Flexible PCB Material

What is a Flexible PCB

The circuit board that offers bendability and stretchability to a certain degree is known as a Flexible PCB. This PCB also delivers rigidity to the device, but flexibility is its USP (Unique Selling Point).

Flexible circuit board with Excellent Bendability

Principal Flexible PCB Materials

Along with base materials (Polyimide, Polyamide, FR4, etc), Substrate, Stiffeners, Coverlay, Conductors, and Adhesives are the principal flexible PCB materials.

A Polyimide-based Flexible PCB Material

Factors you Need to Consider While Choosing Flexible PCB Materials

• Flexibility
• Thermal stability
• Working temperature
• Material cost
• Robustness
• Electrical properties
• Impedance matching
• Chemical resistance
• Flammability rating
• Dielectric constant
• Loss tangent
• Moisture absorption rate
• Compatibility with modern devices

High-Quality Flexible Circuit Materials

Polyimide vs. Polyamide PCB Materials

Though both of these are polymers, Polyamide and Polyimide differ from each other in terms of their chemical structure. Flexible printed circuits, formed with these effective materials, deliver excellent flexibility, elasticity, and thermal dissipation, making them compatible with modern flex circuits.

Polyamide Flexible Materials

Polyamides, made via the polymerization of the Dicarboxylic and Diamine groups, come in natural and synthetic forms. The most common synthetic Polyamides are Nylon, Carbamide-methanal, Aramides, etc. You can find these polymers in natural sources like silk, wool, etc.

Polyamide Structure

Properties of Polyamide PCB Materials

• High working temperature
• Moderate deflection due to temperature (75⁰C)
• HB flammability rating
• Specific gravity is 1.14
• Improved Modulus of the tensile strength (85 MPa)
• The higher degree of abrasion resistance
• Resistant to chemical solvents
• High IK rating
• Low CTE (Coefficient of Thermal Efficiency)
• Low Frictional Coefficient

Polyimides in Flex Circuits

Polyimide Flexible Material

In contrast to Polyamides, Polyimide materials used in flexible PCBs are basically the polymer of Diamine and Dianhydride groups. Kapton, a very popular Polyimide flexible PCB material, is produced through the polymerization reaction of Pyromellitic dianhydride and Pera-Pera-Oxydianiline.

Here, “Pera” refers to the 4th carbon of the Benzyne cycle used in Anyline.

Polyimide Structure

Properties of Polyimide PCB Materials

• Outstanding thermal stability in high temperatures
• Specific gravity is 1,43
• Moderate mechanical strength
• High TG PCB material
• The flammability rating is V-0 according to the UL-94 standards
• Sound Dielectric constant and loss tangent
• Extremely resistant to abrasion
• Good resistance to corrosion and oxidation
• Low moisture and water absorption rate

Polyimide vs. Polyamide: Head-to-Head Comparisons

Factor	Polyimide	Polyamide
Trade Name	Kapton	Kelver and Nylon
Structure	Consist of Imide group (-CO-N-CO-)	Consist of Amide group (-CO-NH-)
Monomers	Diamine and Dianhydride	DiAmine and Dicarboxylic acids
Thermal Deflection	360⁰C	75⁰C
Tensile Strength	70-86.2 MPa	85 MPa
Water Absorption	0.24 at 23⁰C, 24hr 1.15 at 23⁰C, 50RH	0.3 at 23⁰C, 24hr 3.0 at 23⁰C, 50RH
Elongation During Break	7.5	70
Chemical Resistance	Improved	Improved
Relative Resistance	Good	Good
Flammability (UL94 Standard)	V-0	HB
Compatibility	Lead-free compatible	Lead-free processes
Flexibility	Flexible	Flexible
Specific Gravity	1.43	1.14
Thermal Cycle	Advanced	Improved
Durability	Good	Good

Classifications of Polyimide PCB Materials

Polyimides are found in thermosetting and thermoplastic forms depending on the polymer chain reactions. Aromatic and Aliphatic are also two structural varieties of Polyimide materials used in Flexible Printed circuits.

Besides, you can classify these materials with Dianhydride and Diamine monomers into different types concerning the additives with the base ingredient. Such as

Polyimides with FR additives

This Polyimide comes with a flame-retardant molecule like Bromine within its structure. This inclusion improves their stability against flame to a certain degree but also hampers their thermal stability.

But you need to consider that Polyimides filled with FR additives require less amount of time and heat to make substrate compared to pure Polyimides. That means you can use these polymers in high-volume flexible PCB productions. That’s a massive advantage for professional PCB manufacturers, isn’t it?

