Differences Between FR4 and Polyimide Materials In Flex Circuits

Are you tense about choosing the right flex circuit ingredients?

Then, you must confront the analysis of the difference between FR4 and Polyimide Materials in flex circuits. These materials differ mainly in terms of flexibility and thermal management.

It’s a common debate for professionals and newcomers. We hope this blog can make it clear to you ultimately.

Let’s make a move.

Flex Circuit Materials

Do you know which materials and additives are used in FPCs (Flexible Printed Circuits)?

Before digging deeper into the fundamental difference between FR4 and Polyimide Materials in flex circuits, you must clearly understand Flex circuit materials and their properties.

This knowledge allows you to differentiate between them and make a good choice for your Flex circuit boards. Let’s see the basic materials found in flex circuits.

Base Material

Usually, manufacturers use polyimides and FR4 as base materials in flex circuits. Both of these materials come with flexibility and robustness to a certain degree. That’s why flexible devices perform superbly.

Coverlay Dimensions in Flexible PCBs

Coverlay

This item is basically a polyimide film. Being used as the substitute for the solder mask of rigid circuit boards, coverlay protects the exposed components on the flexible circuit board surface.

Copper

• Copper foil deposited via electrolysis
• Copper foil with rolled form

Flexible PCBs with Suitable Adhesives

Adhesive

This is a connector that makes contact between copper and polyimide or FR4.

Stiffeners

These materials include sufficient robustness to flex circuits where necessary.

FR4 Flex Circuits

Being represented as a grade of PCB materials, FR4 (Flame Resistant Grade 4) materials are base materials in flex printed circuits. According to NEMA (National Electrical Manufacturers Association), this grade refers to the flammability rating of V-0 in the UL94 standards.

Fire Resistant Grade 4 Material in Flexible Printed Circuits

Types of FR4 Flex Materials

• High TG FR4

Made of laminated fiberglass, high TG FR4 materials offer moderate thermal resistance and stability in flex circuits.

• Standard FR4

Filled with Bromine, standard FR4 is reputable for flame retardness. 

• Halogen-free FR4

Being supportive of Green electronics, Halogen-free FR4 flex circuits are widely used where the device comes to human contact randomly.

PCBs without Halogens

Properties of FR4 Circuit Boards

• The safe and effective electric insulator
• Low dielectric constant at a particular signal frequency (2.7803.48 at 1GHz frequency)
• Excellent strength-to-weight ratio
• Low dissipation factor (0.016)
• Moderate durability
• High Young’s Modulus
• The glass transition temperature is 135⁰C
• Hardness 110 (M Scale)
• Complaints of Lead-free processes
• Advanced dimensional stability
• Chemically resistant to corrosion and moisture
• Thermal drainage is not up to the mark
• Low thermal conductivity
• Less expensive

Specifications of FR4 in FPCs

Polyimide Flex Circuits

Polyimide means polymers with imide monomers. In the flexible PCB manufacturing world, synthetic polyimides are used as the board bases. But you can find both synthetic and natural polyimides in electronic sectors.

Polyimide in Flex Circuits with Different Structures

Usually, Maleic Anhydride and Bismaleimide are the most common compounds that are found in imide structures to produce synthetic polyamides. Concerning the wide variety of imide structures, Polyimide flex circuits are found in different types.

Polyimide Flex Circuit Types

The most significant polyimide flexible circuits include-

Low-flow Polyimide Flex Circuits

Consisting of resins and other materials that restrict flow, these polyimide-based flex circuits are not so famous for their flexibility. But, these circuits are cost-effective to a certain extent compared to other polyimide flexible circuits.

Low-flow Multilayer Flex Circuits with Polyimides

3G Polyimide Flex Circuits

These flexible circuits are reputable for their low production time and flammability resistance. Usually, 3G polyimides consist of materials that are resistant to fire to a large extent.

Besides, these materials don’t require an extended time and heat to be cured from damage. That’s why manufacturers prefer 3G polyimides for electronic flexible circuits that deal with high temperatures.

