Introduction
The base material that makes up a flex PCB (flexible printed circuit board) is a flexible dielectric polymer film. This provides the mechanical properties that allow the PCB to bend, twist, and flex repeatedly without damage to the copper traces bonded to it.
Selecting the optimal flex PCB material is an important design decision based on factors like flex life, temperature resistance, chemical resistance, flammability rating, and cost. This article explores the most common flex PCB substrate materials and their properties.
Polyimide
Polyimide film is the most widely used flex PCB substrate material. The two most common brands are:
Kapton®
- Developed by DuPont
- Thin polyimide film available in various thicknesses
- High maximum operating temperature up to 400°C
- Excellent chemical resistance
- UL94 VTM-0 flammability rating
- Tensile strength of 231 MPa
Upilex®
- Ube Industries version of polyimide
- UL recognized version of Kapton
- Slightly lower cost than Kapton
- Otherwise similar properties
Polyimide provides an ideal balance of electrical performance, mechanical flexibility, heat and chemical resistance, and cost. It can withstand millions of dynamic flex cycles, making it the standard for demanding flex PCB applications.
Polyester (PET)
PET (polyethylene terephthalate) films offer a lower cost alternative to polyimide for less demanding flex PCB applications.
Key properties of PET include:
- More economical substrate material
- Maximum operating temperature around 120°C
- Moderate chemical resistance
- UL94 VTM-1 flammability rating
- Lower dielectric strength than polyimide
- Tensile strength of 100-150 MPa
The compromise is PET does not withstand high temperatures or extreme bending as well as polyimide. But the cost savings can make PET suitable for many cost-sensitive flex PCB manufacturer products.
Polyethylene Naphthalate (PEN)
For applications requiring very high temperature resistance, PEN (polyethylene naphthalate) is an advanced flex PCB material option.
Features of PEN include:
- Withstands over 200°C operating temperatures
- Low moisture absorption properties
- High glass transition temperature of 120°C
- Excellent mechanical flexibility
- UL94 VTM-0 flammability rating
- Tensile strength over 200 MPa
PEN is more expensive than polyimide but provides enhanced performance in extreme temperature or moisture environments.
Fluoropolymer Films
Fluoropolymers like PTFE (Teflon®) are chosen when superb chemical resistance is the foremost requirement.
Key attributes of fluoropolymers:
- Extremely chemically inert
- Withstand virtually any chemical exposure
- Operate from -150°C up to 260°C
- Moderate mechanical flexibility
- UL94 VTM-0 flammability
- Expensive compared to other flex materials
The uncompromising chemical resistance makes fluoropolymer films ideal for harsh chemical-laden environments, despite their higher cost.
Flexible Material Comparison
| Parameter | Polyimide | PET | PEN | PTFE |
|---|---|---|---|---|
| Brand Names | Kapton, Upilex | Mylar, Melinex | Teonex | Teflon |
| Max Temp | 400°C | 120°C | 210°C | 260°C |
| Dielectric Strength | 7700 V/mil | 3000 V/mil | 5500 V/mil | 2000 V/mil |
| Chemical Resistance | Excellent | Moderate | Very Good | Extreme |
| Cost | Moderate | Low | High | Very High |
| Key Applications | General flex PCBs | Cost-driven flex | High temp flex | Chemically resistant flex |
Flex PCB Material Construction
In addition to the flexible dielectric substrate, other materials are incorporated in a flex PCB construction:
Conductors
- Copper foil – Most common conductive material
- Alternative metals like aluminum or nickel for specialized applications
Bonding Film
- Acrylic or epoxy adhesive to bond layers together
Cover Film
- Insulating soldermask-like layer to protect conductors
Stiffeners
- Optional thicker areas for component mounting stability
Material Selection Criteria
Key considerations when selecting a flex PCB substrate material:
Flexibility
- Bend radii needed
- Dynamic flexing requirements
Temperature
- Operating temperature range
- Thermal cycling conditions
Chemical Exposure
- Will the flex PCB contact any chemicals?
- Liquids, cleaning agents, etc
Flammability
- UL flammability rating required
Cost
- Budget limitations
Lead Time
- Some materials have longer lead times
Matching the flex PCB substrate to the environmental and mechanical requirements ensures reliable performance.
Flex PCB Material Properties
Mechanical
- Tensile strength
- Elongation at break
- Young’s modulus
- Flex fatigue resistance
- Puncture/abrasion resistance
Electrical
- Dielectric constant (Dk)
- Dielectric strength
- Volume resistivity
Thermal
- Glass transition temperature (Tg)
- Coefficient of thermal expansion (CTE)
- Melt point
- Maximum operating temperature
Chemical
- Chemical resistance
- Outgassing and byproducts
- Moisture absorption
Testing Flex PCB Materials
Ensuring the flex PCB material meets specifications involves testing:
Mechanical
- Tensile testing
- Bend testing
- Flex fatigue cycling
- Crease/fold testing
- Peel strength
- Torque testing
Electrical
- Dielectric constant
- Insulation resistance
- Breakdown voltage
- High potential testing
Environmental
- Thermal cycling
- Temperature/humidity exposure
- Flammability testing
- Chemical compatibility testing
This qualification testing validates the material’s suitability for the application.
Frequently Asked Questions
Can you use FR-4 or other rigid PCB materials for flex PCBs?
FR-4 fiberglass and other common rigid laminates are too brittle to withstand repetitive flexing. Only flexible polymer films specifically engineered for dynamic bending should be used for the base substrate.
What are the tradeoffs between polyimide vs PET for flex PCBs?
Polyimide provides higher performance while PET offers lower cost. Polyimide can operate at much higher temperatures with better chemical resistance, while PET provides a more economical option for less demanding applications.
Does conductor thickness impact flex PCB flexibility?
Yes, thicker copper traces over around 2 oz (70 μm) will begin to reduce flexural endurance. This is because thicker conductors are stiffer and undergo more strain during bending. Thinner traces improve performance but must still carry the required current loads.
Can flex PCB substrates be soldered to?
The flexible films typically cannot withstand soldering temperatures directly unless the material is specifically designed as a bond ply. Adhesive bonding or mechanical fasteners are alternatives for attachment if needed.
What are important considerations when selecting flex PCB materials?
Key considerations include flexibility, temperature range, chemical resistance, flammability rating, lead time, and cost. The substrate material properties must be matched to the electrical, environmental, and mechanical requirements of the end product.
Conclusion
Polymer films like polyimide, PET, and PEN provide the lightweight flexible foundation for dynamic flex PCBs. The substrate material selection strikes a balance between cost, chemical and thermal resistance, flexibility, and other application requirements. Leveraging the benefits of flex PCB technology starts with choosing the optimal base material.
