Flex PCB Explained: The Ultimate Tech Breakthrough

Flexible PCBs are advantageous due to their light, thin, and strong adaptability. They are commonly found in the circuits of electronic devices such as smartphones, wearable devices, and medical equipment, as these devices require PCBs with high space and flexibility requirements. Flexible PCB manufacturers are emerging in these new areas. This article will delve into the types and characteristics of flex PCB boards, as well as their applications.

What are flexible PCBs?

Flexible PCBs can be bent, twisted, or flexed around a device’s body without changing their shape permanently. These printed circuit boards are often produced using polyimide materials, keeping the circuit safe even when altered. Each PCB or board remains reliable no matter how many times it is printed or moved. They represent one of the most significant advances in printed circuit board technology. Flexible PCBs have made two new categories of PCBs, i.e., flexible and rigid-flex PCBs. From LED strips to smartphones, flex PCBs are used in various modern applications. You can also read Phototransistor.

Properties of Flexible PCB Boards

The flexibility of printed circuit boards (PCBs) is determined by a number of features that are present in flexible PCBs.

A few of the properties that determine the flexibility and quality of flexible printed circuit boards are:

• Young’s Modulus: In my PCB work, I check how a material handles stress and strain in the elastic region, using the ratio called Young’s Modulus, before any plastic deformation happens in other regions. This helps keep deformation low and boards flexible without damage. In other words, it measures how much a material can deform (or bend in our case) under a constant applied load. A laminate or a PCB material with a lower Young’s modulus implies the laminate has better flexibility and can flex without causing permanent deformation. Similarly, laminates with higher Young’s modulus have lower flexibility and tend to be stiffer; therefore, a good flexible PCB material is expected to have a lower Young’s modulus.
• Tensile Strength: In flexible PCBs, one critical point is how much tensile stress the board can handle before breaking. The tensile strength is measured by the force applied per unit area, usually in Pascals, or for stronger materials, Megapascals (MPa). A good flex material is expected to withstand higher stress without breaking. This strength shows a correlation with the material’s ability to sustain mechanical pulling across different dimensions. From personal testing, we always check this value across the range of pressure values to meet manufacturers’ factors. Reliable printed circuit boards depend on this key characteristic.
• Flexural Strength: In my work with flex PCBs, I’ve learned that flexural strength is key when boards face frequent or extreme bending. A good PCB material can withstand repeated loads without cracking or permanent deformation. This capacity is often quantified in psi, pascal, or megapascals, depending on the range. If the strain from being subjected to stress stays below the value, the laminate won’t fracture or deform. From my experience, checking the flexural strength per square inch or pound is essential for picking the right materials.
• Bending Cycles – Bending cycles refer to the number of times a flexible PCB can be bent back and forth before it begins to show signs of failure, such as fracturing, delamination, or deterioration of electrical integrity. While this unit does not guarantee that the PCB will show signs of failure after or before the mentioned bending cycles, it gives an estimate of how many times a PCB can be bent safely. A high bending cycle implies the PCB can withstand a higher number of flexes or bends without visible damage. A good flex PCB material is expected to have higher bending cycles. This is another characteristic that flexible printed circuit board manufacturers value.
• Bending Radius – The bending radius is the minimum radius that a flexible PCB can be bent without causing damage, such as cracking, breaking, deforming, or loss of functionality. It is one of the important design criteria that define the flexibility of the PCBs. The bending radius of the PCB material determines how much a flexible printed circuit board can bend. A smaller bending radius implies greater flexibility and tighter flexes and bends. A good flexible laminate should have a lower bending radius.
• Elongation at Break – Elongation at break is the percentage by which a material’s length increases before it breaks when subjected to tensile stress. The ductility and flexibility of a material are both evaluated using this metric. A higher value of elongation at break means the material can undergo greater deformation (stretching or bending) before it tears or breaks. A good flexible PCB is expected to have a high percentage of elongation at break, which means it can endure more significant mechanical stresses and bending without breaking.

Please note that when it is said “a good flexible PCB,” it means the PCB has good flexural properties. It might also be possible that a material may have excellent flexible properties but poor electrical properties.