Polyimide Flex Circuits with FR4 Additives

Polyimide with minimum Flow

If you are looking for a flex PCB Polyimide material with less flexibility and moderate stiffness, this Polyimide will be a perfect choice. Low-flow polyimides have extensive applications in electronic devices that must withstand harsh conditions and demand high durability.

This polymer is filled with resin materials and some other additives, which are responsible for lessening flexibility and stretchability by lowering the flow.

Zero-filler Polyimide

This type of Polyimide material (2G Polyimide) with successive Imide linkage doesn’t contain any filler, like flame retardants. These materials are still relevant to the FPC (Flexible Printed Circuits) world through their high thermal stability and flexibility. But the flame retardant rating is not so impressive in this case.

Adhesive-filled and Adhesive-less Polyimide Materials

This is another famous debate between two unique types of flexible Polyimide materials used in PCBs. Have you ever been in such a debate?

Its answer may be yes or no, but you must be meticulous while making a choice between Polyimides with or without adhesives. That means you must have expertise on this issue. Let’s dig into this comparison deeper.

Adhesive-less Polyimide Flex Core Materials

When you need to counter the reliability problem that occurs in adhesive-filled Polyimides, consider flex core Polyimide materials without adhesives. Here, you can find a polyimide core that helps attach to copper. So, there is no need to use adhesives.

As these Polyimides offer tremendous reliability, with improved thermal stability and easy application, manufacturers rely on these materials in designing multilayer flex circuits. Actually, adhesive-less polyimides are an automatic choice in flex printed circuits, while copper and dielectrics are coated on each other separately.

The most noteworthy positive sides of these Polyimides include-

• Improved Reliability

Adhesive-less Polyimides upgrade the reliability of flex circuits, especially in plated through holes. As no additional layer for adhesives is present here, manufacturers can increase the stability in multilayer circuits.

Besides, it helps the circuit board handle stresses and cracks occurring in the Z-axis direction during flex circuit assembly. So, who doesn’t want to get a high-class FPC with improved reliability?

• Impedance Control

To match the impedance in all signal layers, it is crucial to maintain a harmonious structure. Polyimide structures without adhesives are supportive of keeping the construction analogous in signal layers and planes in different layers. That’s why impedance remains controlled, which improves signal transmission to a large extent.

• Compatibility with high layer counts

Multilayer flexible circuit boards demand a certain robustness so that these PCBs can resist excessive thermal or mechanical forces. These polyimides are effective in this case because of their higher reliability and stable features.

• Minimized Thickness

Because of removing adhesive layers, these polymers with Dianhydride and Diamine monomers offer extra space to accommodate PCB components. Thus, component placement becomes feasible, and the device becomes lightweight.

• Flexible

As adhesive-less polyimide flex circuits, consisting of thin cores, have high-temperature resistance, they get more bendable. It helps keep the bending radius minimum with safe hardness. So, their compatibility with stretchable applications increases.

Adhesive-filled Polyimide Flex Core Materials

Usually, flexible polyimide materials in PCB cores with adhesives are primarily found in Epoxy or Acrylic formats. These core materials act superbly to attach copper substrate and polyimide materials to either the front or back side of flex circuit boards.

Flex PCBs with Adhesives

There are some most-desired facilities for using adhesive-based Polyimides in flex-printed circuit boards. Such as

• Improved Durability

It reduces the chance of cracking or breaking of PCB cores and traces because adhesive creates a coating of copper in the PCB via holes. So, it improves durability, no doubt.

• Cost-efficiency

Designing and manufacturing a flexible printed circuit with adhesives and Polyimides are not so complicated. That means the production cost cuts down to a certain degree. That’s why manufacturers prefer adhesive-filled Polyimides to adhesive-less polyimides for FPCs.

• Stability

Adhesives, both Epoxy and Acrylic, are reputable for improving the bonding between copper and dielectric materials. That’s why the peel strength increases and the flexible devices can withstand more mechanical and thermal pressures.

Adhesive-less Polyimides with Improved Reliability

But adhesive-based Polyimides in Flex cores lack reliability in extreme temperatures and moistures. So, manufacturers prefer this type of material for single or double-layer circuit boards in most cases, where budget and peel strength are prioritized.

Moreover, they need to apply certain pressure and thermal forces to allow the adhesives to bind the polyimides and coppers. Sometimes, adhesives lose stiffness under hot weather. So, you must keep these hassles in mind while applying epoxy and Acrylic adhesives to your Flex PCB projects.

