PET Substrate CTE Value Testing: Standards, Methods, and Technical Guidelines

PET (polyethylene terephthalate) film is one of the most widely used substrate materials in flexible electronics, and its coefficient of thermal expansion (CTE) directly determines dimensional stability during thermal processing and long-term reliability. This article presents a complete technical guide to PET substrate CTE value testing, covering sample preparation, applicable standards such as ASTM E831 and ISO 11359-2, measurement procedures, and typical applications in flexible printed circuits and displays.

Technical Overview

PET (polyethylene terephthalate) is a semi-crystalline engineering thermoplastic film that serves as a base substrate in flexible printed circuits (FPC), flexible displays, touch sensors, membrane switches, and printed electronics. Because these devices undergo multiple thermal processing steps, such as lamination, curing, sputtering, and reflow, the dimensional behavior of the PET substrate under temperature change is a critical quality parameter.

The coefficient of thermal expansion (CTE) quantifies the fractional change in length of a material per degree of temperature change. Linear CTE is expressed as:

α = (ΔL / L0) / ΔT, typically reported in ppm/°C (equivalent to ppm/K), where L0 is the initial sample length at the reference temperature, and ΔL is the length change over the temperature interval ΔT.

For PET substrates, several characteristics make CTE testing particularly important:

• Standard biaxially oriented PET film exhibits a CTE of approximately 15 to 30 ppm/°C below the glass transition temperature, while heat-stabilized PET grades can achieve values below 5 ppm/°C or even near zero.
• PET film is anisotropic: the CTE in the machine direction (MD) differs from that in the transverse direction (TD) due to molecular orientation introduced during biaxial stretching.
• PET has a glass transition temperature (Tg) of approximately 76 to 80°C. The CTE below Tg (α1) and above Tg (α2) differ significantly, and α2 can be several times larger than α1.
• CTE mismatch between PET and deposited or laminated layers, such as copper (approximately 17 ppm/°C), ITO, or adhesives, generates thermal stress that can cause curling, warpage, cracking, or delamination.

Accurate CTE measurement therefore supports material qualification, supplier comparison, thermal simulation input data, and process window definition for downstream manufacturing.

Testing Samples

Standard PET Film Samples

Typical commercial PET substrate films range from 12 μm to 250 μm in thickness. For thermomechanical analysis (TMA), strips are cut to the dimensions required by the instrument, commonly 5 to 10 mm in length and 3 to 5 mm in width. Samples must be flat, free of wrinkles, creases, and edge defects, and must be clearly marked with their orientation (MD or TD), since the two directions will yield different results.

Heat-Stabilized PET Film Samples

Heat-stabilized (thermo-stabilized) PET is pre-annealed under tension by the film manufacturer to suppress thermal shrinkage. For these grades, CTE testing should be combined with shrinkage measurement, because the first heating cycle may still show residual shrinkage that masks the true expansion behavior. A pre-test conditioning heat treatment, for example 30 minutes at 120°C or 150°C, may be applied and must be documented in the test report.

Coated, Metallized, and Laminated Samples

Samples may include ITO-coated PET, sputtered or evaporated metallization layers, adhesive-laminated structures, and copper-clad PET laminates for FPC applications. The measured CTE of such multilayer structures is a composite value dominated by the PET layer when the coating is thin. For printed circuit applications, testing is often performed according to board-level methods such as IPC-TM-650 method 2.4.24.

Sample Preparation and Conditioning

• Condition samples at 23 ± 2°C and 50 ± 5% relative humidity for a minimum of 24 hours in accordance with ISO 291 or ASTM D618, unless otherwise specified.
• When moisture influence must be excluded, dry samples in a vacuum oven at 50 to 60°C for 24 hours and test immediately in a dry nitrogen atmosphere.
• Cut samples cleanly with a sharp blade; burrs and non-parallel edges introduce measurement error in the contact direction.
• Prepare a minimum of three specimens per direction (MD and TD) and per material lot to assess variability.

Testing Items

A comprehensive PET substrate CTE test program typically includes the following items:

• Linear CTE below glass transition (α1), usually measured over a defined interval such as 30 to 70°C or 40 to 80°C.
• Linear CTE above glass transition (α2), measured over a defined interval above Tg, for example 100 to 150°C.
• CTE in machine direction (MD) and transverse direction (TD), reported separately due to anisotropy.
• Glass transition temperature (Tg), determined from the inflection point of the dimension-versus-temperature curve.
• Thermal shrinkage at process-relevant temperatures, for example 30 minutes at 120°C, 150°C, or 180°C, in MD and TD.
• Composite CTE of laminated or coated structures, when required for reliability modeling.

