Introduction to Polyimide Materials

Polyimide refers to a family of high performance polymer materials characterized by aromatic heterocyclic structures in the molecular backbone. Polyimides exhibit excellent thermal stability, chemical resistance, mechanical strength, and electrical insulation properties.

Some distinctive attributes of polyimides include:

• High glass transition temperature exceeding 300°C
• Low thermal expansion coefficient
• Resistance to solvents and chemicals
• High tensile strength and modulus
• Excellent dielectric properties

These attributes make polyimides well-suited for demanding applications in aerospace, automotive, electronics, and other industries. Polyimide materials are used in high temperature insulating wire and cable insulation, gaskets, composites, and more.

In electronics, polyimide substrates are an important material for manufacturing flexible printed circuit boards (PCBs) used in consumer, medical, industrial, and military products.

What is a Polyimide PCB?

A polyimide PCB uses polyimide as the flexible insulating dielectric substrate onto which the conductive copper traces are bonded. The thin, lightweight polyimide material allows the PCB to flex and conform to surfaces in applications like wearable devices. Polyimide PCBs can have one or more conductive layers separated by polyimide dielectric layers.

Some key properties of polyimide PCB substrates:

• High thermal stability and heat resistance
• Withstands temperatures exceeding 260°C
• Low dielectric constant between 3.2-3.8
• Low dissipation factor or loss tangent
• Excellent chemical resistance
• High tensile strength provides durability
• Good electrical insulation properties

These attributes make polyimide a popular choice for flexible PCB substrates despite the higher cost compared to standard FR-4 material.

Why Use Polyimide for Flexible PCBs?

Here are some of the key advantages of using polyimide materials for flexible PCB substrates:

Temperature Resistance – Polyimide has much higher maximum operating temperatures typically over 260°C compared to FR-4’s 130-150°C limit. This allows use in high temperature environments.

Flexibility – The elastic polyimide material can be bent and flexed repeatedly without damage. This makes it ideal for dynamic flexing applications.

Chemical Resistance – Polyimide offers broad chemical resistance and does not degrade or dissolve easily when exposed to solvents or acids. FR-4 absorbs moisture and degrades faster.

Dielectric Strength – Excellent insulating properties allow thinner dielectric layers between copper conductive layers, enabling smaller traces and spacing.

Radiation Resistance – Polyimide retains its properties when exposed to ionizing radiation, unlike most other polymers. This makes it suitable for aerospace, scientific, and nuclear applications.

Light Weight – Low density results in very light weight, flexible PCBs compared to rigid FR-4 boards. This helps in wearable and portable devices.

Low Outgassing – Polyimide emits minimal gaseous compounds in vacuum environments, unlike other polymers. This is vital for space and satellite systems.

Thanks to these characteristics, polyimide is often the material of choice for mission-critical, high performance flexible PCB applications despite the higher cost.

Types of Polyimide Materials for PCBs

Several grades of polyimides are used as base materials for printed circuit board substrates:

Kapton

• Kapton film was developed and trademarked by DuPont
• Provides thermal stability up to 400°C
• Used as insulation on magnet wire as well as flexible PCB substrate
• Multiple film thicknesses available, typically 7 to 125 microns
• Colorless, transparent appearance

Apical Polyimides

• Developed by Kaneka High-Tech Materials
• Includes grades like Apical AV, AP, AX, NP, HP
• Low dielectric constant and high insulation resistance
• Apical AV rated for continuous 400°C operation

UPILEX Polyimides

• UPILEX R and UPILEX S grades from Ube Industries
• High mechanical strength and dielectric properties
• UPILEX S offers a UL-94 V-0 flammability rating

Durimide Polyimides

• Durimide 7000 series photodielectric polyimides
• High resolution, photo-imaging properties
• Durimide 7510 offers low loss and stable dielectric constant

Other Brands

• PI-2611 from HD Microsystems
• P84 from Evonik
• EX-1514 and EX-1515 from Guangzhou Kingboard

These and other polyimide materials offer maximum operating temperatures between 300°C to 400°C, well above standard PCB substrates. This makes them preferable for rugged, high temperature environments.

Manufacturing Process for Polyimide PCBs

Polyimide PCBs leverage specialized fabrication processes tailored to the material properties and requirements:

Substrate Preparation

• Casting or coating polyimide resin onto carrier films
• Peeling polyimide films at target thickness
• Cutting sheets into panel sizes

Surface Treatment

• Corona discharge or chemical etching activates surfaces
• Improves bonding with copper foil layers
• Can also coat with adhesive promoters

Metallization

• Bond sheets of rolled copper foil onto polyimide
• Adhesives used to bond metal and dielectric layers
• For multilayer PCBs, additional alternating layers added

Imaging and Patterning

• Coat with photoresist material
• Expose selective areas to UV using photomasks
• Develop to remove exposed or unexposed resist
• Etch away copper in exposed areas

Finishing

• Strip photoresist
• Annular ring rim plating around drilled holes
• Carbon ink coating for conduction between layers -legend printing
• Passivation and protective coatings
• Electrical testing

The specialized polyimide PCB fabrication process results in high performance, reliable boards.

