PET copper clad laminate coefficient of thermal expansion (CTE) testing is a critical quality control procedure for evaluating the dimensional stability of flexible printed circuit substrates under thermal stress. This comprehensive guide covers the experimental principles, sample preparation, testing standards such as ASTM E831 and IPC-TM-650, measurement procedures using thermomechanical analysis (TMA), and typical CTE values for PET-based copper clad laminates. Understanding these test methods helps engineers select reliable materials for flexible electronics, membrane switches, and high-density interconnect applications.
Technical Overview
PET copper clad laminate (CCL) is a composite material consisting of polyethylene terephthalate (PET) film laminated with electrodeposited or rolled copper foil on one or both sides. It is widely used as a base substrate for flexible printed circuits (FPC), membrane switches, and low-cost flexible electronic assemblies. The coefficient of thermal expansion (CTE) describes the fractional change in dimension per unit change in temperature, expressed in ppm/°C, and it is one of the most important parameters governing the dimensional stability of the laminate during soldering, lamination, and long-term thermal cycling.
The CTE experiment for PET copper clad laminates is typically performed because PET film exhibits strongly anisotropic and temperature-dependent expansion behavior. Below its glass transition temperature (Tg, approximately 78–80°C for standard PET), the in-plane CTE of PET film is typically 15–20 ppm/°C, which is reasonably well matched to copper at approximately 17 ppm/°C. However, above Tg, the PET molecular chains gain mobility and the CTE can rise dramatically to 80 ppm/°C or higher, creating significant thermal mismatch stress at the copper–polymer interface. A CTE mismatch between layers can cause warpage, delamination, copper cracking, and misregistration of circuit features during thermal processing.
Because the laminate is a layered composite, the experiment must distinguish between the in-plane CTE (X and Y directions), which controls dimensional stability and registration, and the through-thickness CTE (Z-axis), which affects plated through-hole and via reliability in multilayer constructions. Thermomechanical analysis (TMA) is the standard technique for these measurements.
Testing Samples
Bare PET Film Reference Samples
Bare PET film of the same grade and thickness used in the laminate is tested first to establish the baseline substrate expansion behavior. Typical specimens are cut to approximately 4–6 mm in width for through-thickness TMA testing, or 10 mm × 10 mm squares for in-plane measurement fixtures, depending on the instrument configuration.
Copper Clad Laminate Samples
Laminate specimens are cut from production panels in both machine direction (MD) and transverse direction (TD), because PET film is biaxially oriented and shows direction-dependent expansion. Both single-sided and double-sided constructions should be evaluated. The copper surface must be clean, flat, and free of oxidation that could affect probe contact.
Ettched Test Coupons
In some studies, the copper is selectively etched away from defined regions of the coupon so that the expansion of the bare PET window, the fully clad region, and the transition zone can be compared within a single specimen. This approach directly reveals the mechanical constraint imposed by the copper foil.
Sample Conditioning
- Condition all specimens at 23 ± 2°C and 50 ± 5% relative humidity for at least 24 hours before testing.
- Dry moisture-sensitive specimens at 60°C for 2 hours if the test protocol requires a moisture-free baseline, since absorbed water can plasticize PET and alter the apparent CTE.
- Cut specimen edges cleanly with a sharp blade or punch; rough or angled edges introduce geometric errors into length measurements.
- Verify specimen thickness at multiple points with a micrometer; record the average initial thickness and length (L0) for the CTE calculation.
Testing Items
The CTE experiment for PET copper clad laminate typically includes the following measurement items:
- In-plane CTE below Tg (alpha 1), measured from approximately 30°C to 70°C.
- In-plane CTE above Tg (alpha 2), measured from approximately 85°C to 150°C or the maximum continuous-use temperature.
- Through-thickness (Z-axis) CTE across the same temperature windows.
- Glass transition temperature (Tg) determined from the inflection point of the expansion curve.
- Dimensional change after thermal cycling, comparing heating and cooling curves to evaluate hysteresis and permanent shrinkage of the oriented PET film.
- Comparison of machine direction versus transverse direction expansion to quantify anisotropy.
- Effect of copper cladding ratio on the effective laminate CTE, comparing bare film, etched coupons, and fully clad samples.
Testing Methods
Thermomechanical Analysis (TMA) Principle
A thermomechanical analyzer measures the dimensional change of the specimen while it is heated at a controlled rate under a small static load. A quartz expansion probe with a flat tip applies a force of typically 5–20 mN, and a linear variable differential transformer (LVDT) or capacitive sensor records displacement with sub-micron resolution. The slope of the displacement-versus-temperature curve, divided by the initial specimen length, yields the CTE.
Applicable Standards
- ASTM E831 – Standard Test Method for Linear Thermal Expansion of Solid Materials by Thermomechanical Analysis.
- ISO 11359-2 – Plastics, thermomechanical analysis, determination of coefficient of linear thermal expansion and glass transition temperature.
- IPC-TM-650 Method 2.4.24 – TMA procedure for CTE and Tg of printed board materials.
- ASTM D696 – Dilatometer method, used as an alternative for larger specimens or verification measurements.
