PI Film vs PET Film: Which Insulating Film Is Better for Your Electronics Application?

Introduction

Polyimide (PI) film and polyester (PET) film are the two most widely used insulating film materials in the electronics and electrical industries. PI film is renowned for its exceptional high/low-temperature performance and dimensional stability, finding extensive use in flexible printed circuits (FPC), aerospace wire insulation, and high-end motor insulation. PET film dominates consumer electronics, packaging, and general industrial insulation with its excellent cost-performance ratio. The price gap between the two can reach 5–20×, making material selection critically impactful on cost control. This article provides a systematic comparison across four dimensions: temperature resistance, electrical properties, mechanical properties, and cost.

1. Material Properties Comparison

2. In-Depth Performance Comparison

2.1 Temperature Resistance

The most outstanding characteristic of PI film is its extreme temperature stability. It can be used long-term from –269°C (liquid helium temperature) to +400°C, and can withstand temperatures above 500°C for short periods (before carbonization), with a UL temperature index of 220°C (Class H insulation material). PET film’s long-term service temperature is only –70 to +150°C; noticeable thermal shrinkage begins above 160°C, and melting/flow occurs above 180°C. This gap determines PI’s irreplaceability in extreme temperature environments such as aerospace, automotive engine compartments, and downhole logging.

2.2 Electrical Insulation Properties

Both films achieve dielectric strengths above 200 kV/mm, ranking among excellent insulation grades. PET’s dielectric strength is slightly higher than PI (280–350 vs. 220–300 kV/mm), giving it an edge in general electrical insulation. PI’s dielectric constant (3.4–3.8) is slightly higher than PET (3.0–3.4), and its dissipation factor is also somewhat higher, but the impact on signal integrity in high-frequency/high-speed circuits remains within an acceptable range. Notably, PI film’s dielectric properties remain stable across a wide temperature range (–200 to +300°C), which PET cannot match.

2.3 Mechanical Properties & Dimensional Stability

PI film’s elastic modulus (2.5–3.5 GPa) is slightly lower than PET (3.0–4.5 GPa), but its elongation at break is also lower (40–80% vs. 80–150%), exhibiting higher dimensional stability — after 2 hours at 230°C, PI’s dimensional change rate is <0.3%, while PET shows significant shrinkage. PI's coefficient of thermal expansion (CTE) can be tuned via molecular design to approach that of metals (~20×10⁻⁶/°C), which is critical in high-density interconnect (HDI) and chip packaging for reducing thermally induced stress failures.

2.4 Water Absorption & Environmental Durability

PI film’s water absorption (1.5–3.0%) is significantly higher than PET (0.4–0.8%), which is PI’s primary weakness — after moisture absorption, dielectric constant increases and slight dimensional expansion occurs, requiring pre-baking treatment in high-precision applications. PET has low moisture absorption and performs more stably in humid environments. However, in radiation resistance, PI film performs exceptionally well (withstanding doses >10⁷ Gy), making it suitable for space environments; PET degrades rapidly under UV and γ-ray exposure, rendering it unsuitable for outdoor or aerospace applications.

3. Application Scenarios

3.1 Where PI Film Excels

• Flexible Printed Circuits (FPC): Smartphones, wearables — leveraging high-temperature resistance (SMT reflow 260°C) and dimensional stability
• Aerospace wire & cable insulation: Satellites, rockets — leveraging extreme temperature resistance, radiation resistance, and low outgassing
• Motor and transformer insulation: NEV drive motors (Class H+ temperature rating) — leveraging long-term 200°C+ temperature capability
• Semiconductor packaging: COF (Chip-on-Film), TAB carriers — leveraging low CTE matching silicon chips
• Thermal/acoustic insulation: High-speed rail, aircraft interiors — leveraging low outgassing and flame resistance (self-extinguishing)
• High-temperature labels/tapes: PCB processing carriers — leveraging chemical resistance + high-temperature resistance

3.2 Where PET Film Excels

• Consumer electronics insulation & structure: Cell battery separators, capacitor films — leveraging high dielectric strength and low cost
• General wire & cable insulation: Appliance wiring, low-voltage cables — leveraging good insulation and cost-performance
• Industrial tape substrates: Electrical tapes, packaging tapes — leveraging high tensile strength and low cost
• Food packaging: Retort pouches, vacuum packaging — leveraging high barrier properties, transparency, and heat-sealability
• Solar panel backsheets: PV modules — leveraging weather resistance (with coated treatment) and insulation
• Flexible display substrates (modified PET): Low-end flexible screens — leveraging high transparency and low cost

3.3 Hybrid Approach

In certain applications, PI and PET can be used in combination. Typical example: FPC stiffeners — PI in dynamic bending zones, PET in static reinforcement zones, balancing reliability and cost. Another case: motor insulation systems — PET for slot insulation (cost-optimized), PI for inter-turn insulation (temperature guarantee); hybrid design can reduce material costs by 30–50%.

4. Cost-Effectiveness Assessment

PI film costs 10–20× more than PET — the biggest barrier in material selection. However, from a TCO perspective: in applications requiring >150°C temperature resistance, radiation resistance, or extreme dimensional stability, PI is the only choice — no “alternative” exists. In general applications with <130°C temperature requirements, PET has sufficient performance headroom, and using PI constitutes over-engineering. The key decision criteria: Does the operating temperature exceed 150°C? Is extreme dimensional stability required? Is it used in space/radiation environments? If any answer is “yes,” PI is irreplaceable; if all are “no,” PET is the optimal solution.

5. Selection Guide

Conclusion

PI film and PET film are two important nodes in the electronic insulation material spectrum, not competitive substitutes. If your application involves “high temperature (>150°C) + extreme environment + high dimensional stability,” choose PI film. If your application is “ambient/medium temperature + general electrical insulation + cost-sensitive,” choose PET film.

For cost-sensitive applications requiring moderate temperature resistance, PEN (polyethylene naphthalate) film is a worthwhile compromise — temperature resistance up to 200°C, priced at 1/3–1/2 of PI, with performance between PI and PET.

Procurement advice: Clarify the part’s maximum operating temperature (note: material temperature, not ambient), use it to screen against the two films’ long-term temperature limits; then evaluate lifespan requirements (PI life is typically 3–10× that of PET); finally perform a TCO calculation. Don’t blindly select PI because of its “premium” label, and don’t risk using PET in high-temperature conditions because of its low cost — let data drive the decision.