Corrosion Evaluation of Thermal Conductive Pads for Power Batteries: Testing Items, Methods, and Sta

Corrosion evaluation of thermal conductive pads for power batteries is a critical reliability assessment that verifies whether silicone or silicone-free gap filler materials can safely contact copper busbars, aluminum cooling plates, and flexible printed circuits without causing ionic contamination, siloxane deposition, or contact resistance degradation. This article explains the corrosion mechanisms of battery thermal interface materials, typical sample types, key testing items, standardized test methods, and acceptance criteria used by accredited third-party laboratories and battery manufacturers worldwide.

Technical Overview

Thermal conductive pads, also known as thermal gap pads or thermally conductive gap fillers, are soft polymer composites installed between battery cells, cooling plates, and busbars in electric vehicle (EV) power battery packs and energy storage systems (ESS). These materials typically consist of a silicone, acrylic, polyurethane, or epoxy matrix loaded with high-volume-fraction ceramic fillers such as alumina, aluminum nitride, boron nitride, or zinc oxide. While their primary function is heat dissipation, their direct and long-term contact with electrically sensitive metal surfaces introduces a distinct corrosion risk that must be quantified before design release.

Corrosion mechanisms associated with thermal pads include the release of low-molecular-weight volatile siloxanes from silicone-based formulations, the extraction of ionic impurities such as chloride and sulfate ions by condensed moisture, galvanic coupling between dissimilar metals across the conductive filler network, and electrochemical migration under applied voltage. In battery applications, these mechanisms can increase contact resistance at busbar joints, tarnish nickel- and copper-plated terminals, corrode flexible printed circuit (FPC) traces, and contaminate optical and connector surfaces. A systematic corrosion evaluation therefore combines chemical characterization, accelerated environmental exposure, and electrical performance verification.

Testing Samples

Silicone-Based Thermal Gap Pads

Silicone pads remain the most widely used thermal interface materials in battery packs due to their softness, compression compliance, and wide operating temperature range. They are the primary focus of volatile siloxane and oil-bleed corrosion testing, since platinum-cured and peroxide-cured silicones can release cyclic siloxanes (D3 to D20) and silicone oil fractions at elevated temperatures.

Silicone-Free Thermal Pads and Gap Fillers

Acrylic, polyurethane, and olefin-based silicone-free materials are increasingly adopted near connectors, sensors, and FPCs where siloxane contamination must be eliminated. These materials are evaluated for residual monomer release, acidic by-products, and ionic cleanliness rather than siloxane volatility.

Thermal Gels, Putties, and Phase Change Materials

Dispensable thermal gels and putties used in cell-to-pack (CTP) architectures, as well as phase change materials, are tested for oil separation, ion leaching, and flow-related contact with live terminals under compression and thermal cycling.

Metal Coupons and Mating Surface Specimens

To simulate real interfaces, the evaluation includes bare and plated coupons of the metals actually used in the pack: copper (busbars and FPC traces), aluminum (cell cans and liquid cooling plates), nickel (cell tabs and terminals), tin and silver platings, and anodized or painted aluminum surfaces.

Testing Items

A complete corrosion assessment of battery thermal pads typically covers the following items:

• Volatile siloxane content (cyclic D3 to D20 series) and total volatile matter at battery-relevant temperatures
• Ion impurity analysis: anions (chloride, bromide, sulfate, nitrate, fluoride) and cations (sodium, potassium, ammonium, calcium) by ion chromatography
• pH and conductivity of aqueous extracts from the pad material
• Copper strip and copper mirror corrosion rating after exposure to the pad or its extract
• Corrosion of aluminum, nickel, tin, and silver coupons in direct contact with the pad under damp heat conditions
• Contact resistance change of busbar joints and terminal interfaces before and after aging
• Surface insulation resistance (SIR) and electrochemical migration resistance under applied bias
• Galvanic corrosion current measurement for pad-to-metal couples
• Water absorption, oil bleed, and weight loss after thermal aging
• Salt spray and cyclic corrosion resistance for pack-level or enclosure-adjacent configurations

Testing Methods

Chemical and Extractable Characterization

Volatile siloxanes are quantified by gas chromatography coupled with mass spectrometry (GC-MS), with headspace sampling at temperatures such as 105 °C or 150 °C to simulate worst-case pack conditions. Ion chromatography (IC) performed on water extracts determines chloride, sulfate, and other ionic species down to ppm levels, following methods aligned with IPC-TM-650 extraction procedures. Inductively coupled plasma optical emission spectroscopy (ICP-OES) may be used to screen for leachable metal ions. Total volatile content is assessed by gravimetric weight-loss measurement after extended conditioning at elevated temperature, in line with the general principles of ASTM D5239 and ASTM E1868.

