum: Tin provides a moderately compatible surface for contact with aluminum, making tinned copper straps suitable for aluminum bonds with appropriate protective measures.

Stainless steel to stainless steel: Austenitic stainless steels are compatible with each other. Passive stainless is quite noble and should not be coupled to active metals without protection.

Mitigation Techniques

When dissimilar metals must be bonded, several techniques can control galvanic corrosion:

Barrier materials: An intermediate material compatible with both metals can break the galvanic couple. Plating, cladding, or separate hardware items can serve this function.

Sealants and coatings: Excluding the electrolyte prevents galvanic action. Complete sealing of the joint periphery is essential; partial sealing can actually concentrate attack at any gaps.

Sacrificial protection: Including a more active metal (such as a zinc coating or zinc hardware) causes the sacrificial metal to corrode preferentially, protecting both parent metals. This technique requires periodic replacement of the sacrificial element.

Favorable area ratio: If corrosion of one metal is acceptable, design the bond so the acceptable-loss metal is the larger-area cathode. This slows attack on the anode and distributes any cathode-side effects over a larger area.

Environmental control: Reducing humidity, controlling condensation, and preventing salt spray exposure all reduce electrolyte availability and slow galvanic corrosion.

Surface Cleaning

Before any mechanical preparation, surfaces must be free of bulk contamination:

Degreasing: Oils, greases, and other organic films must be removed completely. Solvent cleaning with appropriate cleaners (isopropyl alcohol, acetone, or commercial degreasers) is typically required. Some facilities use vapor degreasing for critical bonds. Residues from cleaning solvents themselves must be allowed to evaporate completely.

Paint removal: All paint, primer, and conversion coatings must be removed from the bond area. Chemical strippers, mechanical abrasion, or both may be needed. The stripped area should extend slightly beyond the final bond area to ensure no paint creeps under the bonding surface.

Corrosion product removal: Rust, aluminum oxide, or other corrosion products must be removed to reach sound base metal. Heavy corrosion may require aggressive mechanical treatment.

Mechanical Surface Preparation

After cleaning, surfaces typically require mechanical treatment to remove oxide layers and create an appropriate texture:

Abrasive methods: Wire brushing, sanding, or abrasive blasting removes oxide layers and creates a mildly roughened surface that promotes metal-to-metal contact. For aluminum, stainless steel brushes or non-metallic abrasives should be used to avoid embedding iron particles that cause subsequent corrosion. The abrasive should be clean and free of contamination.

Scratch brushing: For copper and copper alloys, scratch brushing with a rotary brass brush effectively removes tarnish and creates good bonding surfaces. The brushing action should be sufficient to expose bright metal.

Scotch-Brite or equivalent pads: Non-woven abrasive pads provide a consistent, controlled surface finish suitable for many bonding applications. They are less aggressive than wire brushes and leave a finer finish.

Surface finish requirements: The prepared surface should show bright, clean metal with a slightly roughened texture. A surface that is too smooth may not provide sufficient contact area, while a surface that is too rough may trap contaminants and make assembly difficult.

Timing and Handling

Prepared surfaces begin to degrade immediately upon exposure to air:

Aluminum: Aluminum begins forming an oxide layer within seconds of exposure. The initial oxide is thin and easily penetrated during assembly, but thickens with time. Aluminum surfaces should be assembled within 4 hours of preparation for best results; some standards require assembly within 2 hours.

Copper: Copper tarnishes more slowly than aluminum oxidizes but is still time-sensitive. Prepared copper surfaces should be assembled within 8 hours or protected with an appropriate compound.

Steel: Bare steel in humid environments can begin rusting within minutes. Steel surfaces for bonding should be prepared and assembled as quickly as possible, or treated with a protective primer compatible with subsequent bonding.

Handling precautions: After surface preparation, the bond area must not be touched with bare hands, which deposit oils and salts. Clean gloves should be worn during assembly. If a prepared surface is accidentally contaminated, it must be re-prepared.

Joint Compound Application

Conductive joint compound should be applied immediately after surface preparation:

A thin, uniform layer of compound is spread over both mating surfaces. The compound should fill surface irregularities but not be so thick that it prevents metal-to-metal contact. Excess compound will squeeze out during assembly and should be wiped away.

The compound type should be appropriate for the materials and environment. Some compounds are formulated specifically for aluminum, others for copper, and general-purpose compounds serve many applications. Always verify compatibility with the applicable specification.