Here are side-by-side comparisons between these two rivalries for your convenience-

Factor	Adhesive-less Polyimide	Adhesive-based Polyimide
PTH Reliability	Excellent	Not impressive
Z-axis Stress Resistance	Better	Moderate
Compatibility	Multilayer PCBs	Single-layer, Double-layer PCBs
Impedance Control	Better	Good
Durability	OK	Good
Flexibility	High	Moderate
Copper Peel Strength	Moderate	High
Cost	Little bit costly	Economical

FR4 Flexible Material: Classification and Properties

Though FR4 is widely recognized for its rigidity, manufacturers use this material in flexible PCBs on many occasions. It happens due to the durability, thermal management, flammability, and moderate flexibility of FR4 (4th Grade Flame Retardant) materials.

FR4 Materials in Flexible Printed Circuits

FR4 is basically fiberglass materials filled with Epoxy laminated sheets. Here, fiberglass renders dimensional stability, whereas Epoxy includes rigidity and resistance against fire. So, it is clear that you must use FR4 flexible PCB materials in electronic appliances that require robustness and flame retardness simultaneously.

Features of FR4 Flexible PCB Materials

• Improved Dielectric Properties

FR4 offers high dielectric strength. Dielectric strength refers to the potential required to break the insulation of a conductor. Because of the controlled and low dielectric constant of FR4, it can hold its insulation against high current forces. That’s why it is a popular electric insulator. 

• Flame Resistant

FR4 materials, filled with halogens like Bromine, Fluorine, Iodine, etc, have high electrical arc resistance. That means these materials can withstand fire and flame for an extended time.

In some cases, Aluminium Trihydrate is doped into the FR4 structure to make these materials more flame resistant inorganically. Both of these ways (Organic and forced) are workable enough to meet your purpose.  

• Cost-friendly

FR4 is a very economical flexible PCB material compared to other options. That’s why the manufacturing cost, in most cases, favors PCB makers.

• Dimensional Stability

Laminated Epoxy sheets, a vital component of FR4 materials, include sufficient stiffness to the flexible circuits. So, these PCBs can survive successfully against thermal forces and mechanical pressures, which indicates improved dimensional stability of FR4-based flexible PCBs.  

• Chemical Resistant

Flexible circuits consisting of FR4 can resist chemical interventions in harsh weather and extreme humidity because of their minimal moisture absorption rate. Most noteworthy, this feature helps keep the electrical properties consistent.

Classifications of FR4 Materials

• Standard FR4 Material

•  High TG FR4 Material

• G-10 Fr4 Material

• Copper-free Fr4 Material

Difference Between Polyimide and FR4

Factor	Polyimide	FR4
Rockwell Hardness (M Scale)	70-80	110
Dielectric Constant (DK)	4.2 at 1 GHz	2.78-3.48 at 1 GHz
Modulus of Tensile Strength	231 MPa	70 MPa
Loss Tangent	4	0.016
Operating Temperature Range	-220 to 300⁰C	-50 to 110⁰C
Young’s Modulus	4 GPa	24 GPa
Chemical Resistance	Improved	Moderate
Resistivity	Better	Good
TG Value	195-220⁰C (High TG)	135⁰C (Mid TG)
Dielectric Constant (DK)	4.2 at 1 GHz	2.78-3.48 at 1 GHz
Thermal Conductivity	0.2	2.2-2.5
Durability	Improved	Good
Flexibility	Flexible	Rigid
Specific Gravity	1.6	1.8-1.9
Thermal Cycle	Advanced	Good
Arc Resistance (sec)	143	125
Lead-free Compatibility	OK	OK
Cost	Expensive material	Low-cost material

Notable Applications of Flexible PCB Materials

• Computer products
• Aerospace
• Military Industry
• Robotics
• Home appliances
• Laboratory Equipment
• Testing Equipment

Flex PCB Materials- How to Stackup

Flexible PCB stack-up mostly depends on the layer count and material types. You need to change the strategy concerning the number of layers.  Here, a basic stack-up of 4-layer flex circuit boards is given so that you can have ideas on this topic.

Layers	Materials	Thickness
Topmost Coverlay	Polyimide	12.5 micrometer
	Adhesive	15 micrometer
Topmost Copper Layer	Copper	12 micrometer
	Adhesive	0 micrometer
	Polyimide	25 micrometer
	Glue	13 micrometer
Copper Layer	Copper	12 micrometer
	Adhesive	0 micrometer
	Polyimide	25 micrometer
	Adhesive	0 micrometer
Copper Layer	Copper	12 micrometer
	Glue	13 micrometer
	Polyimide	25 micrometer
	Adhesive	0 micrometer
Bottom Copper Layer	Copper	12 micrometer
Bottom Coverlay	Adhesive	15 micrometer
	Polyimide	12.5 micrometer

Watch this video on how to use flex PCB materials while making the PCB-

MV Flex Circuit is a worthwhile flexible PCB manufacturer. Contact us to experience the best flexible PCB materials for your flex PCB applications.