2G Polyimide Flex Circuits  

2G polyimides are thermally more stable and heat resistive compared to other polyimides used in flex circuits. These circuits are made of pure polyimides where there is no presence of Bromines and other flame-resistant additives. That’s why 2G polyimides can render such stability, both thermally and electrically.

Flexible Circuits with Filled Polyimide

Here, polyimide is found along with a filler that is used to minimize the effect of resin shrinkage. Besides, Filler is useful for curing cracks while drilling flexible circuit boards, while Polyimide ensures flammability resistance and stability.

4G Polyimide Flex Circuits

4G Polyimides offer moisture sensitivity with the adhesiveness of copper foil. Besides, these Polyimides ensure overall stability in terms of thermal management and design of flex circuits.

Properties of Polyimide Flexible Circuits

High Flexibility

Polyimide materials with numerous Imide structures are widely used in flex circuits because of their high flexibility. Compared to other flexible circuit materials, Polyimide offers less hardness and elasticity.

Besides, the additives used here are also supportive of flexible electric circuits to a large extent.

Improved Robustness

Despite its noteworthy flexibility, Polyimide flex circuits provide certain robustness to the device. Its Rockwell hardness on the M scale is almost 70-80, which is negligible.

Moreover, the tensile strength (231 MPa) and specific gravity (1.6) are also impressive in terms of rigid-flex or rigid circuits.

Chemically Resistant

This property makes Polyimide materials suitable for the intense, foggy environment. Their chemical resistivity is adequate to handle moisture and toxic chemicals. Their moisture absorption rate is almost 0.02-0.04, which is good enough to make flex circuits compatible with extreme weather.

Polyimide PCBs with Improved Durability

Controlled DK

Dielectric Constant(DK) refers to the ratio of a medium’s relative permittivity over a vacuum’s permittivity. It mainly indicates the charge density and its impacts on the conductor.

Polyimide’s DK value at 1 GHz frequency is almost 4.8. This value is controlled enough to be used in multilayer flex circuits, as well as single-layer flex and rigid-flex circuits.

It is supportive of maintaining a controlled impedance throughout the circuit. Besides, low and controlled DK significantly improves their usability and thermal performance.

Improved Usability

Polyimide materials are compatible with various sectors’ FPCs (Flexible Printed Circuits) due to their fantastic flexibility, thermal management, and chemical resistance. You can find Polyimides in industries where mobility and stretchability are highly required.

That’s why the following sectors are filled with Polyimide flex circuit boards-

• Computer products
• Medical equipment
• Military instruments
• Aerospace products
• Consumer goods
• Automotive Industries

Polyimide Circuit Boards with Advanced Compatibility

Advanced Thermal Management

Next to flexibility, thermal management is the most appealing feature of Polyimide flex circuits. The thermal stability, TG value, thermal conductivity, etc, support the criteria the flex circuits need to fulfill.

Moreover, the flammability of the 2G Polyimide (purely filled with polyimides) is HB standard. The arc resistance is around 143 secs in this case. This is highly commendable to resist stresses due to fire.

High Durability

As mentioned above, Polyimide materials are reputable for handling extreme thermal pressure and hazardous chemicals. Besides, their high flexibility allows them to deal with mechanical forces.

That’s why Polyimides are highly durable, and their shelf life extends for a long period. This long operating period minimizes the impact of cost in the long run.

FR4 vs. Polyimide- Head-to-Head Comparison

Factors that Differentiate Between FR4 and Polyimide Materials in Flex Circuits

Flexibility

In terms of flexibility, polyimide flex circuits are far away from FR4 circuits. Mainly, the principal difference between these two is flexibility.

So, if you desire a purely flexible circuit for your gaming devices or computer products, you must choose polyimide flexible circuit boards.

But FR4 will be suitable where rigidity with certain flexibility is mandatory. That’s why you must make a choice sincerely.

Cost Performance

Polyimide circuits are costly, no doubt. But we think the performance you get from these PCBs deserves that cost. In the long run, performance matters over cost.

So, polyimide flex circuits are appropriate for a long-term and sustainable project. You will never regret paying a high price for these materials.

