High-Temperature Coefficient of Thermal Expansion (CTE) Testing of PET Ceramic Substrates

The coefficient of thermal expansion (CTE) of PET ceramic substrates is a decisive parameter governing dimensional stability, warpage behavior, and interconnect reliability in electronic packaging and thermal management applications. High-temperature CTE determination, performed primarily by thermomechanical analysis (TMA) and push-rod dilatometry in accordance with ASTM E831, ISO 11359-2, and IPC-TM-650, provides accurate expansion data across the entire operating range of ceramic-filled PET composites. This article explains the testing principle, sample requirements, measured parameters, experimental methods, and typical applications of high-temperature CTE measurement for PET ceramic substrates.

Technical Overview

PET ceramic substrates are polymer-ceramic composite materials in which a polyethylene terephthalate (PET) film, resin, or matrix is filled, coated, or laminated with ceramic components such as alumina (Al2O3), fused silica (SiO2), or other functional ceramic fillers. These substrates combine the flexibility, low weight, and processability of PET with the enhanced thermal conductivity, dielectric tuning, and dimensional stability contributed by the ceramic phase, making them widely used in flexible circuits, LED modules, and high-frequency electronic assemblies.

The coefficient of thermal expansion (CTE) quantifies the fractional change in length of a material per unit temperature change, defined as alpha = (1/L0) x (dL/dT), and is conventionally expressed in ppm/degree Celsius (10^-6/K). Because PET exhibits a glass transition at approximately 70 to 85 degrees Celsius, its expansion behavior is strongly temperature dependent: below Tg the CTE of neat PET film is typically on the order of 15 to 25 ppm/degree Celsius, while above Tg it can rise to well over 60 ppm/degree Celsius. Incorporating ceramic fillers at loading levels of 40 to 70 weight percent suppresses the effective CTE, often bringing it into the range of roughly 8 to 20 ppm/degree Celsius and significantly reducing the step change at the glass transition.

High-temperature CTE measurement is essential because substrate expansion mismatches with copper foil (about 17 ppm/degree Celsius), solder joints, silicon die (about 2.6 ppm/degree Celsius), and ceramic heat sinks drive warpage, delamination, and thermo-mechanical fatigue in service. Accurate expansion data above ambient temperature, through the glass transition, and up to the upper service or processing limit of the material allows designers to model reliability and select substrates whose expansion behavior matches the surrounding assembly.

Testing Samples

Ceramic-Filled PET Composite Films

These are flexible substrate sheets in which ceramic powder is dispersed throughout the PET matrix. Specimens are cut along both the machine direction (MD) and transverse direction (TD) to characterize CTE anisotropy introduced by film orientation during processing. Typical thickness ranges from 0.05 to 0.5 mm.

Ceramic-Coated and Laminated PET Substrates

These samples consist of PET film carrying a ceramic coating, a sintered ceramic layer, or a bonded ceramic-polymer laminate. The composite expansion response reflects the relative thickness and modulus of each layer, so specimen preparation must preserve the as-manufactured layer structure without edge damage.

Metallized and Multilayer Specimens

Copper-clad PET ceramic laminates and multilayer builds can be tested either in the as-received state, to obtain the effective laminate CTE relevant to circuit performance, or after etching away the metal, to isolate the substrate contribution per IPC test practice.

Specimen Geometry and Conditioning

For TMA, specimens are typically 2 to 10 mm in length with two flat, parallel ends and a thickness of up to several millimeters. For push-rod dilatometry, specimens of 10 to 25 mm in length are preferred. All samples should be conditioned at 23 +/- 2 degrees Celsius and 50 +/- 5 percent relative humidity for 24 to 48 hours, or pre-dried where moisture effects are a concern, since absorbed moisture softens PET and shifts the apparent transition behavior.

Testing Items

The high-temperature CTE test program for PET ceramic substrates typically reports the following parameters:

• Linear CTE below the glass transition (alpha 1), measured over a specified temperature window below Tg.
• Linear CTE above the glass transition (alpha 2), reflecting the softened polymer state critical for soldering and processing excursions.
• Glass transition temperature (Tg), identified from the inflection in the expansion curve.
• Total thermal expansion or shrinkage (delta L / L0) over the full test range.
• Through-thickness (z-axis) CTE for laminates, which governs via-hole and interlayer reliability.
• In-plane CTE anisotropy, comparing machine direction and transverse direction values.
• Secant (mean) CTE between defined temperature pairs, as required by many material specifications.

