Flexible PCB Laminate Materials: The Ultimate Guide

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Oakley Mae

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About Flexible PCB Laminate Materials, The foundation of a good flexible PCB lies in the choice and quality of its laminate materials.

They are the ones that determine whether your PCB will work well or not.

Consequently, having the right information is paramount for working on a flex PCB project. 

Also, you must have knowledge of the exact materials to use and any alternatives if some are unavailable.

Luckily, we have all you need in today’s ultimate guide. Let’s get going.

Table of Contents

Materials Used in Flexible PCB Laminates

Multiple considerations come into play when selecting laminates for flexible printed circuit boards.

The materials need to be versatile, have either static or dynamic flexing, and be easily made into the desired product.

Let’s look at some materials used in flexible PCB laminates. 

Conductors

Generally, most conductors are metals with good electrical properties such as conductance and low resistance.

And one such popular material is copper. However, not all types of copper metal can withstand the periodic bending and movement of the flex circuit.

For instance, the electro-deposited (ED) copper foil cannot withstand such conditions. 

Consequently, manufacturers use them in rigid PCBs or circuit boards that only need to flex during installation.

On the other hand, rolled annealed (RA) copper foils do not suffer from hardening and fatigue over a long period.

Therefore, use it when your flexible PCB undergoes periodic bending and rolling.

Substrates

Typically, we use the following three materials as substrates in flexible printed circuit boards:

Polyimide

Polyimide is a popular material owing to its high-temperature resistance, cheap cost, and high tensile strength.

Moreover, it is readily available and tough enough that you can’t stretch it by pulling it apart using your hands. 

As a result, this property makes it ideal for PCBs manufactured for environments with broad temperature fluctuations.

On the downside, polyimide has low moisture resistance making them unideal for specific high-humidity environments. 

Liquid Crystal Polymer (LCP)

Installing a flex PCB cable into a motherboard socket

(Installing a flex PCB cable into a motherboard socket)

An alternative to polyimide is the more expensive liquid crystal polymer.

The substrate material has a high-temperature resistance, high tensile strength, and excellent moisture resistance.

Moreover, its melting point (320 degrees Celsius) is higher than that of polyimide, which ranges from 250 to 300 degrees Celsius.

Such properties make LCP a popular material for intensive circuit systems. 

Polyester (PET)

Polyester is another popular substrate material that is durable and moisture-resistant.

Moreover, it’s highly malleable; you can make it into any shape you want.

However, the material has low temperature and moisture resistance.

Due to this, PCB manufacturers use PET in circuits that do not generate much heat and in some low-cost electronics. 

Adhesives

Printed circuit board manufacturers use adhesives to bind the laminates since they can’t achieve a strong bond with the annealed copper.

Typically, epoxy or acrylic adhesives are popular with most manufacturers as they don’t compromise the flex PCBs’ structural flexibility. 

However, modern advancements in PCB technology are slowly pushing away the use of adhesives.

Some applications like HDI circuits require fine pitches, hence the use of laminates that don’t require adhesives. 

Why Choose Polyimide Substrate Material

Engineers controlling a robotic arm machine that uses flex PCBs

(Engineers controlling a robotic arm machine that uses flex PCBs)

Different substrate materials have unique characteristics that make them suitable in certain circumstances.

Moreover, their unique properties impact the PCB’s functionality and production method.

One such substrate material is polyimide. In this section, we’re looking at why manufacturers prefer it over other substrate materials.

High Thermal Stability

Most materials that can be flex PCB substrates fail due to their thermal instability at high temperatures.

Interestingly, polyimide has excellent thermal stability at high temperatures.

And such quality is essential since the PCB fabrication process relies on heat.

Additionally, flex PCBs work in high-resistance systems that generate much heat.

As a result, their excellent thermal conductivity works well with thermal pads in dissipating heat from the system.

Excellent Mechanical Strength and Flexibility.