Polyimide PCB vs FR-4 PCB Comparison

Here is a comparison between typical properties of polyimide PCB substrates versus standard FR-4 material:

In summary, polyimide PCBs offer higher performance but at a higher cost compared to standard FR-4 material. Polyimide is preferred when maximum temperature resistance, flexibility, and reliability are needed despite the higher price tag.

Single vs Double vs Multilayer Polyimide PCBs

Polyimide PCBs can have different layer configurations:

Single Layer – One layer of copper traces on a polyimide substrate. Used for simple, low-cost flex PCBs.

Double Layer – Two conductive layers bonded to both sides of the polyimide dielectric. Provides more routing capacity.

Multilayer – Two or more conductive layers alternating with polyimide dielectric. Allows complex designs with high component density but increases fabrication difficulty and cost.

The cross-section diagram illustrates the layer structure for single, double, and multilayer polyimide PCB configurations.

Typical Properties of Polyimide PCBs

Typical characteristics and parameters of polyimide PCB substrates:

• Dielectric layer thickness from 25 to 125 μm
• Copper foil layers from 12 to 35 μm thickness
• Single, double, and multilayer variants
• Flexible boards down to 12.5 μm thickness
• Operating temperatures exceeding 260°C
• High tensile strength of 200-300 MPa
• Low thermal expansion coefficient around 20 ppm/°C
• Thermal conductivity around 0.12 W/m-K
• Dielectric constant between 3.2 to 3.5
• Loss tangent under 0.003
• Dielectric breakdown voltage of 500-1000 V/mil
• Water absorption under 2%

These properties provide a robust foundation for creating high performance flexible printed circuits.

Applications of Polyimide PCBs

Some common application areas for polyimide PCBs include:

Aerospace – Avionics systems, engine controls, guidance systems. Withstands temperature swings.

Military – Missile guidance, radars, ruggedized electronics. High reliability.

Medical – Implantable devices, sensors, radiation resistance. Biocompatible.

Automotive – Flexible cabling for doors, seats, dashboard. Vibration and chemical resistance.

Wearables – Flexible circuits for smartwatches, fitness bands. Dynamic flexing.

Robotics – Joining connectors, flexible cabling. High tensile strength.

Spacecraft – Low outgassing, radiation hardness needed.

Industrial – Process control systems, measurement instrumentation. Temperature resilience.

Consumer Electronics – Mobile phones, tablets, laptops. Lightweight and portable.

Polyimide delivers enhanced performance and resilience across a wide spectrum of demanding environments.

Pros and Cons of Polyimide PCBs

Some key advantages and disadvantages of polyimide PCB substrates:

Pros

• Extremely high temperature rating
• Lightweight and ultra-thin flexible boards
• Excellent chemical resistance
• High tensile strength and durability
• Resistance to radiation effects
• Low outgassing properties
• Dynamic flexing capabilities

Cons

• Much more expensive than FR-4 boards
• Fabrication is more complex
• Limited number of qualified manufacturers
• Lead times typically longer than FR-4
• More challenging rework and modifications
• Anisotropic expansion requires careful design

Polyimide PCBs deliver substantially higher performance albeit at a premium price point. This makes them more suitable for specialized, high-value applications.

Trends in Polyimide PCB Technology

Some current trends shaping polyimide flexible PCB substrate technology:

• Demand for thinner, lighter circuits from smartphones and wearables
• Polyimide films down to 6 microns enabling ultra-thin circuits
• Shift to finer features and spacing, higher layer counts
• Flex-rigid boards with polyimide and FR-4 sections
• Additives to reduce CTE for improved reliability
• Laser direct imaging for higher precision patterning
• 3D printing using polyimide inks allowing flexible geometries
• Higher frequency designs requiring advanced dielectric materials
• Research into novel polyimide blends and formulations

These trends in polyimide PCB technology help accelerate electronics miniaturization and drive performance for cutting edge applications.

Frequently Asked Questions

Q: How do polyimide PCBs compare to FR-4 PCBs?

A: Polyimide PCBs offer much higher temperature ratings, greater flexibility, chemical resistance and reliability but at a higher cost than standard FR-4 PCB material.

Q: What are some key benefits of polyimide PCB substrates?

A: Key benefits include extremely high heat resistance, lightweight and flexible circuits, resilience to harsh environments, and excellent electrical insulation properties.

Q: What types of polyimide grades are used in PCBs?

A: Common polyimide grades include DuPont Kapton, Kaneka Apical, Ube UPILEX, HD Microsystems PI-2611, and Evonik P84 among others.

Q: Are polyimide PCBs suitable for high frequency, high speed designs?

A: Yes, the stable electrical properties, controlled dielectric constant, and low loss of polyimide makes it suitable for high frequency applications.

Q: What are some typical applications of polyimide PCBs?

A: Polyimide PCBs are widely used in aerospace, military, automotive, medical, wearable electronics, industrial, and consumer products.

Conclusion

Polyimide PCB substrates provide an elite class of flexible printed circuits thanks to the exceptional thermal, chemical, and electrical insulation properties of the material. Polyimide allows reliable functionality in extreme environments and mission-critical applications ranging from jet fighters to pacemakers to spacecraft. With continuing improvements in precision manufacturing and formulations, polyimide PCBs deliver the advanced capabilities needed in emerging electronics designs, albeit at a premium cost only warranted for specialized, high-performance systems.