Testing Methods

Thermomechanical Analysis (TMA)

TMA is the primary method for PET ceramic substrates. A flat-ended expansion probe applies a small static force, typically 10 to 50 mN to avoid indentation of the softened polymer above Tg, while the specimen is heated at a controlled rate of 5 degrees Celsius per minute under nitrogen purge. Length change is recorded with a linear variable differential transformer, and CTE is extracted from the slope of the expansion curve using tangent or secant analysis. TMA testing follows ASTM E831, ISO 11359-2, and IPC-TM-650 method 2.4.24 for printed board materials.

Push-Rod Dilatometry

For thicker, more rigid, or ceramic-rich specimens, horizontal or vertical push-rod dilatometers provide measurement capability from room temperature up to 1000 degrees Celsius or higher under ASTM E228 or ISO 7991. This method is preferred when characterizing the ceramic layer itself or evaluating dimensional stability during simulated high-temperature processing.

Laser Interferometry and Optical Dilatometry

Non-contact interferometric techniques, covered by ASTM E289, offer sub-micron resolution suitable for very thin PET films and for resolving small CTE differences between material lots. Optical dilatometry and video extensometry are useful when probe loading must be eliminated entirely.

Calibration and Data Processing

System calibration is performed against certified reference materials such as sapphire and fused silica over the relevant temperature range. Where thermal history effects are significant, a first heating scan may be run to erase residual stress, followed by a controlled second scan used for CTE reporting. The mean CTE between two temperatures is calculated as (L2 - L1) / (L0 x (T2 - T1)), while the instantaneous CTE is obtained from the derivative of the expansion curve. Typical measurement uncertainty is on the order of plus or minus 0.1 to 0.5 ppm/degree Celsius, or 5 to 10 percent of the reported value, whichever is greater.

Applications

High-temperature CTE data for PET ceramic substrates directly supports design, qualification, and reliability engineering across multiple industries:

• Flexible and rigid-flex printed circuits, where CTE matching with copper and coverlay films prevents warpage and cracked traces.
• LED packaging and thermal management, where ceramic-filled PET substrates must maintain dimensional stability during solder reflow at peak temperatures of 240 to 260 degrees Celsius.
• Automotive electronics, including cabin and under-hood modules exposed to sustained elevated temperatures and thermal cycling.
• High-frequency and 5G antenna substrates, where ceramic loading adjusts dielectric properties while the measured CTE ensures stable resonator geometry.
• Chip-on-board and semiconductor packaging, where substrate expansion relative to silicon die controls solder joint and bond wire fatigue life.
• Finite element analysis and lifetime modeling, for which temperature-dependent CTE curves are a mandatory material input.

FAQ

What temperature range is used for high-temperature CTE testing of PET ceramic substrates?

PET-based substrates are typically tested from room temperature to 200 or 260 degrees Celsius, staying below the melting point of PET at approximately 250 to 260 degrees Celsius. Ceramic layers or highly ceramic-loaded composites can be evaluated up to 300 to 1000 degrees Celsius using dilatometry.

Why do test reports show two CTE values?

PET undergoes a glass transition during heating, producing a distinct change in expansion slope. The CTE below Tg (alpha 1) and above Tg (alpha 2) describe the rigid and softened polymer states, and both are needed to predict behavior across real processing and service profiles.

Which standard should be applied?

ASTM E831 or ISO 11359-2 is appropriate for TMA measurement of most specimens, IPC-TM-650 method 2.4.24 is specific to printed board and substrate materials, and ASTM E228 covers push-rod dilatometry for higher-temperature or ceramic-dominant samples.

How are specimens prepared to ensure valid results?

Specimens must have flat, parallel, and perpendicular end faces, be free of delamination or edge chips, and be conditioned in a controlled atmosphere before testing. Machine direction and transverse direction samples should both be tested for oriented films.

What accuracy can be expected?

With proper calibration and conditioning, TMA and dilatometry repeatability within the same laboratory is typically better than plus or minus 3 percent. Interlaboratory agreement depends on probe load, heating rate, and temperature range definition, so these parameters must always accompany the reported CTE values.