Despite being lightweight, polyimide substrates have a reliable load-bearing capacity suitable for flexible PCBs.

Consequently, they last longer when you use them, even in harsh environments.

Second, they have one of the best flexibility-to-stress ratios of most PCB materials.

In other words, you’ll find that the elongation while bending is directly proportional to the force you apply. 

Excellent Electrical Properties

Flexible cable PCB

(Flexible cable PCB)

We use a significant percentage of flexible PCBs in electrical systems.

Consequently, manufacturers prefer materials like polyimide for their desirable electrical properties.

For instance, the polyimide’s dielectric constant delivers recommendable insulation.

Moreover, the material is resistant to radiation in electric environments.  

Chemical Tolerant

Given that you’ll use your flex PCB in many different environments, your board must have high chemical resilience.

And none equals polyimide’s resistance to conditions like corrosion.

Moreover, they are solvent resistant owing to their long molecular structure. 

Advantages of Flexible Aluminum PCBs

The difference between aluminum PCBs and other printed circuit boards is that they have an aluminum base.

The design is because aluminum has high thermal conductivity and quickly dissipates heat. 

Let’s look at the advantages of using an aluminum base over other metals here. 

  • Lightweight: Weight is an essential consideration in most flexible PCBs. Unlike rigid PCBs, flexible circuit boards are lightweight and usually found in lightweight systems. As a result, manufacturers prefer an aluminum base to provide good tensile strength without adding unnecessary weight. 
  • Good Heat dissipation: The dielectric layer that pairs with the aluminum base effectively dissolves heat. Similarly, aluminum has significant heat dissipation properties that prevent flex PCBs from overheating. 
  • Cheap: Aluminum is a cheap material in comparison to some base materials. As a result, manufacturers prefer it for use in circuits that need a cheap, reliable, but flexible base material.
  • Long-lasting: When designing long-lasting flexible PCBs for high-power circuitry, it’s better to use aluminum. It will retain its structural strength and flexibility for a long time.   

What to Consider When Choosing the Suitable Laminate Material for a Flex PCB

A couple thinking before concluding

(A couple thinking before concluding)

Printed circuit board manufacturers consider many factors when selecting materials for making PCBs.

For instance, you must know the materials’ decomposition temperature, thermal conductivity, and heat dissipation. 

Additionally, be aware of its coefficient of thermal expansion and relative permeability.

There’s a lot of information that you need to know before working on a PCB.

Luckily, some circuit board datasheets have the right data on the materials to make PCBs with specific properties.  

Additionally, ensure that the substrate has materials with the same dielectric constant and coefficient of thermal expansion.

Failure to do so will result in a board misbehaving when subjecting it to an electric current. 

Secondly, use materials with low moisture absorption, especially the PCB in highly humid areas.

Thirdly, avoid FR4 as a material for circuit boards in high-frequency systems.

The reason is that FR4 is a material with a high and undesirable dielectric loss.

FAQs

What materials are used in rigid-flex PCBs?

Most materials for rigid-flex PCBs are the same as those for flex circuit boards.

For instance, manufacturers use polyimide or polyester for PCBs in demanding environments.

For less demanding environments, they use the cheaper woven fiberglass substrate.

Other materials are conductors like copper, coatings, and adhesives. 

How thick is a flex PCB layer?

Generally, flex PCBs are thin since the base material is also thin for flexibility.

Consequently, their thickness ranges from 0.1mm to 0.5mm.

However, depending on their application, some flex PCBs can have a thickness of as much as 3.0mm. 

Conclusion

Flex-printed circuit boards have become an integral part of modern-day electronics.

Most electronics manufacturers prefer to avoid using many wires as connections between essential system components. 

However, manufacturing these flex PCBs requires a deep understanding of the properties of laminate materials to ensure they work well.

Luckily, we’ve learned what you need to know to kickstart your journey into the electronic world of flex PCBs.