Fastener Materials

Fastener material must be compatible with the bonded materials:

For aluminum structures: Cadmium-plated steel fasteners are traditional but increasingly restricted. Stainless steel fasteners are common but require careful attention to galvanic isolation. Aluminum alloy fasteners avoid galvanic concerns but have lower strength. Zinc-plated steel provides sacrificial protection.

For copper structures: Brass, bronze, or stainless steel fasteners are appropriate. Avoid zinc-plated or aluminum hardware in direct contact with copper.

For steel structures: Zinc-plated or cadmium-plated steel fasteners match the base metal. Stainless steel fasteners can be used if galvanic effects are managed.

Corrosion-resistant alloys: In severe environments, fasteners of Monel, Inconel, or titanium may be specified despite higher cost.

Fastener Types and Torque

Proper torque is essential for achieving and maintaining low-resistance bonds:

Standard threaded fasteners: Bolts or screws with nuts provide reliable, adjustable clamping force. Torque values should be specified based on fastener size and material. Under-torqued fasteners provide insufficient contact pressure; over-torqued fasteners can strip threads or damage soft materials.

Lock washers and locking features: Vibration can loosen fasteners, increasing contact resistance and eventually causing complete bond failure. Lock washers (split, external tooth, or internal tooth), nylon insert lock nuts, or thread-locking compounds prevent loosening. Self-locking fastener features may be required by specification.

Flat washers: Washers distribute clamping force over a larger area, reducing surface damage and providing more consistent contact pressure. For bonds to aluminum, large-diameter washers help prevent the fastener from pulling into the soft aluminum.

Star washers: Serrated or star washers are sometimes specified for electrical bonds because their teeth bite into mating surfaces, penetrating oxide layers and ensuring metal-to-metal contact. However, they can damage soft materials and may not be appropriate for all applications.

Hardware Configuration

The arrangement of hardware affects both mechanical and electrical performance:

Direct metal-to-metal contact: Hardware should be configured so the bonded surfaces are in direct contact with no intervening insulating materials. This typically means flat washers only under bolt heads and nuts, not between bonding surfaces.

Contact area: Multiple smaller fasteners often provide better electrical performance than a single large fastener because they create more separate contact points. However, this must be balanced against installation complexity.

Fastener spacing: For bonds under hardware that must resist high currents (such as lightning bonds), fastener spacing affects current distribution. Closely spaced fasteners ensure current can flow through any part of the bond area without excessive concentration.

Specialized Bonding Hardware

Several specialized products address specific bonding needs:

Self-grounding connectors: Some electrical connectors incorporate features that automatically establish an electrical bond when mated, eliminating the need for separate bonding jumpers for shielded cable terminations.

Bond-thru bushings: These feedthrough devices provide electrical continuity through a shield wall while maintaining shielding integrity. They are used where cables must pass through shielded enclosures.

Conductive gaskets: When enclosures must be opened regularly for maintenance, conductive gaskets maintain EMC bonding across the seam without permanent fastening. Gaskets are made from metal mesh, conductive elastomer, or beryllium copper fingers.

EMI/RFI terminal lugs: Specialized lugs designed for RF bonding often incorporate serrated or knurled surfaces to penetrate oxide layers and ensure low-impedance contact.

Components of Bond Impedance

The total bond impedance includes several components in series:

Strap impedance: This includes the DC resistance of the strap conductor plus inductive reactance at the frequency of interest. For braided straps, the weave geometry affects both resistance and inductance.

Hardware impedance: Bolts, nuts, and washers add resistance and inductance to the path. The current must flow through the fastener if it is part of the electrical path, adding the fastener's impedance.

Contact resistance: The interface between the strap and bonded surfaces presents contact resistance that depends on contact pressure, surface condition, and materials. This is often the largest component of bond resistance.

Spreading resistance: Current flowing into or out of a localized contact point must spread through the bulk material, adding resistance that depends on material conductivity and geometry.

Frequency Effects

Bond impedance is strongly frequency-dependent:

Low frequency (DC to approximately 10 kHz): Bond impedance is dominated by the sum of resistances: strap resistance, contact resistance, and spreading resistance. Inductance is negligible.

Transition region (approximately 10 kHz to 1 MHz): Inductive reactance becomes comparable to resistance. Total impedance begins to increase with frequency. Skin effect starts to affect conductor resistance.

