PEEK insulation film dielectric loss testing measures the dissipation factor (tan δ) and relative permittivity of polyetheretherketone films used in high-frequency and high-temperature electrical systems. This article explains the applicable standards, sample preparation, test instrumentation, and typical results for PEEK dielectric films. The information helps engineers qualify materials for flexible electronics, aerospace wiring, EV power systems, and RF applications.
Technical Overview
PEEK (polyetheretherketone) is a semi-crystalline, high-performance engineering thermoplastic with an aromatic backbone that provides a glass transition temperature of approximately 143°C, a melting point of approximately 343°C, and a continuous service temperature of up to 250°C. When converted into thin insulation films with thicknesses ranging from 25 μm to 250 μm, PEEK combines high dielectric strength (typically 150–200 kV/mm for thin gauges), low moisture absorption (below 0.5%), inherent flame retardancy, and excellent chemical resistance.
Dielectric loss, expressed as the dissipation factor tan δ, quantifies the portion of electrical energy dissipated as heat within a dielectric material under an alternating electric field. It is defined as the ratio of the imaginary part to the real part of the complex permittivity (tan δ = ε'' / ε'). For PEEK film, the relative permittivity is typically 3.2–3.3 and the dissipation factor generally falls between 0.001 and 0.005 at room temperature across the 1 kHz to 1 MHz range. Because tan δ increases with temperature—particularly near and above the glass transition—characterizing dielectric loss under realistic operating conditions is essential for reliable design.
- Signal attenuation in high-frequency transmission lines increases directly with tan δ
- Dielectric heating and thermal runaway risk in power electronics depend on loss levels
- Energy efficiency and thermal stability of capacitive components are governed by loss behavior
- Material qualification, incoming inspection, and batch-to-batch consistency verification require accurate tan δ data
Testing Samples
Neat (Unfilled) PEEK Film
Standard-grade PEEK film, produced by extrusion or film casting, is the most common test article. Typical thicknesses include 25, 50, 75, 100, 125, and 250 μm. Specimens are usually cut as discs or squares with dimensions between 50 mm and 100 mm to suit the electrode fixture geometry.
Modified and Filled PEEK Films
Specialty grades—such as films with enhanced thermal conductivity, improved corona resistance, barrier coatings, or surface treatments for adhesion promotion—should be tested in their as-supplied condition. Fillers and additives can measurably raise permittivity and dissipation factor, so each formulation requires individual characterization.
Specimen Preparation and Conditioning
Specimens must be flat and free of wrinkles, pinholes, scratches, and surface contamination. Unless otherwise specified, samples are conditioned at 23 ± 2°C and 50 ± 5% relative humidity for at least 24 hours in accordance with ISO 291 or ASTM D618. For low-loss or humidity-sensitive applications, dry-state testing after vacuum drying at approximately 120°C may be added. Thickness is measured at a minimum of five positions using a micrometer or non-contact gauge with 1 μm resolution, and the average value is used in permittivity calculations. Electrodes are applied as sputtered gold, evaporated aluminum, or conductive silver paste, with a guarded electrode pattern where three-terminal measurement is required.
Testing Items
A comprehensive dielectric characterization program for PEEK insulation film typically includes the following measured parameters:
- Relative permittivity (dielectric constant, εr) at fixed frequencies such as 1 kHz, 10 kHz, 100 kHz, and 1 MHz
- Dissipation factor (dielectric loss tangent, tan δ) at the same frequency points
- Frequency sweep characterization across a broadband range, for example 20 Hz to 120 MHz
- Temperature-dependent permittivity and loss from −60°C to 250°C to capture relaxation behavior near Tg
- Complementary items: dielectric breakdown strength (ASTM D149), volume and surface resistivity (ASTM D257 / IEC 62631-3-1), and comparative tracking index where required
Testing Methods
Parallel-Plate Electrode Method (ASTM D150 / IEC 60250)
The reference method for PEEK film uses a guarded parallel-plate electrode system connected to a precision LCR meter or impedance analyzer. The sample is clamped between electrodes, and capacitance C together with dissipation D is measured. Relative permittivity is calculated as εr = C·t / (ε0·A), where t is film thickness, A is the guarded electrode area, and ε0 is the vacuum permittivity. The guard electrode eliminates surface leakage currents and minimizes edge capacitance errors, which is critical for low-loss materials such as PEEK.
