What Are Flexible PCBs, and How Are They Manufactured?

Flexible printed circuit boards (PCBs) are a type of electronic interconnection that is growing in popularity. This approach to connecting components in a circuit offers design options and more robustness. Also referred to as flexible electronics, flexible circuit boards, flexible printed circuits (FPCs), or flex circuits, the conducting paths of the circuit are built on a flexible plastic substrate — usually made of polyimide, PEEK, or polyester — and the components are soldered to exposed copper pads.

Flexible PCBs can have single, double, or multiple layers of conducting copper circuits. Because the substrate is flexible, it has advantages over rigid PCBs in packaging.

The flexible PCB market grew to $21.8 billion in revenue for 2023, making up 30% of the $73 billion market for all types of PCBs. Flex circuits support the same electronic components as rigid boards, and like that more-popular technology, the manufacturing processes, materials, and packaging of electronic assemblies drive flex circuit board configuration.

The difference is the bendability of the substrate. Designers must understand these factors to avoid challenges and take advantage of the benefits of this (literally) flexible way of realizing circuit designs.

Flexible PCBs are constructed from layers of material used to conduct, insulate, or bond. Based on the number of conducting layers, FPCs can be divided into single, double, or multilayer types.

In addition to these layers, other important features of a flexible PCB are areas where the outside cover layer of insulating material is removed to expose solder pads and copper-plated holes — called vias — that are used to conduct between layers.

A single-layer configuration consists of a flexible dielectric substrate, an adhesive layer, a conducting layer, another adhesive layer, and a flexible dielectric cover layer.

Double-layer configurations have a stack of adhesive, conductor, adhesive, and dielectric on both sides of the base dielectric. Plated through-holes serving as vias are added to connect the conducting layers.

Multilayer configurations stack this laminate structure with as many layers as needed, and vias can be done through or blind.

Flexible PCBs can also be attached to rigid PCBs by embedding layers inside the rigid PCBs. This configuration is called a rigid-flex PCB and is often a way to take advantage of each format at the same time or to connect multiple rigid PCBs with a flex circuit instead of wires or a wire harness.

The advantages of flexible PCBs also make them a good match for designs that use high-density interconnects (HDIs). HDIs allow engineers to create rigid-flex PCBs with more compact circuitry, thinner layers, and micro-vias, all while having a flexible portion in the PCB design.

The smaller form factor makes HDIs a strong choice for applications with complex or tight packaging needs. The tight packaging makes the flexible PCBs not just the preferred choice to offer greater design options but often the only way to connect HDIs to other components.

Conversely, when a configuration requires a rigid board for a component like a chip or a delicate sensor, those components can be placed on an HDI connected to a FPC.

Below is a brief description of the components used to create the stackup of a flexible PCB.

Flexible Dielectric Layer

This is the base material of a flexible PCB. It defines the characteristic physical properties of the assembly, especially the shape and rigidity of the final structure. The substrate is usually made of polyimide (PI) because of its flexibility, resistance to chemicals, and advantageous thermal properties. 

Another popular material is polyester (PET), but other dielectric polymers can be used when special properties are necessary. The thickness of this layer is typically from 12 μm to 125 μm. The thicker the layer, the more rigid the structure. The substrate acts as a core layer and an insulating layer, like a solder mask in rigid PCB, on top of the conducting layer.

Bonding Adhesive Layer

The laminate structure uses an adhesive layer because the conducting layer does not bond directly to the base layer. Designers should pay attention to the bonding strength and maximum temperature of the adhesive material because these values limit mechanical and thermal loading.

Conductive Layer

The conductive layer of the laminate stack is usually made of copper. However, other conducting metals can be used when necessary. In most applications, the copper layer is created from a foil that is bonded to the substrate and then etched to create the desired circuitry. The metal foil can come in a variety of thicknesses as well. Copper foil is usually rolled to produce wrought copper foil or electrodeposited. Conducting inks can also be used to print the traces.

Plated Copper

When the design requires a connection through layers, holes are drilled through the laminate and plated with copper to create vias.

Surface Finish

One downside of highly conductive metals like copper is that they easily oxidize. To combat this, a thin coating of material is applied as a surface finish. These materials also aid solder bonding. The most common types of surface finish are electroless nickel/immersion gold (ENIG), organic solderability preservative (OSP), immersion silver, immersion tin, and gold.

Stiffeners

Sometimes, mechanical stiffness is required in a region of the flexible PCB. The stiffener can be a piece of FR4 — the material rigid PCBs are made from — or a thicker layer of polyimide. A common application of an FR4 stiffener is to support a rigid connector or to stop bending under a large component that has been soldered to the circuit to reduce the stress on the solder joint.

Using flexible PCBs offers many advantages. The substrate's mechanical and thermal properties open up multiple design and performance opportunities. Most of the advantages come from the flexibility of the circuit board materials and the ability to easily create complex shapes with laser cutting. Also, the substrate material often has better thermal properties than the materials used in rigid PCBs. The most significant benefits are listed below.

Efficient Use of Space

Flexible PCBs are thinner, easily cut into complex shapes, and can be bent to fit the shape of the device they are placed inside. Also, especially in rigid-flex circuits, components can be placed in different orientations and still connect.

Robust in Harsh Environments

The polymer material used in flex designs is resistant to harsh chemicals. It can also withstand high temperatures and has better heat dissipation capabilities than rigid PCBs.

Improved Durability

When designed properly, flex circuits can withstand a high number of flexing cycles without failing the conducting circuitry.

Resistance to Shock and Vibration

The lower stiffness and mass result in less energy entering and transmitting through an electronic assembly using flexible PCBs. Also, the material can survive significant levels of strain caused by shock and vibration.