Experimental Procedure
- Calibrate the TMA for temperature and length using certified reference materials such as indium (melting point) and aluminum or sapphire (expansion reference).
- Measure the initial specimen length or thickness (L0) precisely at ambient temperature.
- Place the specimen on the quartz platform and lower the expansion probe with the specified static force; allow the system to stabilize for 5 minutes.
- Purge the furnace with nitrogen or dry helium at 30–50 mL/min to prevent copper oxidation at elevated temperatures.
- Run a first heating cycle from room temperature to the maximum test temperature (typically 150–170°C, kept below the PET melting range of 250–260°C) at 5°C/min, then discard this run to erase thermal history and release fabrication stress.
- Run the second heating cycle under identical conditions and use this curve for CTE calculation.
- Perform linear regression on the expansion curve over defined temperature windows below and above Tg to obtain alpha 1 and alpha 2.
- Calculate CTE using the formula: CTE = (dL / dT) / L0, expressed in ppm/°C or µm/(m·°C).
- Repeat the measurement on at least three specimens per condition and report the mean and standard deviation.
Typical Experimental Results
For a standard 50 µm PET film laminated with 18 µm electrodeposited copper, representative results include an in-plane CTE of approximately 15–20 ppm/°C below Tg and 60–100 ppm/°C above Tg in the bare film. The copper clad laminate shows a reduced effective in-plane CTE of approximately 15–25 ppm/°C below Tg because the copper layer mechanically constrains the polymer, while the Z-axis CTE remains significantly higher, often 80–150 ppm/°C below Tg and several hundred ppm/°C above Tg. The measured Tg from the expansion inflection typically falls between 75°C and 85°C.
Error Sources and Precautions
- Probe penetration into soft PET above Tg can be mistaken for expansion; minimize the static force and verify with the first-cycle comparison.
- Moisture absorbed by PET lowers the apparent Tg and shifts the CTE; control conditioning rigorously.
- Curled or warped FPC specimens create poor probe contact; flatten samples gently under light pressure at room temperature before testing.
- Residual stresses from lamination cause irreversible shrinkage on first heating; always use the second heating run for reporting.
- Temperature gradients within thick specimens can be reduced by using slower heating rates of 3–5°C/min.
Applications
The PET copper clad laminate CTE experiment supports material qualification and product reliability across multiple industries:
- Flexible printed circuits for consumer electronics, where CTE data ensures registration accuracy during roll-to-roll processing and solder reflow.
- Membrane switch and keypad manufacturing, where thermal stability guarantees long-term tactile performance and adhesion retention.
- Flexible photovoltaic module substrates and lithium battery interconnects, where laminates endure repeated thermal cycling in the field.
- Automotive interior electronics, where temperature swings from -40°C to +85°C demand predictable dimensional behavior.
- Display driver interconnects and camera module flex assemblies, where fine-pitch traces require minimal thermal misalignment.
- Material development programs, where CTE results guide the selection of adhesive systems, filler loading, and copper foil thickness to minimize thermal mismatch.
Engineers use the measured CTE values to build finite element thermal stress models, define solder reflow profiles, and set process window limits for lamination temperature and pressure. Laminate suppliers also publish CTE specifications derived from this experiment as part of datasheet characterization.
FAQ
What is a typical CTE value for PET copper clad laminate?
Below Tg, the in-plane CTE of PET copper clad laminate is typically 15–25 ppm/°C, closely matching the 17 ppm/°C of copper foil. Above Tg, the effective in-plane CTE rises sharply, and the Z-axis CTE is substantially higher, often exceeding 100 ppm/°C even below Tg.
Why does the CTE of PET increase dramatically above its glass transition temperature?
Above Tg, the amorphous regions of the PET polymer gain segmental mobility, allowing chains to rearrange and expand much more freely. In oriented PET film, relaxation of molecular orientation also contributes to additional dimensional change.
Which test standard should be used for the experiment?
ASTM E831 or ISO 11359-2 covers general TMA CTE measurement of plastics, while IPC-TM-650 Method 2.4.24 is specifically designed for printed circuit board base materials and is commonly referenced in the electronics industry.
How does the copper foil affect the measured laminate CTE?
Copper, with its low CTE and high elastic modulus, mechanically constrains the PET layer, reducing the effective in-plane CTE of the laminate compared with bare PET film. The degree of reduction depends on the copper-to-polymer thickness ratio.
Why is the first heating run discarded in the test procedure?
The first heating cycle relieves residual stresses introduced during film orientation and lamination and produces irreversible shrinkage. A second heating run reflects the equilibrium, repeatable expansion behavior of the material.
What specimen size is required for TMA CTE testing?
For through-thickness measurements, specimens 3–6 mm wide with a flat, parallel top and bottom surface are sufficient. In-plane measurements require longer strips or dedicated in-plane fixtures, typically 10 mm or greater in the measurement direction.
What temperature range should the experiment cover?
A practical range is 25°C to 150–170°C, which captures both alpha 1 below Tg and alpha 2 above Tg while remaining safely below the PET melting point of approximately 250–260°C.