Polyimide (PI) insulation films are widely used in flexible printed circuits, motor insulation, aerospace wiring, and power electronics because of their outstanding dielectric strength and thermal stability. Dielectric withstand voltage testing is the core experiment used to verify whether a PI film can endure specified operating and transient voltages without electrical breakdown. This article explains the sample types, testing items, test methods, evaluation criteria, and typical applications involved in PI insulation film withstand voltage experiments.
Technical Overview
Polyimide (PI) film is a high-performance engineering plastic film produced by polycondensation and imidization of dianhydride and diamine monomers. Commercial grades such as Kapton, Upilex, and Apical combine a continuous service temperature of approximately 200 to 260 degrees Celsius, excellent mechanical toughness, chemical resistance, and a high intrinsic dielectric strength, making them the material of choice for thin-film electrical insulation. Because PI films are frequently deployed as the primary insulation barrier in high-voltage and high-density electronics, their ability to withstand voltage without breakdown must be verified experimentally before design release and mass production.
A withstand voltage experiment (often called a hipot or proof-voltage test) applies a specified test voltage across the film for a defined duration and evaluates pass or fail based on whether breakdown or excessive leakage current occurs. A closely related experiment, the dielectric breakdown test, raises the voltage until the film fails in order to measure breakdown voltage and calculate dielectric strength in kV/mm. For PI films, dielectric strength typically falls in the range of roughly 150 to 300 kV/mm, decreasing as film thickness increases due to the greater probability of defects and field distortion in thicker specimens. Key reference standards include IEC 60243-1, ASTM D149, UL 746A, and IPC-TM-650, along with material specifications such as GB/T 5593 and GB/T 13542.6 for electronic-grade and electrical-insulation-grade polyimide films.
Testing Samples
Standard Polyimide Films
Unmodified, uncoated PI films in common thicknesses from 7.5 micrometers up to 125 micrometers form the baseline sample group. These specimens are usually cut into squares or rectangles large enough to provide adequate creepage distance beyond the electrode edges, typically at least 25 mm wider than the electrode diameter on every side. Representative thickness points such as 12.5, 25, 50, 75, and 125 micrometers allow the laboratory to characterize the thickness dependence of breakdown voltage.
Specialty and Modified PI Films
Advanced product variants are tested separately because fillers and coatings change dielectric behavior. This group includes corona-resistant PI films containing inorganic nanoparticles for inverter-driven motor insulation, black or opaque PI films for optical shielding, filled thermally conductive grades, and adhesive-coated coverlay films used in flexible printed circuit manufacturing, where the adhesive layer is often the weakest dielectric link.
Aged, Conditioned, and Processed Samples
To assess long-term reliability, test pieces are frequently subjected to conditioning before the withstand voltage experiment. Typical pre-treatments include thermal aging at 200 to 300 degrees Celsius, humidity exposure such as 85 degrees Celsius and 85 percent relative humidity, mechanical creasing or tensile straining, and chemical exposure to etchants or flux residues. Finished articles such as etched flexible circuits, insulated busbars, or film-wound capacitor sections may also be tested as built to verify insulation integrity after all manufacturing processes.
Testing Items
Depending on the purpose of the experiment, one or more of the following dielectric characteristics are measured:
- AC dielectric breakdown voltage, the power-frequency voltage at which the film fails.
- DC dielectric breakdown voltage, relevant for battery, traction, and HVDC insulation applications.
- Dielectric strength, the breakdown voltage divided by specimen thickness, expressed in kV/mm or V per mil.
- Proof withstand voltage pass/fail result at a specified voltage and duration, such as 1 to 5 kV AC applied for 60 seconds.
- Leakage or conduction current during the withstand test, with trip limits commonly set at 0.5 to 5 milliamperes depending on test area and capacitance.
- Corona inception and extinction voltage, or partial discharge magnitude, which is critical because partial discharge erosion, not instantaneous breakdown, governs the life of thin films under AC stress.
- Statistical breakdown parameters, including mean, minimum, standard deviation, and Weibull characteristic values obtained from multiple specimens.
Testing Methods
Sample and Electrode Preparation
Specimens are conditioned at 23 plus or minus 2 degrees Celsius and 50 plus or minus 5 percent relative humidity for at least 24 hours unless a different environment is being studied. Testing follows IEC 60243-1 or ASTM D149 using symmetrical cylindrical electrodes, typically 25 mm in diameter with rounded edges, or ball electrodes for thin films. To prevent surface flashover and external corona from masking the true bulk breakdown strength, the electrode assembly and specimen are immersed in clean insulating oil such as transformer oil. Every specimen is inspected for pinholes, wrinkles, and contamination before testing, and thickness is verified at several points with a low-pressure micrometer.
