PET insulation film breakdown voltage testing is a critical quality control procedure that evaluates the dielectric strength and electrical reliability of polyester films used in capacitors, motors, transformers, and flexible electronics. This comprehensive technical guide covers the applicable testing standards, sample preparation requirements, test methodologies, and industry applications for accurate breakdown voltage measurement of PET insulation films.
Technical Overview
PET (polyethylene terephthalate) insulation film is a biaxially oriented polyester film widely used as a dielectric and electrical insulation material due to its excellent dielectric strength, high volume resistivity, low dielectric loss, superior mechanical toughness, and strong resistance to heat and chemical degradation. Breakdown voltage testing, also referred to as dielectric strength testing, determines the maximum voltage a PET film specimen can withstand before electrical failure occurs, making it one of the most important quality indicators for insulation performance.
When an electric field is applied across a PET film, the material polarizes and sustains the voltage up to a critical threshold. Beyond this threshold, a sudden and irreversible conductive channel forms through the film, resulting in dielectric breakdown. The breakdown voltage (Vbd) divided by the film thickness yields the dielectric strength, typically expressed in kV/mm or V/μm. Commercial PET insulation films generally exhibit dielectric strengths in the range of 150 kV/mm to 300 kV/mm, with thinner films demonstrating higher breakdown strength per unit thickness due to fewer weak points and defect probability effects across the dielectric path.
Breakdown voltage testing is essential for product qualification, incoming material inspection, process validation, and failure analysis. Because breakdown events are inherently statistical in nature, influenced by film defects, pinholes, gels, thickness uniformity, moisture content, and testing conditions, rigorous standardized procedures are required to obtain reproducible and comparable results.
Testing Samples
Metallized Capacitor Grade PET Film
Thin PET films ranging from 2 μm to 12 μm, plain or metallized with aluminum or zinc, used as the dielectric layer in film capacitors. These samples require extremely careful handling and are tested to verify energy density capability and self-healing performance under high electric stress.
Standard Electrical Insulation PET Film
Commercial insulation-grade films with thicknesses from 12 μm to 350 μm, typically supplied in roll or sheet form. Common products include films of 23 μm, 36 μm, 50 μm, 75 μm, 100 μm, 125 μm, 188 μm, and 250 μm nominal thickness used for general insulation purposes.
Motor and Slot Insulation Film
PET films used in combination with nonwoven materials (such as NMN composite structures) for slot liner, phase insulation, and interlayer insulation in electric motors and generators. These samples are tested in both original and thermally aged conditions.
Flexible Electronics and Battery Insulation Film
PET films used as base substrates for flexible printed circuits, barrier films in display modules, and insulation layers in lithium-ion battery cells and packs, where reliable dielectric performance directly affects device safety.
Testing Items
The breakdown voltage test program for PET insulation film typically includes the following measurement items:
- AC breakdown voltage (rms) at commercial power frequency (50 Hz or 60 Hz)
- DC breakdown voltage for applications involving DC voltage stress
- Dielectric strength (breakdown field strength) calculated as breakdown voltage divided by specimen thickness, in kV/mm or V/μm
- Breakdown voltage after thermal aging to evaluate long-term insulation endurance
- Breakdown voltage under elevated temperature or humid environments
- Statistical analysis of breakdown data using Weibull distribution to characterize scale parameter (63.2% breakdown probability) and shape parameter (scatter of data)
- Thickness uniformity verification and defect screening of incoming film lots
Testing Methods
Breakdown voltage testing of PET insulation film is performed according to recognized international and national standards, primarily ASTM D149, IEC 60243-1, and GB/T 1408. These standards define the electrode system, voltage application procedure, ambient conditions, and data evaluation methods.
Specimen Preparation and Conditioning
Specimens are cut to dimensions of approximately 100 mm × 100 mm or larger to prevent flashover around the electrode edges. Film thickness is measured at multiple points using a precision micrometer or optical gauge to ensure representative thickness values. Prior to testing, specimens are conditioned at a standard atmosphere of 23 ± 2 °C and 50 ± 5% relative humidity for a minimum of 24 hours, unless otherwise specified. Specimens exhibiting visible creases, pinholes, contamination, or edge damage are excluded from the test series.