High frequency (above approximately 1 MHz): Bond impedance is dominated by inductance. The impedance increases linearly with frequency (20 dB per decade). Skin effect fully developed; current flows only on conductor surfaces.

A bond that measures 1 milliohm at DC might present 10 ohms of impedance at 100 MHz. This dramatic increase explains why short, wide straps are essential for high-frequency bonding.

Reducing Bond Impedance

Several design choices minimize bond impedance:

Minimize strap length: Keeping the bonding path as short as possible is the single most effective way to reduce inductance and thus high-frequency impedance.

Maximize strap width: Wide straps have lower inductance than narrow straps, though the benefit diminishes beyond length-to-width ratios of about 3:1.

Parallel paths: Multiple bonding straps in parallel reduce total inductance. Two straps have half the inductance of one strap, assuming they are separated enough to avoid mutual inductance effects.

Direct contact: Eliminating the strap entirely through direct metal-to-metal contact between structures provides the lowest possible impedance. This is only practical when structures are in intimate contact without relative movement.

Quality contact interfaces: Proper surface preparation and adequate contact pressure minimize contact resistance, which dominates low-frequency impedance.

Measuring Bond Impedance

While DC resistance is easily measured with standard instruments, characterizing high-frequency bond impedance requires different techniques:

Network analyzer: A vector network analyzer can measure bond impedance versus frequency by treating the bond as a two-port network. This provides complete characterization but requires careful fixture design to isolate the bond from test cable effects.

Transfer impedance method: By injecting a known current through the bond and measuring the resulting voltage with a high-impedance probe, bond impedance can be determined at specific frequencies.

Time-domain reflectometry: TDR measurements can characterize bond impedance by analyzing reflections from impedance discontinuities. This is particularly useful for identifying the location of high-impedance elements in a bonding path.

Verification Intervals

The frequency of bond verification depends on the criticality of the bond and the severity of the environment:

Critical bonds: Bonds essential for safety (lightning protection, equipment safety grounds) or mission success (primary EMC bonds on critical systems) may require annual or more frequent verification. Aerospace vehicles often verify critical bonds during each major maintenance cycle.

Standard bonds: Most electrical bonds in controlled industrial environments can be verified every 2-5 years as part of routine maintenance. More frequent verification may be warranted if problems are discovered.

Non-critical bonds: Bonds that serve secondary functions or are highly redundant may be verified less frequently, perhaps every 5-10 years or only when problems are suspected.

Event-triggered verification: Bonds should be re-verified after any event that might have affected them, such as nearby maintenance work, lightning strikes, flooding, or physical damage to bonded structures.

Verification Procedures

A systematic verification procedure ensures consistent, meaningful results:

Record review: Before measuring, review previous measurements for this bond. Note any trends or previous anomalies. Identify the applicable specification and acceptance criteria.

Visual inspection: Examine the bond for signs of corrosion, mechanical damage, loose hardware, or environmental contamination. Document any abnormalities with photographs.

Resistance measurement: Using calibrated instruments and proper four-wire technique, measure bond resistance. Record the measurement along with instrument identification, test current, ambient conditions, and operator name.

Comparison with criteria: Compare the measured value with both the specification limit and historical values. A bond that meets the limit but shows a significant increase from previous measurements may need attention.

Documentation: Record all verification results in a permanent log. Include pass/fail determination and any recommended follow-up actions.

Trending and Analysis

Systematic tracking of bond resistance over time provides valuable insight:

Trend analysis: Plotting bond resistance versus time reveals degradation patterns. A bond showing steady increase may require remediation before it exceeds limits. Sudden changes may indicate discrete events requiring investigation.

Population analysis: Comparing similar bonds across an installation identifies anomalies. A bond that measures higher than similar bonds in the same environment may have a defect even if it meets absolute requirements.

Correlation with events: Tracking bond performance against environmental factors (humidity, temperature, vibration) can reveal sensitivity to specific conditions and guide preventive maintenance.

Specification review: Long-term data may indicate that original specifications were inappropriate, either too stringent (causing unnecessary maintenance) or too lenient (allowing degraded bonds to remain in service). This data supports specification refinement.

Bond Cleaning and Rejuvenation

Minor degradation can often be corrected without disassembling the bond:

Torque verification: Loose hardware is a common cause of increased bond resistance. Re-torquing fasteners to specification often restores acceptable resistance. This should be done carefully to avoid further disturbing corroded interfaces.

