This PET ceramic substrate coefficient of thermal expansion test report provides a complete technical evaluation of the thermal dimensional stability of PET-based ceramic substrates used in electronic packaging and flexible circuit manufacturing. The report covers sample descriptions, test items, thermomechanical analysis methods, representative results, and acceptance criteria in accordance with ASTM E831 and IPC-TM-650 standards.
Technical Overview
This test report documents the coefficient of thermal expansion (CTE) measurement program performed on PET ceramic substrates intended for high-reliability electronic applications. The coefficient of thermal expansion, expressed in parts per million per degree Celsius (ppm/°C), quantifies the fractional dimensional change of a material per unit temperature change and is one of the most decisive parameters governing the long-term reliability of laminated structures.
PET (polyethylene terephthalate) films and ceramic layers, such as alumina or aluminum nitride, exhibit fundamentally different expansion behaviors across the operating temperature range. When these materials are combined in a composite substrate, the mismatch in CTE generates internal stresses during solder reflow, thermal cycling, and power cycling. These stresses can manifest as warpage, interfacial delamination, conductor cracking, and solder joint fatigue. Accurate CTE characterization is therefore essential for material qualification, stack-up design, and process window definition.
All measurements described in this report were performed by thermomechanical analysis (TMA) in expansion mode, following ASTM E831, ISO 11359-2, and IPC-TM-650 Method 2.4.24. The test was conducted in an ISO/IEC 17025-accredited laboratory environment with calibrated reference materials traceable to national standards.
Testing Samples
PET Base Film Reference Sample
Biaxially oriented PET film with a nominal thickness of 125 µm was tested as the polymer reference layer. Three specimens were cut from different positions of the film roll to capture machine-direction and transverse-direction anisotropy. Each specimen measured 6 mm × 6 mm in the X–Y plane to fit the TMA expansion probe platform.
Ceramic Substrate Sample
Ninety-six percent alumina ceramic substrates with a nominal thickness of 0.38 mm and 0.635 mm were prepared as the rigid ceramic reference. Specimens were cut by laser scribing, and all edges were deburred and cleaned with isopropyl alcohol to eliminate mechanical damage that could affect probe contact.
PET–Ceramic Composite Substrate
The finished product under evaluation consisted of a laminated structure of PET adhesive film bonded to a metallized ceramic substrate. Five specimens were taken from different panels of the production lot to assess lot uniformity. All samples were conditioned at 23 ± 2 °C and 50 ± 5% relative humidity for a minimum of 24 hours prior to testing.
Testing Items
The test program covered the following thermal expansion parameters for each sample group:
- Coefficient of thermal expansion below the glass transition temperature (α1, ppm/°C)
- Coefficient of thermal expansion above the glass transition temperature (α2, ppm/°C)
- Glass transition temperature (Tg) determined from the TMA inflection point
- Through-thickness (Z-axis) CTE for the laminated composite structure
- Total percent expansion over the specified temperature range
- Repeatability assessment across multiple specimens and test cycles
Testing Methods
Instrumentation and Standards
A silica-based thermomechanical analyzer equipped with a flat expansion probe was used for all measurements. The instrument was calibrated for temperature using indium and zinc melting point standards, and for expansion coefficient using a certified aluminum oxide reference. Tests were run under a nitrogen purge of 45 to 55 mL/min to prevent oxidative degradation of the PET layer at elevated temperatures.
Test Conditions
Specimens were tested from −50 °C to 200 °C at a heating rate of 5 °C/min, consistent with ASTM E831. A static probe force of 0.05 N was applied to ensure firm contact without inducing creep in the polymer layer. Each specimen was first heated to 120 °C and cooled to erase prior thermal history, then measured over two consecutive heating cycles, with the second cycle reported as the result.
Representative Test Results
The alumina ceramic substrate exhibited a nearly linear expansion behavior with a mean CTE of 7.0 ppm/°C between 25 °C and 200 °C, confirming expected behavior for 96% alumina. The PET film showed a mean α1 of 17.5 ppm/°C below its glass transition at approximately 78 °C, followed by a marked increase to an α2 of 95 ppm/°C above Tg. The PET–ceramic composite substrate displayed a composite α1 of 11.8 ppm/°C, with the ceramic layer constraining the polymer expansion as predicted by rule-of-mixtures estimation.
- PET base film: α1 = 17.5 ppm/°C, α2 = 95 ppm/°C, Tg = 78 °C
- Alumina ceramic substrate: CTE = 7.0 ppm/°C (25 °C to 200 °C)
- PET–ceramic composite: α1 = 11.8 ppm/°C, Z-axis CTE = 42 ppm/°C
- Inter-specimen variation was below 4%, indicating good process uniformity
Analysis and Acceptance Criteria
The measured composite CTE fell within the design window of 9 to 14 ppm/°C, which keeps the substrate expansion closely matched to copper conductor and solder interconnect behavior. No delamination, blistering, or irreversible dimensional change was observed after two full thermal cycles, confirming interfacial adhesion integrity. The sample lot was therefore judged to have passed the CTE qualification requirement.
Applications
CTE-qualified PET ceramic substrates support reliability-critical applications where thermal cycling and dimensional stability govern performance. Typical end uses include:
- LED packaging modules requiring matched expansion between ceramic heat spreaders and polymer circuit layers
- Flexible hybrid electronics combining rigid ceramic islands with PET interconnect films
- Automotive power devices exposed to −40 °C to 125 °C operating environments
- Thick-film heater circuits and sensor platforms for medical and industrial instrumentation
- RF and microwave substrates where dimensional stability controls impedance consistency
FAQ
Q: Why is the CTE mismatch between PET and ceramic a concern?
A: PET expands roughly two to three times more than alumina below Tg, and more than ten times above Tg. This mismatch creates shear stress at the bonding interface during thermal excursions, which can cause delamination, warpage, and cracked conductors if not controlled through material selection and stack-up design.
Q: What temperature range should be used for CTE testing?
A: The range must bracket the intended service environment and any process excursions. For PET-based composites, testing from −50 °C to 200 °C captures the glass transition region, where the expansion behavior changes significantly, as well as typical solder reflow preheat conditions.
Q: How are CTE values calculated from TMA data?
A: The CTE is obtained from the slope of the dimensional change versus temperature curve over a defined interval. The mean CTE between two temperatures equals the total expansion divided by the temperature span and the initial specimen length, and values below and above Tg are reported separately as α1 and α2.
Q: What sample preparation is required before testing?
A: Specimens must have flat, parallel surfaces perpendicular to the probe, be free of burrs and residual stress, and be conditioned in a controlled environment for at least 24 hours. A thermal preconditioning cycle is also applied to remove fabrication history that would otherwise distort the first measurement.
Q: How often should CTE testing be repeated?
A: CTE qualification is required for initial material release, after any change in resin formulation, ceramic composition, lamination process, or supplier, and periodically per the incoming inspection plan defined in the product quality agreement.