Electrode Configuration and Test Medium
The standard electrode system consists of a cylindrical upper electrode with a 25 mm diameter and a lower electrode of 25 mm or 75 mm diameter with rounded edges (radius approximately 3 mm), both polished to a smooth finish. The specimen is mounted between the electrodes, and the assembly is immersed in clean transformer oil or insulating medium to suppress surface flashover and corona discharge, ensuring true volumetric breakdown through the film. Electrode pressure is controlled to guarantee uniform contact without damaging thin specimens.
Voltage Application Procedures
Three standardized voltage application methods are commonly used:
- Short-time (rapid-rise) method: the voltage is raised uniformly from zero at a rate of typically 500 V/s or 2 kV/s until breakdown occurs, and the breakdown voltage is recorded directly
- Slow rate-of-rise method: voltage is increased from an initial value of 50% of the expected breakdown voltage at a specified slow rate until breakdown
- Step-by-step method: voltage is applied in discrete increments held for a fixed dwell time (typically 60 seconds) until failure occurs, and the withstand step is reported
For routine quality control of PET film, the short-time rapid-rise method at power frequency is the most widely applied approach, with at least 10 specimens tested per sample lot to enable statistical evaluation.
Data Evaluation and Reporting
Individual breakdown voltage values, mean value, standard deviation, and minimum value are calculated for each specimen set. Dielectric strength is obtained by dividing the breakdown voltage by the measured thickness at the breakdown point. For design purposes, Weibull statistical analysis is recommended to determine the characteristic breakdown voltage and to establish reliable lower-bound design values. A complete test report documents the film type, nominal and measured thickness, electrode geometry, test medium, voltage rise rate, number of specimens, individual and statistical results, and any deviation from the standard procedure.
Reference Performance Values
Typical breakdown voltage values for high-quality PET insulation film under standard test conditions are approximately 7 kV to 9 kV for 23 μm film, 12 kV to 15 kV for 50 μm film, and 18 kV to 22 kV for 100 μm film. Corresponding dielectric strength values generally range from 250 V/μm to 300 V/μm for thin films and gradually decrease as thickness increases. Actual acceptance criteria should always be established based on product specifications and application requirements.
Applications
Breakdown voltage testing of PET insulation film supports quality assurance and design validation across numerous electrical and electronic industries:
- Film capacitor manufacturing, where dielectric strength determines rated voltage, energy density, and product reliability
- Wire and cable industry for insulating wrap and barrier layer verification
- Electric motor and generator production for slot insulation, phase insulation, and composite laminates (NMN, DMD structures)
- Transformer and inductor manufacturing for interlayer and winding insulation
- Lithium-ion battery packs for cell-to-cell and cell-to-busbar insulation barriers
- Flexible printed circuits and flexible display modules for base film qualification
- Incoming inspection and supplier qualification in electronics assembly plants
- Failure analysis and root cause investigation of electrical insulation failures
FAQ
What is the difference between breakdown voltage and dielectric strength?
Breakdown voltage is the actual voltage at which the film fails, expressed in volts or kilovolts. Dielectric strength is the breakdown voltage divided by the specimen thickness, expressed in kV/mm or V/μm, allowing comparison between films of different thicknesses.
Which standards govern PET film breakdown voltage testing?
The principal standards are ASTM D149 for dielectric breakdown voltage and dielectric strength of solid insulating materials at commercial power frequencies, IEC 60243-1 for electric strength test methods, and GB/T 1408, the Chinese national standard equivalent. Test methods and electrode specifications should always be stated when reporting results.
Why are multiple specimens tested instead of a single sample?
Dielectric breakdown is a statistical phenomenon governed by the weakest defect within the stressed electrode area. Testing at least 10 specimens per lot provides a representative distribution and enables Weibull analysis for reliable quality assessment and design margins.
Why is the test performed in insulating oil?
Immersion in transformer oil prevents premature surface flashover and corona discharge along the air gap between electrodes, ensuring that failure occurs through the bulk of the PET film rather than across its surface, which yields true volumetric breakdown data.
How do temperature and humidity affect test results?
Elevated temperature reduces the dielectric strength of PET film, typically measurably above 100 °C, while absorbed moisture from high-humidity environments can lower breakdown voltage and increase data scatter. Standard conditioning at 23 °C and 50% relative humidity ensures comparable results.
Can DC breakdown voltage be tested instead of AC?
Yes. DC testing is performed for applications subjected to DC stress, such as battery insulation and DC-link capacitors, using a defined ramp rate and polarity. DC breakdown voltages are typically higher than AC rms values for the same film, so results must be labeled with the test mode.