Contact cleaning: If access allows, the visible portions of bond interfaces can be cleaned with appropriate solvents and abrasives. This may remove surface contamination that has increased resistance.

Sealant renewal: Degraded or cracked sealant should be removed and replaced to prevent further moisture intrusion. New sealant should completely encapsulate the bond perimeter.

Protective coating touch-up: Any protective coatings that have been damaged should be repaired to prevent continued environmental attack.

Bond Refurbishment

More significant degradation requires partial or complete disassembly:

Hardware replacement: Corroded or damaged hardware should be replaced with new fasteners of the correct type. Lock washers and locking features should be replaced; they lose effectiveness after use.

Surface re-preparation: With the bond disassembled, bonding surfaces can be fully re-prepared. This involves cleaning, mechanical treatment to remove corrosion and expose fresh metal, and application of fresh joint compound.

Strap replacement: Damaged, corroded, or fatigued bonding straps should be replaced. New straps should match original specifications for material, size, and construction.

Re-assembly: Following proper assembly procedures, the bond is reassembled and re-verified. All original protective measures should be applied.

Complete Bond Replacement

In some cases, refurbishment is impractical and complete replacement is necessary:

Severe base metal corrosion: If the bonded structures have corroded significantly, the bond may need to be relocated to sound metal. This may require design changes and approval from engineering.

Structural changes: Modifications to bonded structures may require new bond configurations. Original bonds may become inaccessible or inappropriate after structural changes.

Upgraded requirements: Changed operational requirements or updated specifications may require bonds with better performance than originally installed. This may require larger straps, additional bonds, or different mounting arrangements.

Documentation and Tracking

All maintenance activities should be documented thoroughly:

Work performed: Record exactly what was done to each bond, including cleaning methods, materials replaced, and assembly parameters.

Materials used: Record the specific joint compounds, sealants, and hardware used, including lot numbers where applicable. This supports traceability and helps diagnose any future problems.

Post-maintenance verification: Record verification measurements taken after maintenance is complete. These become the new baseline for future trending.

Configuration control: If bond configuration was changed, update applicable drawings and documentation to reflect the as-maintained configuration.

Lightning Protection Bonds

Bonds in lightning protection systems must survive the extreme currents and voltages of lightning strikes:

Current capacity: Lightning bonds must carry peak currents up to 200 kA for the most severe lightning environments. Straps must have sufficient cross-section to avoid fusing or mechanical damage from electromagnetic forces.

Multiple parallel paths: Critical lightning current paths should include redundant bonds so that failure of any single bond does not compromise protection.

Spark gap consideration: Where bonds might arc during a strike, the arc products should not create fire or damage hazards. Bond locations near flammable materials require special attention.

Verification after strikes: Bonds in areas that experienced lightning strikes should be re-verified promptly. Lightning damage may not be visible but can significantly degrade bond performance.

Fuel System Bonding

Bonds in and around fuel systems must prevent electrostatic discharge that could ignite fuel vapors:

Continuous bonding: All metallic components in the fuel system must be bonded to ensure charge equalization. This includes tanks, pipes, fittings, valves, and access panels.

Low resistance requirements: Fuel system bonds typically require less than 1 ohm resistance to ensure rapid charge dissipation. More stringent requirements may apply in specific applications.

Intrinsically safe materials: Some fuel environments require tools and materials that cannot produce incendive sparks. Bonding work in these areas requires appropriate precautions.

Static grounding: During fuel transfer operations, temporary bonds may be required between vehicles and ground systems. These must be established before fuel flow begins and maintained throughout the operation.

Composite Structure Bonding

Modern aircraft and other structures increasingly use composite materials that are electrically non-conductive:

Conductive treatments: Composite structures may include embedded metal mesh, conductive coatings, or applied foil to provide electrical continuity. Bonds must connect to these conductive elements.

Dedicated bonding provisions: Because conductive paths through composites may be limited, specific bonding locations must be designed into the structure. Bonding jumpers may be required where structural joints provide inadequate electrical continuity.

Expanded metal foil: Expanded aluminum or copper foil bonded to composite surfaces provides a bonding surface. The attachment of bonding straps to this foil requires special techniques to ensure reliable connection.

Hybrid structures: Where composite and metallic structures meet, careful attention to bonding is required. The interfaces between materials are often high-resistance paths requiring dedicated bonding provisions.