The Types of Multilayer Flex Circuits and Their Characteristic Advantages

Key Takeaways

• Learn about the types of flex circuits.

• Gain a greater understanding of multilayer flex circuit functionality.

• Develop a better understanding of the advantages multilayer flex circuits inherently afford.

Flex circuits see widespread use in modern technology. 

The phrase, “bend but do not break” is an adage that is still relevant today. This is perhaps most evident in our approach to advancement in today's technology. Technology, as a whole, tends to push the envelope in terms of designs, which promote further breakthroughs and advances. Many in the science and technology industries will attest that flexibility is the cornerstone of many design concepts, such as the curved 4K TVs and curved PC monitors, that were previously thought to be impossible.

However, this incorporation of flexibility in monitors would not be possible without flexible PCBs implemented into their designs.

What Are Flex Circuits?

A flex circuit consists of two or more copper conductive layers that utilize either rigid or flexible insulation material as an insulator between each of its layers. It also has plated through-holes that extend through both its flexible and rigid layers. Also, there is a selective application of coverlay or coverfilm on the flex layers in the flexible regions only.

Note: We apply coverlay or coverfilm during the manufacturing process of flexible printed circuit boards. The coverlay encapsulates and protects the external circuitry of a flexible circuit board. In summary, coverlay serves in the same capacity as a solder mask we typically encounter on a rigid PCB.

There are various configurations of both flex circuits and rigid-flex circuits, which afford a wide range of applicational utilization. The different configurations are as follows:

• Single-layer flex circuit: These adhere to the IPC 6013 - Type 1 standard and consist of a single copper conductive layer fixed between two polyimide insulating layers. Suitable applications include dynamic flex or bend-to-fit applications.

• Double-sided flex circuit: These adhere to the IPC 6013 - Type 2 standard and consist of two copper conductive layers with insulating polyimides between external insulating polyimide layers. They also utilize plated through-holes for circuit connectivity between the layers. Suitable applications include dynamic flex and bend-to-fit applications, depending on construction.

• Multilayer flex circuit: These adhere to the IPC 6013 - Type 3 standard and consist of three or more conductive flexible layers with flexible insulating layers between each layer and external insulating polyimide layers. They also utilize plated through-holes for circuit connectivity between the layers. Suitable for bend-to-fit applications only. Additionally, they use high-speed controlled impedance in a surface stripline or microstrip configuration.

• Rigid-flex circuit: These adhere to the IPC 6013 - Type 4 standard and consist of two or more copper conductive layers and utilize rigid or flexible insulation materials as insulators between its layers. They also utilize plated through-holes that extend through both its flexible and rigid layers. Furthermore, there is a selective application of coverlay or coverfilm on the flex layers in the flexible regions only. Suitable for dynamic flex or bend-to-fit applications and with 1-2 flex layers only. Additionally, they use high-speed controlled impedance in a surface stripline or microstrip configuration.

Multilayer Flex Circuit

As stated, there are various configurations for the flex circuit. However, for the consideration of this article, we will focus on the multilayer flex circuit only for now. A multilayer flex circuit combines several double or single-sided circuits with shielding, complex interconnections, and surface-mounted technologies (SMT) into a multilayer design.

During the production process, the multiple layers can be continuously laminated or not. This distinction is critical because continuous lamination is typically not suitable for designs aimed at maximum flexibility.

In general, multilayer flex circuits are a practical solution when addressing design challenges like specified impedance requirements, eliminating crosstalk, extreme component density, shielding inadequacies, and unavoidable crossovers.

Benefits of Multilayer Flex Circuits over Standard Cabling and Rigid PCBs

• Increased circuit density

• Elimination of mechanical connectors

• Unmatched design flexibility

• Reduced weight and size

• Increased operating temperature range

• A reduction in wiring errors

• Increased signal quality

• Increased impedance control and reliability

The Other Benefits of Multilayer Flex Circuits

• Reduced assembly errors: Multilayer flex circuits help to eliminate human error by avoiding the use of hand-built wire harnesses via design accuracy and production automation. Furthermore, multilayer flex circuits only route to the points necessitated by the plan's design.

• Reduced assembly cost and times: Multilayer flex circuits do not require as much manual labor during assembly, thus reducing production errors. Multilayer flex circuits have an inherent ability to integrate fit, function, and form. They reduce the high cost of wrapping, soldering, and routing wires.

• Design freedom: Design freedom comes in the form of not being confined to only two dimensions, like the case with rigid PCBs. Their flexibility affords operation in the harshest environments and virtually endless applicational options.

• Flexibility during installation: As the name implies, flexibility is inherent and it introduces a 3rd dimension for use in designs and applications. You can manipulate flex circuits throughout the installation process without losing electronic functionality.

• High-density applications: Multilayer flex circuits accommodate high-density component placements. This, of course, leads to more space for other possible features.

• Improved airflow: Their streamlined designs afford better airflow, which translates to lower operating temperatures and increased product lifecycle.

• Better heat dissipation: With their compact design and increased surface to volume ratio they provide improved heat dissipation.

• Improved system reliability: Multilayer flex circuits have fewer interconnections, thus reducing failures and improving reliability.

• Durable and reliable: Multilayer flex circuits are durable and they can flex up to 500 million times before failure. Additionally, they can withstand extreme thermal conditions.

• Less complex circuit geometry: Multilayer flex circuit technology utilizes the direct placement of surface-mount components onto the circuit. This results in a more streamlined design.

• Reduced weight and package size: As I am sure you are aware, multiple systems using rigid boards weigh more and require more space. However, multilayer flex circuits are streamlined and use thin dielectric substrates, which eliminates the need for bulky rigid PCBs. Also, their flexibility and elasticity translate into smaller package sizes.

Multilayer flex circuits will continue to remain competitive and stay in demand with the ever-increasing miniaturization trend. Furthermore, their reduced weight, increased reliability, and their ability to perform in extreme environments makes them poised for now and the future.

Multilayer flex printed circuit boards are a mainstay of future technologies.

Successfully designing flex circuits or semi-rigid flex circuits relies on having a robust PCB design and analysis software that is capable of working with this technology. An example of a PCB design system that has the features and capabilities to design flex circuits, including real-time 3D design and analysis, is OrCAD PCB Designer from Cadence. With OrCAD you will have the high-powered professional design tools that you need to successfully design your flex and rigid-flex circuits.

If you’re looking to learn more about how Cadence has the solution for you, talk to us and our team of experts.