Different Types of Flexible PCBs by Material

While there are fewer options for material types than rigid PCBs, the available options for flexible PCBs are generally sufficient for applications requiring flexible printed circuit boards. A few of the flexible materials used for PCBs in the flex PCB fabrication are:

1. Polyimide (PI) flexible PCBs

Polyimide is one of the most common materials used for flex PCB boards due to its excellent thermal stability, mechanical properties, and flexibility. Plastic circuit boards made of polyimide are an excellent option for high-reliability flexible PCB applications. They are the most widely used flexible PCB materials by flex PCB manufacturers. Please note that polyimide PCBs are also available for rigid PCBs. They have the following properties.

1. Dielectric Constant (Dk): 3.4 – 3.6
2. Dissipation Factor (Df): 0.002 – 0.004 at 1 MHz
3. Tensile Strength: 150–250 MPa
4. Elongation at Break: 50% – 70%
5. Thermal Conductivity: 0.12 – 0.25 W/m·K
6. Flexibility: High flexibility, capable of being bent, twisted, and folded multiple times without damage.
7. Temperature Range: -200 °C to +300 °C

2. Polyester (PET) flexible PCBs

Polyester is a popular material for flexible PCB boards in flex PCB fabrication. It is recognized for its affordability and excellent electrical insulation properties. PET is one of the cheapest flexible PCB materials available. That’s why it is popular with flex PCB manufacturers. However, for the trade-off of cost, it has mediocre flexibility. It has the following properties.

1. Dielectric Constant (Dk): 3.1 – 3.3
2. Dissipation Factor (Df): 0.005 – 0.007 at 1 MHz
3. Tensile Strength: 100 – 200 MPa
4. Elongation at Break: 90% – 120%
5. Thermal Conductivity: 0.15 – 0.3 W/m·K
6. Flexibility: Moderate flexibility, suitable for applications where less bending is required.
7. Temperature Range: -70 °C to +150 °C

3. Polyethylene Naphthalate (PEN) flexible PCBs

It is a similar material to PET but with much better chemical and thermal properties. Most flexible PCB suppliers have this material available for general-type PCB production. It has comparable flexibility to PET. It has the following properties.

1. Dielectric Constant (Dk): 3.0 – 3.2
2. Dissipation Factor (Df): 0.004 – 0.006 at 1 MHz
3. Tensile Strength: 120–230 MPa
4. Elongation at Break: 60% – 90%
5. Thermal Conductivity: 0.15 – 0.4 W/m·K
6. Flexibility: Moderate to high flexibility; better thermal stability than PET.
7. Temperature Range: -50 °C to +180 °C

4. Liquid Crystal Polymer (LCP) flexible PCBs

LCP is one of the most expensive flexible PCB materials. It has limited flexibility but excellent electrical properties. LCP PCBs are mostly used in high-frequency applications that require moderate flexing. It has the following properties:

1. Dielectric Constant (Dk): 2.9 – 3.2
2. Dissipation Factor (Df): 0.002 – 0.004 at 1 MHz
3. Tensile Strength: 150–220 MPa
4. Elongation at Break: 5% – 20%
5. Thermal Conductivity: 0.25 – 0.3 W/m·K
6. Flexibility: Moderate flexibility, suitable for applications with limited bending.
7. Temperature Range: -100 °C to +250 °C

5. Teflon (PTFE) flexible PCBs

Teflon is used in high-frequency applications due to its excellent dielectric properties and chemical resistance, making it often used by the Flex PCB suppliers. It has, however, reduced durability, meaning it has high flexibility, but it cannot withstand repeated bending cycles. It has the following properties.

1. Dielectric Constant (Dk): 2.0 – 2.2
2. Dissipation Factor (Df): 0.0005 – 0.002 at 1 MHz
3. Tensile Strength: 20–40 MPa
4. Elongation at Break: 300% – 500%
5. Thermal Conductivity: 0.2 – 0.3 W/m·K
6. Flexibility: High flexibility but less durable in repeated bending compared to polyimides.
7. Temperature Range: -200 °C to +260 °C

Conclusion

Flexible PCBs have transformed modern electronics by offering unmatched design versatility, space efficiency, and mechanical flexibility. Their ability to bend, twist, and conform to complex shapes makes them indispensable in compact and dynamic devices like smartphones, wearables, medical instruments, and LED systems. With various material options such as Polyimide, PET, PEN, LCP, and Teflon, engineers can tailor flex PCBs to meet the specific mechanical, thermal, and electrical demands of their applications. As technology continues to evolve, the role of flexible PCBs will only expand, making them a vital part of next-generation electronic innovations.