Step-by-Step Procedure for Measuring the Coefficient of Thermal Expansion of PET PCB Boards

The coefficient of thermal expansion (CTE) is a critical parameter that determines the dimensional stability and long-term reliability of PET-based printed circuit boards. This article provides a comprehensive, step-by-step technical guide for measuring the CTE of PET PCB substrates using thermomechanical analysis (TMA) and dilatometry, covering sample preparation, conditioning, test conditions, calculation methods, and relevant industry standards such as ASTM E831, ISO 11359-2, and IPC-TM-650.

Technical Overview

PET (polyethylene terephthalate) is a thermoplastic polyester film widely used as a flexible substrate material for printed circuit boards, membrane switches, and flexible electronic assemblies. Because PET is a semi-crystalline polymer, its dimensions change measurably with temperature, and this behavior is quantified by the coefficient of thermal expansion (CTE). For PET-based PCB laminates, the in-plane CTE typically ranges from approximately 15 to 30 ppm/°C, which is considerably higher than that of rigid FR-4 laminates and closer to the CTE of copper (approximately 17 ppm/°C).

Accurate CTE determination is essential for several reasons. First, a large mismatch between the CTE of the PET substrate and the copper traces or solder joints generates thermomechanical stress during thermal cycling, which can cause trace cracking, delamination, or pad lifting. Second, in multilayer flexible circuits, CTE mismatch between adhesive layers, copper, and PET film drives via and interconnect failures. Third, CTE data is required to predict dimensional changes during reflow soldering and to control layer-to-layer registration accuracy in high-density flex designs.

CTE is defined as the fractional change in length per unit change in temperature and is expressed in units of ppm/°C (×10⁻⁶/°C). For polymeric substrates, two distinct linear regions are observed: alpha 1 (CTE below the glass transition temperature, Tg) and alpha 2 (CTE above Tg, which is typically two to four times higher). PET films also exhibit anisotropic expansion behavior, meaning the machine direction (MD) and transverse direction (TD) CTE values may differ due to film stretching during manufacturing.

Testing Samples

Bare PET Film Substrate Samples

Representative samples of the raw PET film should be tested to establish the baseline expansion behavior of the polymer substrate. For in-plane CTE measurement, specimens are cut as rectangular strips measuring approximately 3 to 7 mm in width and 10 to 20 mm in length, with the long axis aligned parallel to either the machine direction or the transverse direction of the film. Film thicknesses of 25 μm to 125 μm are typical for flexible PCB applications.

Copper-Clad PET Laminates

Samples of the adhesive-laminated copper-clad PET (with common copper weights such as 18 μm or 35 μm) should be tested to evaluate the effective composite CTE. If the application requires the substrate-only contribution, the copper foil may be chemically etched away prior to testing while leaving the adhesive layer intact, since the adhesive also influences the overall expansion behavior.

Finished Flexible PCB Samples

Circuit-patterned samples may be tested to verify the CTE of the actual product configuration. Care must be taken to cut specimens from regions with uniform construction, avoiding areas containing stiffeners, connectors, or dense copper planes that would locally alter the expansion response.

Z-Axis (Through-Thickness) Samples

For through-thickness CTE measurement, which is relevant for via reliability, stacked specimens or thicker laminate sections are prepared so that the total thickness is sufficient for reliable measurement, typically at least 0.5 mm. Multiple film layers may be laminated together, or a disc-shaped specimen may be punched with a diameter that fits the TMA platform, commonly 3 to 10 mm.

Testing Items

The following parameters are determined during a complete CTE characterization of PET PCB materials:

• In-plane CTE in the machine direction (X-axis) below and above Tg
• In-plane CTE in the transverse direction (Y-axis) below and above Tg
• Through-thickness CTE (Z-axis) below and above Tg
• Glass transition temperature (Tg) identified from the inflection point of the dimension-versus-temperature curve
• Dimensional change rate and irreversible shrinkage after the first heating cycle
• CTE hysteresis behavior comparing the first and second heating scans

Testing Methods

Method Selection and Applicable Standards

Thermomechanical analysis (TMA) is the preferred method for thin PET films and PCB laminates because it requires small specimens and provides excellent sensitivity at the microstrain level. The TMA method is performed in accordance with ASTM E831 or ISO 11359-2 for general polymer CTE determination, while IPC-TM-650 method 2.4.24 is applied specifically for the Z-axis CTE of printed board materials. Push-rod dilatometry (ASTM E228 or ASTM D696) may be used for thicker or stiffer specimens, and optical or strain-gauge methods can be used as verification techniques for in-plane expansion.

Step 1: Sample Preparation

Cut the specimen strips using a sharp die, razor blade, or precision cutter to avoid edge defects and residual stress. Verify that the cut edges are straight and perpendicular, as burrs or ragged edges introduce measurement artifacts. Measure the initial length (L₀) of each specimen at reference temperature using a calibrated optical comparator or micrometer, recording the value to within ±0.01 mm. Mark the orientation of each specimen relative to the machine direction of the film.

Step 2: Specimen Conditioning

Condition all specimens in a controlled environment of 23 ± 2 °C and 50 ± 5% relative humidity for a minimum of 24 hours (40 hours after machining in some specifications) to ensure moisture and temperature equilibrium. This step is critical because PET absorbs a small amount of moisture, and unconditioned specimens can show anomalous dimensional drift during the early portion of the test.

Step 3: Instrument Calibration

Calibrate the TMA furnace temperature using indium and zinc melting point standards, and verify the length-change measurement using a calibrated reference specimen of known CTE, such as quartz or a certified reference material. Perform a baseline run with a zero-expansion reference (fused silica) to correct for instrument expansion of the probe and platform assembly.

