Thermal Expansion Performance Evaluation of PET PCB Boards

PET PCB boards built on polyethylene terephthalate film provide a low-cost, lightweight platform for flexible circuits, but the polymer substrate expands measurably with heat, making thermal expansion performance evaluation essential for product reliability. This article explains the thermal expansion characteristics of PET substrates, the sample types and test items involved, the standard measurement methods such as TMA-based CTE testing, and the application scenarios where this evaluation matters most.

Technical Overview

Polyethylene terephthalate (PET) film is one of the most widely used base materials for low-cost flexible printed circuits, membrane switches, and printed antennas. Like all polymer substrates, PET expands and contracts with changes in temperature, and this dimensional movement is quantified by the coefficient of thermal expansion (CTE). For biaxially oriented PET films commonly used in circuit fabrication, the in-plane CTE below the glass transition temperature is typically in the range of 15 to 25 ppm/°C, which is reasonably close to that of copper foil at approximately 17 ppm/°C. This partial match helps limit stress at the copper-to-substrate interface during moderate temperature excursions.

The critical limitation of PET is its relatively low glass transition temperature of approximately 70 to 80°C. Above Tg, the molecular chains gain mobility, and the CTE can increase severalfold, while the material begins to soften and lose mechanical strength. In a finished circuit, excessive thermal expansion relative to the copper conductor can produce warpage, adhesive delamination, trace cracking, and loss of registration between layers or assembled components. Evaluating the thermal expansion behavior of PET PCB boards therefore focuses on determining CTE below and above Tg, identifying the softening onset, and verifying dimensional stability through repeated thermal cycles.

Testing Samples

Bare PET Substrate Film

Unclad PET film, typically in thicknesses of 50 μm, 75 μm, 125 μm, or 188 μm, is tested to establish the baseline thermal expansion characteristics of the raw dielectric material. Samples are cut in both the machine direction and transverse direction, because oriented PET films exhibit anisotropic expansion behavior.

Copper-Clad PET Laminate

Copper-clad samples consist of electrodeposited or rolled-annealed copper foil bonded to PET film through an acrylic or epoxy adhesive layer. These laminates are evaluated to determine how the composite structure expands as a whole and how the CTE mismatch between copper, adhesive, and PET affects flatness and adhesion at elevated temperature.

Finished PET Flexible Circuits

Etched and patterned circuits, including those with coverlayers, stiffeners, or carbon-based printed resistors, are tested in their end-use configuration. This sample type reveals the combined effects of patterning, layer count, and protective coatings on overall thermal expansion and long-term reliability.

Testing Items

A comprehensive thermal expansion evaluation of PET PCB boards typically covers the following parameters:

• In-plane coefficient of thermal expansion (X and Y axes) below the glass transition temperature (alpha 1)
• In-plane coefficient of thermal expansion above the glass transition temperature (alpha 2)
• Z-axis thermal expansion for multilayer or adhesive-containing constructions
• Glass transition temperature and softening onset point
• Dimensional stability before and after thermal exposure or thermal cycling
• Peel strength retention of copper foil after thermal aging
• Resistance to delamination, blistering, and wrinkling under heat
• Continuity of conductive traces after repeated expansion and contraction cycles
• Influence of moisture absorption on expansion behavior, since PET absorbs humidity and swells

Testing Methods

Thermomechanical Analysis (TMA)

TMA is the primary method for measuring CTE and Tg of PET materials, following IPC-TM-650 method 2.4.24 and ASTM E831. A sample of known length is placed in the TMA furnace, and a low-force probe tracks dimensional change while temperature ramps at a controlled rate, commonly 5 or 10°C/min. The slope of the expansion-versus-temperature curve yields alpha 1 below Tg and alpha 2 above Tg, and the inflection point identifies the glass transition. Because PET softens near its Tg, the maximum test temperature is limited to prevent probe penetration.

Dynamic Mechanical Analysis (DSC and DMA)

Differential scanning calorimetry per IPC-TM-650 method 2.4.25 and dynamic mechanical analysis per ASTM E1640 are used to confirm the glass transition temperature and to assess changes in modulus and damping behavior across the service temperature range. These results help define the practical upper operating temperature of the PET circuit.

Thermal Cycling and Thermal Shock

Finished circuits are subjected to repeated cycling between low and high temperatures, for example -40°C to +85°C for 100 to 500 cycles, with resistance monitoring and visual inspection for cracking, delamination, or adhesive failure. Thermal shock testing with rapid transitions accelerates failure mechanisms driven by CTE mismatch between copper and PET.

Dimensional Stability Measurement

Flat specimens are marked with reference targets, measured precisely, then exposed to defined thermal conditions and remeasured in accordance with ASTM D1204. The permanent change in dimensions after conditioning indicates whether the film or laminate will hold registration during subsequent processing steps and during the product lifetime.

Applications

Evaluation of thermal expansion performance directly determines where PET PCB boards can be deployed with confidence. Typical application areas include:

• Membrane switches and sealed user-interface keypads for appliances and industrial equipment
• RFID antennas and smart labels requiring low cost and moderate thermal exposure
• Flexible interconnects and jumper circuits in consumer electronics
• Medical diagnostic sensors, disposable test strips, and wearable devices
• Automotive interior electronics such as seat heater controls and dashboard switch panels
• Electroluminescent backlighting circuits and low-temperature heating elements
• Battery pack sensing circuits and thermal management wiring in sealed assemblies

In each of these applications, verified CTE data and thermal cycling results allow designers to set safe operating envelopes, select compatible assembly methods, and avoid field failures caused by substrate movement.

FAQ

What is the typical CTE of a PET PCB substrate?

Biaxially oriented PET film used for flexible circuits generally shows an in-plane CTE of about 15 to 25 ppm/°C below its glass transition temperature. Above Tg, the CTE can rise to several times this value, which is why the operating temperature of PET circuits is normally kept well below 105 to 120°C.

Why does thermal expansion of PET increase sharply above Tg?

Above the glass transition temperature of roughly 70 to 80°C, the amorphous regions of the polymer gain segmental mobility. The free volume increases rapidly with temperature, producing a much higher expansion rate along with softening and reduced modulus.

Can PET PCB boards withstand soldering temperatures?

Standard reflow and wave soldering temperatures of 217 to 260°C exceed the limits of PET, which melts at approximately 250 to 260°C. PET circuits are instead assembled using anisotropic conductive film, heat-seal connectors, low-temperature solders, conductive adhesives, or mechanical crimping.

How does PET compare with polyimide for flexible circuits?

Polyimide offers a far higher temperature capability, stable CTE, and solderability, while PET provides significantly lower cost, good dielectric properties, and adequate performance for moderate-temperature applications. Thermal expansion evaluation helps confirm whether PET is sufficient for a given design without the added cost of polyimide.

Which test method is most authoritative for CTE evaluation?

Thermomechanical analysis is the industry-standard technique, performed per IPC-TM-650 method 2.4.24 or ASTM E831. It simultaneously provides alpha 1, alpha 2, and the glass transition temperature from a single controlled heating run.