Broadband Impedance Analysis
A precision impedance analyzer extends characterization from power frequencies up to approximately 100–120 MHz. Short, open, and load compensation routines remove fixture residuals, and fringe-field corrections are applied when air-gap or non-contacting electrode configurations are used. This method maps the full frequency response of tan δ, revealing dipolar relaxation peaks that fixed-frequency tests would miss.
Microwave Resonator Methods
For 5G, radar, and mmWave applications, dielectric properties at gigahertz frequencies are measured using a split-post dielectric resonator (SPDR) covering roughly 1–15 GHz, or cavity perturbation techniques at higher frequencies. These resonant methods achieve the high sensitivity needed to resolve loss tangents below 0.005 on thin polymer films.
Temperature- and Frequency-Dependent Testing
Specimens are tested inside an environmental chamber while temperature is ramped stepwise, with dwell times long enough to reach thermal equilibrium at each point. The resulting tan δ versus temperature curves identify the glass transition region and confirm that the material meets loss limits at maximum continuous operating temperature. Combined frequency–temperature maps allow construction of master curves for lifetime and performance modeling.
Applications
Verified low dielectric loss is a key acceptance criterion for PEEK film in demanding electrical and electronic systems. Typical end uses include:
- Flexible printed circuits and high-speed interconnects, where low tan δ preserves signal integrity
- Aerospace and defense wire and cable insulation requiring stable dielectric behavior from −60°C to 250°C
- Electric vehicle traction motor slot insulation, battery pack barriers, and busbar wrapping films
- 5G antenna substrates, RF connectors, and millimeter-wave components evaluated at gigahertz frequencies
- Oil and gas downhole sensor encapsulation exposed to high temperature and aggressive chemicals
- Semiconductor test sockets, wafer carriers, and cleanroom-compatible insulating layers
In each of these applications, dielectric loss testing provides the quantitative data needed to predict heat generation, signal attenuation, and long-term reliability, and it supports supplier qualification, design verification, and failure analysis activities.
FAQ
What is the typical dissipation factor of PEEK insulation film?
At 23°C, PEEK film generally shows tan δ between 0.002 and 0.005 from 1 kHz to 1 MHz, with a relative permittivity of approximately 3.2–3.3. Exact values vary with crystallinity, moisture content, film thickness, and test frequency, so measurement under the actual service conditions is recommended.
Which standards govern PEEK dielectric loss testing?
The primary standards are ASTM D150 (AC loss characteristics and permittivity of solid electrical insulation), IEC 60250 and its successor IEC 62631-2-1, and GB/T 1409 for the Chinese market. IPC-TM-650 test methods may apply when the film is evaluated for printed circuit applications.
What sample dimensions are required?
Discs or squares of 50–100 mm across are typical, with thicknesses between 25 μm and 250 μm. Thickness uniformity is critical because it enters the permittivity calculation directly and is usually the dominant source of measurement uncertainty.
How does temperature affect the dielectric loss of PEEK film?
Tan δ remains relatively low and stable below the glass transition, then rises noticeably as temperature approaches 143°C and can increase by roughly an order of magnitude well above Tg due to enhanced dipolar and interfacial mobility. Testing at the maximum operating temperature is essential for high-temperature designs.
How does PEEK film compare with polyimide film?
Polyimide films show very low loss at 1 kHz but exhibit a stronger rise in tan δ toward the megahertz range and absorb significantly more moisture, which further degrades dielectric performance. PEEK maintains comparatively flat, low loss at high frequencies and after humidity exposure, making it attractive for high-frequency and humid-environment applications, while polyimide retains advantages in maximum continuous temperature capability.
What measurement uncertainty can be expected?
With a guarded electrode fixture, certified thickness measurement, and proper open/short compensation, typical expanded uncertainty is approximately 3–5% for permittivity and an absolute value of 0.0005–0.001 for tan δ. Careful electrode contact, contamination control, and stable conditioning are necessary to achieve these levels.
Can testing be extended to gigahertz frequencies?
Yes. Split-post dielectric resonator and cavity perturbation methods routinely characterize PEEK films from about 1 GHz up to 20 GHz and beyond, supporting qualification for 5G, radar, and millimeter-wave components.