Step 4: Specimen Loading and Preload Application

Place the conditioned specimen on the TMA stage and lower the expansion probe onto the specimen surface at its center. Apply a small static preload force, typically 0.02 to 0.1 N, sufficient to maintain probe contact without indenting or deforming the PET film. For thin films, use a macro-expansion probe or flat quartz probe to distribute the load and prevent localized creep. Allow the specimen to equilibrate at the initial temperature (typically 20 to 30 °C, or −50 °C for sub-ambient programs) for 5 to 10 minutes before starting the ramp.

Step 5: Temperature Programming

Select a heating rate of 3 to 5 °C/min, which is the standard rate that balances thermal equilibrium with practical test duration, although rates up to 10 °C/min are permitted in some specifications. Program the temperature range from ambient up to a maximum of 150 to 200 °C, ensuring the upper limit remains safely below the melting temperature of PET (approximately 250 °C) and avoids degradation. If the glass transition region must be fully characterized, extend the hold at the upper limit and run a second heating cycle after controlled cooling at 5 °C/min to obtain a reproducible CTE value free of thermal history effects.

Step 6: Data Acquisition

Record the dimensional change (ΔL) as a function of temperature continuously throughout the scan. The resulting thermomechanical curve displays the specimen length change on the vertical axis and temperature on the horizontal axis. Identify the linear expansion regions below Tg (alpha 1) and above Tg (alpha 2) and mark the glass transition as the temperature at the inflection point where the slope changes.

Step 7: CTE Calculation

Calculate the mean coefficient of linear thermal expansion using the formula CTE = ΔL / (L₀ × ΔT), where ΔL is the length change measured over the temperature interval ΔT, and L₀ is the original specimen length at the reference temperature. Perform a least-squares linear regression on the dimension-versus-temperature data within each defined temperature window (for example, 30 to 80 °C for alpha 1) and take the slope as the CTE value. Report the result in ppm/°C to two significant figures, and always report the temperature interval over which each CTE value was calculated because CTE varies with temperature.

Step 8: Reporting and Acceptance Criteria

Report the specimen description, orientation, thickness, conditioning history, test standard used, heating rate, preload force, temperature range, Tg, alpha 1 CTE, alpha 2 CTE, and the average and standard deviation of at least three replicate specimens. For PET flexible PCB substrates, typical acceptance criteria specify an in-plane alpha 1 CTE below approximately 30 ppm/°C, with tighter requirements for high-density applications where registration tolerance is critical.

Quality Control Considerations

• Run at least three specimens per direction and per material lot to characterize variability
• Use the second heating scan when thermal history effects must be eliminated from the reported value
• Verify that no specimen slippage, probe penetration, or softening occurred by inspecting the curve for discontinuities
• Confirm that the measured Tg of PET (typically 70 to 85 °C) falls within the expected range as a validity check on the test
• Maintain nitrogen purge flow (approximately 50 mL/min) through the furnace to prevent oxidative degradation of the PET at elevated temperatures

Applications

CTE measurement data for PET PCB boards directly supports design and manufacturing decisions across multiple flexible electronics sectors:

• Flexible printed circuits and flat flexible cables (FFC) in smartphones, laptops, and camera modules, where CTE data ensures reliable copper trace adhesion during thermal cycling
• Membrane switches and flexible keyboards that must withstand repeated heating from LEDs and environmental exposure
• RFID antenna inlays on PET substrates, where dimensional stability preserves antenna resonance tuning
• Automotive sensor circuits and lighting modules subjected to −40 °C to +125 °C operating environments
• Medical sensors and wearable devices where laminated PET assemblies experience body-heat and sterilization temperature swings
• Photovoltaic junction box flex circuits and display driver attachments requiring controlled solder joint stress
• Incoming material qualification and supplier lot acceptance testing for copper-clad PET laminate procurement

FAQ

What is a typical CTE value for PET PCB substrates?

Oriented PET films used in flexible PCBs typically exhibit an in-plane alpha 1 CTE of approximately 15 to 25 ppm/°C in the machine direction and 20 to 30 ppm/°C in the transverse direction. Above Tg, the alpha 2 CTE can rise to 60 to 150 ppm/°C, which is why operating temperature must remain well below the glass transition for dimensional stability.

Why is the Z-axis CTE usually higher than the in-plane CTE?

In film-based laminates, molecular chain orientation during film extrusion and stretching constrains in-plane expansion while allowing greater chain spacing changes through the thickness. As a result, the Z-axis CTE of PET-based laminates, often 50 to 100 ppm/°C or more below Tg, substantially exceeds the in-plane values, which affects plated through-hole and via reliability.

Which standard should be followed for PET PCB CTE testing?

ASTM E831 and ISO 11359-2 govern TMA-based CTE measurement of solid plastics, IPC-TM-650 method 2.4.24 covers the Z-axis CTE of printed wiring board materials, and ASTM D696 or E228 apply to dilatometer methods. Most PCB material datasheets reference IPC-TM-650 for qualification testing.

Why do the first and second heating scans give different CTE results?

The first heating scan of PET releases residual stresses introduced during film manufacturing, lamination, and specimen cutting, and may include irreversible shrinkage and moisture-driven dimensional changes. A second heating scan after controlled cooling provides a reproducible, thermally stabilized CTE value, and both results are frequently reported to document the stress-relief behavior.

Can CTE testing predict soldering performance of PET boards?

CTE data combined with Tg and decomposition temperature provides a strong indicator of thermomechanical behavior during reflow. Because PET softens near its Tg of 70 to 85 °C, most PET-based circuits are assembled with low-temperature solders or conductive adhesives, and CTE data helps engineers select assembly processes that keep expansion-induced strain within acceptable limits.