On the contrary, FR4 is less expensive than polyimides. If  you have budget constraints and intend to get a moderate service from your flexible printed circuits, you can consider FR4 over polyimide.

Rigidity

There are some flex circuits used in aerospace and robotics that demand a certain rigidity. In those cases, FR4 materials are stronger candidates compared to Polyimides. The tensile strength of FR4 flex circuits is around 70 MPa. Besides, The Rockwell hardness is 110 on the M scale.

These ratings indicate superior rigidity of FR4 materials in flexible and rigid-flex circuits. Though polyimide flex circuits’ performance is not so bad in terms of rigidity, manufacturers usually prefer FR4 in most cases.

Thermal Stability

In terms of thermal stability, Polyimide circuits are the best option. Polyimide with numerous structures offers effective thermal cycling and glass transition temperature. Usually, Polyimides are high TG materials.

That means these polymers can withstand extreme thermal pressures. The operating temperature of polyimides varies from -220⁰C to 300⁰C, which is fantastic.

Besides, their flammability rating is also impressive. That’s why professionals look for Polyimides flex circuits for heat-sensitive applications.

FR4 is also reputable for thermal management. But its operating temperature is lower than polyimides. FR4 can operate within -50⁰C to 110⁰C, with a TG value of 135⁰C.

These ratings are good for flex circuits but not good enough to beat polyimide’s thermal management.

But the thermal conductivity varies from 2.2-2.5 in the case of FR4 materials. Their CTE (Coefficient of Thermal Expansion) values are supportive of flex devices. So, you can consider FR4 in some cases, along with Polyimides.

Electrical Performance

Both of these flex circuit materials have controlled dielectric constants and low loss tangents. That’s why you can choose any of them when their electrical performance is concerned.

The dissipation factor of FR4 is very low. As the DF value indicates the lack of efficiency in electrical signal transmission, we can consider FR4 as a more electrically efficient flex circuit material than 2G or 3G polyimide flex printed circuits.

Chemical Resistance

This is a crucial point when we are analyzing the difference between FR4 and Polyimide materials in flex circuits. Chemical resistance indicates how strongly a circuit can withstand harsh chemicals and moisture.

Polyimide renders an improved resistance against toxic chemicals. Besides, the moisture absorption rate is also significantly low in the case of 2G polyimide flex circuits. This property impacts the working life of circuit boards, which is very important from a marketing point of view.

FR4 (Flame Retardant Grade 4) materials deliver a moderate performance against highly toxic compounds. That’s why the flex circuits filled with FR4 materials consistently can’t deliver outstanding performance in dense conditions. That means Polyimide flex circuits are the winner in this case.

Elasticity

FR4 materials are more elastic than Polyimides. Elasticity depends on Young’s Modulus, which is a ratio of stress and strain within the elastic limit. A higher Young’s Modulus indicates more elasticity.

Stress vs Strain Graph of Polyimide Materials

This rating is almost 24 GPa and 4 GPa, respectively, for FR4 and Polyimides. As Elasticity is a vital issue in flex printed circuits, FR4 will be the better option than Polyimide structures.

Use

If you compare the usability of these two options for a flex circuit board, you can find that both of them are used widely in flexible and rigid-flex circuits. But Polyimides are more usable in aerospace, gaming consoles, and medical equipment because of their high flexibility and elasticity.

In contrast, manufacturers and PCB assemblers prefer FR4, where lead-free compatibility and robustness are mandatory. For example, low-frequency digital applications and DC board applications consist of standard FR4 materials.

Their dimensional stability and effective strength-to-weight ratio also play a crucial role in making FR4 compatible with versatile products.

Shelf life

Polyimides last longer than FR4 in most cases. So, you must choose polyimide flex circuits to experience a long-lasting device with optimum electrical, thermal, and chemical performances.

So, it is crystal clear that Polyimide and FR4 materials include particular positive sides to the flex circuits. But polyimides have a comparatively better contribution in improving the flexible circuit performance than FR4 in many cases.

You must make your choice concerning your device specifications, requirements, and, most importantly, budget. MV Flex Circuit is a worthwhile PCB maker. Contact MVFlex Circuit to get your quote immediately.