Metal Coupon Corrosion Testing

Copper strip corrosion is evaluated according to ASTM D130, in which a polished copper strip is placed in contact with the pad material or its oil fraction at elevated temperature and subsequently rated against the standard tarnish scale from 1a to 4c. Copper mirror tests provide a sensitive screening method for volatile corrosivity and siloxane deposition. Sandwich coupons, in which the pad is compressed between two metal plates, are aged and then examined visually and microscopically, with corrosion products analyzed by SEM-EDS to identify the chemical species responsible for attack.

Accelerated Environmental Exposure

Coupon stacks and realistic interface assemblies are subjected to combined environmental stresses to accelerate corrosion mechanisms:

• Steady-state damp heat at 85 °C and 85% relative humidity for 500 to 1000 hours, per IEC 60068-2-78
• Thermal cycling between −40 °C and 85 °C or 125 °C, per IEC 60068-2-14, to induce condensation cycles and pump moisture into the interface
• Salt spray exposure (neutral, NSS) or cyclic salt spray per ISO 9227 and ASTM B117 where pack-level ingress scenarios must be covered
• Fluid compatibility immersion and contact aging per the principles of ASTM D471 using coolants, electrolyte residues, and cleaning agents relevant to pack manufacturing

Electrical and Electrochemical Evaluation

Contact resistance across pad-contacted joints is measured using a four-wire (Kelvin) micro-ohmmeter before and after each aging sequence, with acceptance typically based on a limited percentage change. Surface insulation resistance and electrochemical migration behavior are assessed on comb-pattern test boards in contact with the pad under bias, following IPC-TM-650 methodology. Galvanic current between dissimilar metal electrodes bridged by the pad is monitored using a zero-resistance ammeter technique to quantify the driving force for bimetallic corrosion.

Post-Test Analysis and Acceptance Criteria

After exposure, specimens undergo visual inspection, optical and scanning electron microscopy, mass change measurement, and EDS or XPS surface analysis of corrosion products. Common acceptance criteria applied by battery OEMs include copper strip ratings no worse than 1b, chloride and sulfate extractables below specified ppm limits (frequently single-digit ppm for chloride), volatile siloxane content below several hundred ppm or fully quantified and risk-assessed, no visible pitting or white/red rust on coupons, and contact resistance change within an agreed band such as ten to twenty percent. Since formal pass or fail thresholds are set by each manufacturer's specification, testing is normally performed against customer-defined criteria or jointly agreed protocols at accredited third-party laboratories.

Applications

Corrosion evaluation of thermal conductive pads directly supports reliability engineering in the following areas:

• Material selection and qualification for EV power battery packs, including prismatic, pouch, and cylindrical cell formats
• Cell-to-pack (CTP) and cell-to-chassis designs where pads contact large-area aluminum cooling plates and structural adhesives
• Busbar insulation and thermal management layers requiring stable long-term contact resistance
• Protection of flexible printed circuits, temperature sensors, and voltage acquisition harnesses routed adjacent to gap fillers
• Grid-scale and residential energy storage systems with 15-to-20-year design life requirements
• Supplier incoming material control, change management, and failure analysis of field-returned battery modules

FAQ

Why is volatile siloxane considered a corrosion and reliability concern in battery packs? Low-molecular-weight cyclic siloxanes can evaporate from silicone pads and redeposit on contacts, connectors, and solderable surfaces. Deposited siloxane films increase contact resistance, impair soldering and coating adhesion, and migrate under bias, so their content is quantified even though the compound itself is not a classical electrolytic corrosive agent.

Which metals are most vulnerable to corrosion from thermal pads? Copper busbars and FPC traces are most sensitive to tarnishing and contact resistance increase, followed by nickel-plated tabs and terminals. Aluminum surfaces are primarily at risk from chloride-induced pitting when ionic impurities combine with moisture, while silver-plated surfaces are susceptible to sulfide tarnish.

How long does a typical corrosion evaluation take? Chemical characterization such as GC-MS siloxane analysis and ion chromatography can be completed within one to two weeks. Coupon aging programs including 1000-hour damp heat, thermal cycling, and post-test microscopy typically extend the full evaluation to six to twelve weeks depending on the test matrix.

Is silicone-free always safer than silicone-based thermal pad material? Not necessarily. Silicone-free pads eliminate siloxane migration risk but may introduce acidic residual monomers, higher ionic cleanliness variability, or inferior compression set behavior. A comparative corrosion evaluation covering both chemical extractables and aged coupon performance provides the objective basis for selection.

Which standards govern this testing? There is no single dedicated international standard for thermal pad corrosion; evaluations combine ASTM D130 for copper strip corrosion, ASTM B117 and ISO 9227 for salt spray, IEC 60068-2-14 and IEC 60068-2-78 for environmental conditioning, IPC-TM-650 for ionic extraction and insulation resistance, ASTM D471 for fluid aging, and GB/T and UL material and battery safety standards as applicable to the end product.