How to Choose a Flex PCB Substrate: PI, PEN or PET?

What Are PI, PEN, and PET?

In the design of a flex PCB substrate, choosing the right material is crucial. The base material directly affects the performance, cost, and durability of your flexible printed circuit. At Flex Plus, we help customers worldwide manufacture flexible PCBs that are reliable and application-optimized.

There are three common flex PCB substrate materials:  PEN (Polyethylene Naphthalate), Polyimide (PI), and PET (Polyethylene Terephthalate). While they may look similar at first glance, each has unique characteristics that make it suitable for specific applications.

PI – Polyimide

Polyimide is the gold standard in high-performance flexible PCBs. It is used in industries like aerospace, automotive, and medical devices, where reliability and thermal resistance are essential.

• Temperature Resistance: Up to ~400°C

• Flexibility: Excellent

• Chemical Resistance: Very high

• Cost: High

PI's exceptional performance makes it the preferred flex PCB substrate when long-term stability under harsh conditions is required.

PEN – Polyethylene Naphthalate

PEN is considered a middle ground between PET and PI. It offers better thermal performance than PET, at a lower cost than PI.

• Temperature Resistance: ~180°C

• Flexibility: Moderate

• Dimensional Stability: Better than PET

• Cost: Moderate

For applications that demand more durability than PET can offer, but don’t require the full capabilities of Polyimide, PEN is a solid choice.

PET – Polyethylene Terephthalate

PET is a thermoplastic polymer widely used due to its low cost and decent dimensional stability. It is typically found in consumer electronics and static flex circuits.

• Temperature Resistance: Up to ~150°C

• Flexibility: Limited (suitable for low-flex designs)

• Moisture Resistance: Moderate

• Cost: Low

PET is ideal for mass production where cost-efficiency is a priority and extreme thermal or mechanical stress is not expected.

Comparing the Pros and Cons of PI, PEN, and PET

Selecting a flex PCB substrate requires careful consideration of your product’s demands. Here is a detailed comparison of the pros and cons of each material:

Polyimide

Pros:

• Excellent thermal resistance (ideal for soldering processes)

• Outstanding flexibility, especially in dynamic flex applications

• Superior chemical and solvent resistance

• Excellent electrical insulation properties

Cons:

• Higher cost

• May absorb moisture if untreated

• Requires more complex manufacturing processes

PEN

Pros:

• Better thermal performance than PET

• More dimensionally stable

• Good chemical resistance

• Balanced performance-to-cost ratio

Cons:

• Not as heat resistant or flexible as PI

• More expensive than PET

• Not ideal for high-reliability or harsh environments

PET

Pros:

• Very cost-effective for mass production

• Good dimensional stability in normal conditions

• Easy to process and handle

• Adequate for static flex applications

Cons:

• Low thermal endurance—not suitable for reflow soldering

• Limited mechanical flex life

• Less chemically resistant than PI or PEN

Quick Comparison Table

Feature	Polyimide (PI)	PEN	PET
Max Temperature	~400°C	~180°C	~150°C
Flexibility	Excellent	Good	Moderate
Moisture Resistance	Very High	High	Moderate
Chemical Resistance	Excellent	Good	Average
Cost	High	Medium	Low
Typical Use	Medical, Aerospace	Consumer, Displays	LED strips, sensors

How to Choose the Right Flex PCB Substrate for Your Project?

As a flex PCB manufacturer, Flex Plus often advises customers on how to choose the best flex PCB substrate. Here’s a decision-making guide based on real-world application needs.

1. Consider the Operating Temperature

If your circuit must endure high temperatures, such as in solder reflow or automotive environments, Polyimide is your best bet. PET will not hold up in such conditions, while PEN provides limited resistance.

2. Evaluate Flexibility Needs

For dynamic flexing applications, such as folding devices or wearable technology, PI offers the best performance. PET can crack under repeated flexing, while PEN performs better but still has limitations.

3. Factor in Cost and Budget

If you are working with strict cost constraints and your application doesn’t require high-end specs, PET is a practical solution. For middle-range requirements, PEN strikes a good balance. If performance and longevity matter most, investing in Polyimide is a smart choice.

4. Assess the Environmental Conditions

If your application is exposed to chemicals, humidity, or vibration, Polyimide provides superior resistance. PEN also performs well in moderate environments, while PET is suitable for stable, clean-use conditions.

5. Determine Manufacturing and Assembly Needs

If your project involves soldering or surface mount technology (SMT), Polyimide is typically the only safe choice. PET and PEN may deform or degrade during high-heat soldering processes.

Final Thoughts

Choosing the right flex PCB substrate—whether PET, PI, or PEN—depends on your application’s unique demands. Here at Flex Plus, we work closely with clients to understand their product goals, technical requirements, and budget constraints.

Whether you're designing a cost-efficient wearable device or a mission-critical medical sensor, we have the materials, engineering support, and manufacturing experience to deliver reliable and high-performance flexible PCBs.

Looking for expert help? Contact Flex Plus today for custom solutions and high-quality production of your flexible printed circuit boards.