Testing Methods

Thermomechanical Analysis (TMA)

TMA is the primary method for PET film CTE measurement. A sample is placed on a quartz platform and contacted by a flat-tipped expansion probe under a small static force. The sample length change is recorded while the furnace applies a controlled temperature program. Applicable standards include ASTM E831, ISO 11359-2, and IPC-TM-650 method 2.4.24 for printed board materials. Typical test parameters are:

• Static force: 0.01 to 0.05 N, low enough to avoid compressive creep of the polymer but high enough to maintain probe contact.
• Heating rate: 3 to 5°C/min for good thermal equilibrium and resolution of the Tg inflection.
• Temperature range: commonly -50°C to 150°C for full characterization, or room temperature to 120°C for process-relevant α1 determination.
• Atmosphere: nitrogen purge at 20 to 50 mL/min to prevent oxidative degradation.
• Calibration: temperature calibration with indium or zinc standards, and length-change calibration using a reference material such as aluminum (23 ppm/°C).

Dilatometry

Push-rod dilatometry (ASTM E228) offers high precision for rigid solids and can be applied to PET by stacking multiple film layers to achieve the required specimen length. It is less common for thin films because of sample preparation complexity and load-induced deformation, but it serves as a useful cross-check for thicker PET sheets above 250 μm.

Optical and Non-Contact Methods

For ultra-thin films or free-standing measurement without mechanical loading, non-contact optical techniques are increasingly used. These include laser interferometry, digital image correlation (DIC), and optical extensometry, in which fiducial marks on the film are tracked while the sample is heated on a hot stage. Non-contact methods eliminate probe force artifacts and are well suited to films below 25 μm, where even minimal probe load can cause measurable creep.

Data Evaluation and Reporting

CTE is obtained from the slope of the linear regression of specimen dimension versus temperature over the specified interval. The test report should include the mean CTE with standard deviation, the exact temperature interval used for regression, sample orientation, thickness, thermal and humidity conditioning history, heating rate, applied force, instrument and standard referenced, and the number of specimens tested. For heat-stabilized grades, first- and second-heating results should be reported separately, since the second heating typically reflects the fully stabilized CTE.

Applications

• Flexible printed circuits (FPC): PET base films and coverlays require low CTE to prevent copper circuit deformation, delamination, and registration error during lamination and soldering processes.
• Flexible and foldable displays: PET substrates and barrier films used in OLED structures demand near-zero CTE to maintain layer alignment and prevent cracking of brittle inorganic barrier layers.
• Touch panels and sensor films: patterned ITO and metal mesh electrodes on PET depend on dimensional stability to preserve line width, pitch accuracy, and optical uniformity.
• Membrane switches and in-mold electronics: PET films undergo ink curing and thermoforming temperatures, so CTE and shrinkage data define allowable process windows.
• Photovoltaic backsheets and flexible solar modules: PET-based backsheets must match encapsulant and cell CTE to avoid long-term delamination under thermal cycling.
• Printed and additive electronics: RFID antennas, printed heaters, and stretchable interconnects use PET substrates where thermal expansion affects conductive ink adhesion and conductivity.

FAQ

What is a typical CTE value for PET substrate film?

Standard biaxially oriented PET film typically shows an α1 CTE of roughly 15 to 30 ppm/°C below Tg, with MD and TD values differing by up to 30 percent. Heat-stabilized PET grades are engineered to reach 5 ppm/°C or lower, and premium display-grade films approach near-zero expansion.

Why are MD and TD CTE values different?

Biaxial film manufacturing stretches the polymer melt in two perpendicular directions, aligning molecular chains anisotropically. This orientation makes the film expand differently along the machine direction versus the transverse direction, so both orientations must always be measured and reported.

Which standards govern PET substrate CTE testing?

The most commonly applied standards are ASTM E831 and ISO 11359-2 for thermomechanical analysis of polymer CTE, ASTM E228 for dilatometry, and IPC-TM-650 method 2.4.24 for CTE and Tg of printed board materials including PET-based flexible circuits.

What temperature range should be used for testing?

The range should cover the actual service and process conditions. A typical program runs from room temperature (or -50°C for full characterization) to 150°C, with α1 regression performed below Tg (for example 30 to 70°C) and α2 regression above Tg (for example 100 to 150°C).

How does moisture affect the CTE test result?

PET absorbs a small amount of moisture, which plasticizes the polymer and can shift the Tg inflection and slightly alter the measured slope. Standard conditioning at 23°C and 50% RH ensures comparability, while dried-sample testing in nitrogen is used when intrinsic material data are required.

What is the difference between CTE and thermal shrinkage?

CTE describes reversible dimensional change with temperature, whereas thermal shrinkage is an irreversible contraction caused by relaxation of oriented molecules, typically dominant above 100°C for non-stabilized PET. Heat-stabilized films reduce shrinkage dramatically, which is why both parameters are usually measured together.