Less Mass

In applications where every gram counts, flexible PCBs provide electronic circuits with less mass than rigid boards.

Simplified Assembly

When replacing traditional wiring or wire harnesses, flexible PCBs can greatly reduce assembly costs. They also don’t need to be held down with screws or clamps and can be glued in place or left to float inside an enclosure.

At first glance, there are numerous reasons to use flexible or rigid-flex PCBs. However, standard rigid circuit boards are still the dominant platform for electronic circuits because many applications don’t benefit from a flexible substrate. Below is a breakdown of the most significant challenges of flexible PCBs.

Cost

The material and manufacturing costs of flexible PCBs are higher than those of the ubiquitous rigid PCB. These costs will eventually decline as flex circuits become more popular. However, the difficulty of creating a stackup of flexible materials and the price of the base material will keep the costs higher.

Bending Under Components and of Traces

The biggest advantage of flexible PCBs can also be a disadvantage. Flexing the material under a rigid component can cause significant strain on the solder that attaches the component to the conducting layer. Stiffeners are a good solution to this problem, but adding them increases costs.

The same bending that can strain solder joints can also cause failure in the conducting circuits if the routing of the traces does not take stress concentration into account. The proper use of electronic reliability simulation can identify problem areas in a design and help drive the PCB design to avoid failures from too much bending.

Flex PCB Manufacturing Challenges

It’s very easy to automate the construction and population of a rigid PCB. Thin, flexible circuits are harder to hold within manufacturing tolerances, and automating the placement and soldering of components is more difficult.

Signal Integrity Issues

Signal integrity is the ability of an electrical signal to travel through a circuit without degradation or distortion. The thin material used in flexible PCBs can allow electromagnetic interference (EMI) from one circuit to another. Also, the bending of the substrate can cause changes in signal reflection and impedance mismatches. These challenges can be overcome by following good design practices and proper routing driven by signal integrity simulation.

Flexible PCBs are finding their way into a wide variety of applications in which the advantages of the technology outweigh the higher cost — and an optimized design can avoid the challenges. 

The most common application of flexible PCBs is in electronic devices for consumers, in which the shape and size require the circuit to conform to the device’s shape while keeping the size as small as possible. From calculators to mobile phones, designers are balancing performance and aesthetics through the judicious use of flexible electronics.

Replacing expensive wire harnesses with a vibration- and heat-resistant flexible PCB that can also fit in a tight corner makes the technology ideal for many automotive applications. Another plus for car designers is the ability to connect components that are not sitting on the same plane with connectors at odd angles to one another. Those same advantages drive developers of industrial sensors to also use flexible PCBs to connect components in an efficient manner and survive the heat and vibration of industrial machinery the sensors are monitoring.

Medical devices, especially wearables, also rely on flexible PCBs. This is an ideal way to deal with wearables' weight and size constraints and the need to fit into the organic shape of the human body. As medical devices get smaller and more powerful, companies will opt for flexible or rigid-flexible PCBs.

Multiple trends in the electronics industry are driving greater adoption of flexible PCBs. The ongoing push for greater capability in smaller packages will move designers away from flat, rigid PCBs to simply fit the circuits inside their devices. 

Suppliers of flexible technology are working on new materials to deliver enhanced performance. Increased adoption will also lead to lower prices and faster production as manufacturers deploy better manufacturing processes and greater automation. More suppliers will offer flexible PCB manufacturing, and contract manufacturers will increase their capacity to assemble flexible PCBs into devices.

One area that will drive greater adoption is the growth of flexible displays and ever-smaller industrial and consumer electronics. This technology requires the use of flexible circuits. Likewise, the push for more capable computers and mobile devices in increasingly thin enclosures will continue to drive the technology forward. Lastly, the electrification of automobiles will require more flexible circuitry in tighter and harsher environments.

Realizing this future will require engineers charged with developing packages with flexible and rigid-flexible PCBs to balance often conflicting characteristics. Deploy the Ansys suite of simulation tools to explore design options and drive toward the optimal design. Some of the most commonly used Ansys tools deployed to support flexible PCBs are: 

• Ansys Maxwell®: The gold standard for simulating low-frequency electromagnetics. PCB designers and consumer electronics engineers use Maxwell software to address issues such as electro-magnetically induced mechanical vibration and inductively coupled EMI/EMC.
• Ansys HFSS™: The gold standard for simulating high-frequency electromagnetics, engineers use HFSS software to develop cost-effective, high-performance circuit layouts and antennas built on flexible PCBs. 
• Ansys SIwave™: Dedicated to PCB electromagnetic simulation, SIwave software provides users with a fast and powerful way to import geometry and model signal integrity (SI), power integrity (PI), electromagnetic interference (EMI), impedance, and crosstalk.  
• Ansys Icepak®: The industry standard for PCB thermal simulation and cooling design, it includes the leading multiphysics solvers in a user interface built for PCB simulation. 
• Ansys Sherlock™: Sherlock allows you to import various flexible PCB file formats and provides multiple options to preprocess the file for material, geometry, part info, etc. A common scenario is exporting an edited flexible PCB file from Sherlock software into Ansys Mechanical™ software for in-depth thermo-mechanical analysis.
• Ansys Mechanical: Mechanical software, along with the Sherlock tool, provides a robust solution to analyze reliability and thermo-mechanical aspects of flexible PCBs.

There is a reason so many of the most challenging flexible PCB designs are driven with Ansys simulation: The tools are easy to use, work together, and provide more actionable information. Explore the links in this article or reach out to tell us about your needs and challenges, and our engineers will help you find the right solution.

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