Short-Time Ramp Method
In the rapid-rise method, voltage is increased from zero at a constant rate, commonly 0.5, 1, 2, or 5 kV/s selected according to the expected breakdown level, until breakdown occurs. Breakdown is identified by an abrupt jump in current, operation of the overcurrent trip circuit, or visible puncture and carbonized channel in the film. The voltage at failure is recorded as the breakdown voltage, and dielectric strength is calculated by dividing it by the minimum thickness measured near the breakdown point. At least five to ten specimens are tested, and the scatter is evaluated statistically.
Step-by-Step Method
The step method starts at an initial voltage equal to about 50 percent of the expected breakdown value and then increases in fixed increments, for example 10 percent of the initial value, with each step held for a dwell time of 10 to 60 seconds. This approach better simulates service stressing where voltage plateaus occur, and it yields breakdown values typically lower than the rapid-rise method because the material is stressed longer at each level.
Slow Rate-of-Rise Method
For research-grade characterization, the voltage is raised at a slow uniform rate such as 100 or 500 V/s. This method improves reproducibility and is useful when comparing material formulations, because it reduces the sensitivity of results to ramp-rate-dependent charge and heating effects.
Proof Withstand Test Procedure
In routine qualification and production screening, a fixed test voltage, usually two to three times the rated operating voltage as defined by the product or insulation coordination standard, is applied for a fixed duration, most commonly 60 seconds. The specimen passes if no breakdown occurs and leakage current remains below the specified limit. After testing, the specimen is discharged and grounded, and the test area is inspected for punctures, tracking marks, or thermal damage. Safety requirements include an interlocked enclosed test fixture, insulated tools, high-voltage warning signage, and mandatory discharge of the specimen before handling.
Data Processing and Evaluation
Breakdown data from replicate specimens are analyzed using Weibull or normal distributions to report characteristic values and lower confidence bounds. Results are reported together with the film thickness, electrode geometry, voltage waveform and frequency, ramp rate or dwell time, ambient medium, temperature, and humidity, because all of these parameters measurably influence PI film breakdown voltage. A failed withstand test is investigated by locating the puncture site, examining it microscopically, and reviewing process records to distinguish material defects from test artifacts such as air gaps or flashover.
Applications
Withstand voltage experiment data for PI films directly support design and qualification decisions across many industries:
- Flexible printed circuits and chip-on-film packages, where PI serves as the base dielectric and coverlay insulation between conductive layers.
- Inverter-driven motors and generators, where corona-resistant PI films are used as slot liner, phase insulation, and magnet wire wrap, with turn-to-turn withstand capability verified by testing.
- Transformers, inductors, and film capacitors, where layer-to-layer and wound-section dielectric margins are validated through hipot testing.
- Aerospace and railway wiring, harness protection, and high-temperature cable wraps that must survive specified test voltages after thermal cycling.
- Electric vehicle battery packs and busbar insulation, where DC withstand testing confirms cell-level and pack-level isolation integrity under high DC stress.
- Etched-foil heaters and flexible heating films, where the PI dielectric layer must pass reinforced insulation test levels between live conductors and grounded or touchable surfaces.
FAQ
What dielectric strength can be expected from a typical 25 micrometer PI film?
Standard unfilled PI films of 25 micrometers thickness generally exhibit dielectric strengths in the region of 250 to 300 kV/mm when tested in oil with short-time ramp conditions. The value per unit thickness decreases as film thickness increases, so a 125 micrometer film may measure closer to 130 to 160 kV/mm.
Should the test be performed with AC or DC voltage?
AC testing at 50 or 60 Hz is the conventional choice and matches most power-frequency service conditions, but it also stresses the material through repetitive partial discharge activity. DC testing is appropriate for battery, traction, and HVDC applications and often produces different breakdown values because space charge accumulation alters the internal field distribution. The waveform used in the experiment should always match the waveform defined in the applicable product specification.
Why is the test performed in insulating oil?
For thin films with very high dielectric strength, breakdown of the surrounding air or surface flashover across the specimen edge can occur before the film itself punctures. Immersion in transformer oil suppresses external discharge and ensures that the measured failure truly reflects the dielectric capability of the PI film.
What are the most common causes of withstand test failures in PI films?
Typical root causes include pinholes and gel particles introduced during film casting, contamination by conductive dust or ionic residues, localized thin spots, moisture absorbed during humid storage, mechanical damage such as creasing or scratches, and corona erosion during long-duration AC tests in air. Correlation of the puncture location with process records is essential for effective corrective action.
How does temperature affect the withstand voltage of PI film?
Polyimide retains a large fraction of its room-temperature dielectric strength even at 200 degrees Celsius and above, which is one of its main advantages over polyester and other lower-temperature films. Nevertheless, breakdown voltage decreases gradually with rising temperature, so insulation designs for hot environments should rely on test data measured at the actual service temperature rather than room-temperature values alone.