Flexible PCBs have revolutionized many industries, especially the medical sector, in form-fitting monitoring and biometric patches.

These devices are becoming increasingly vital. What is the technology behind them?

Nearly every electronic device, including phones, TVs, and appliances, are equipped with a printed circuit board (PCB). These boards consist of laminated conductive insulated layers. They serve two purposes: they house electronic components in designated locations on the outer layers and provide reliable electrical connections between component terminals.

Electronic components, resistors, capacitors, microcontrollers, interfaces, etc., are electronically connected via traces, planes, and other features using a chemical etching process that deposits copper layers laminated onto sheets of a non-conductive substrate.

PCB Evolution

PCBs were created in the early 20th century. While they have evolved with the advent of new technologies, their basic design remains the same: a rigid board filled with electronic components.

Industry professionals have made it possible to make smaller, more efficient electronics by leveraging the rapid advancements in semiconductor packaging technology. This technology has led to wearable technologies such as AR/VR glasses and smartwatches.

These new technologies have also led to the advent of form-fitting medical devices that can monitor the wearer's health through direct contact with their skin. Smartphones are now able to use folding displays as well. A new type of PCB was needed that could bend and warp beyond the right angles to realize these devices. Engineers took on the challenge of creating flexible circuit boards with various materials and techniques, including transparent polyester and polyimides (Fig. 1).

A flexible PCB being picked up with forceps.

Different types of flexible PCBs

Today's manufacturers have chosen to use two types of flexible boards: rigid-flex and flexible PCBs. Flexible circuit boards, also known as flexible PCBs or flexible circuit boards, function similarly to their rigid counterparts. The electronics are placed on flexible substrates rather than a platform. Flexible PCBs allow electronics to be formed in different configurations and shapes than a flat surface. These can be broken down into the two most popular types based on their structure and layer.

Rigid-flex PCBs can be described as hybrids. They are both flexible and rigid boards to support a variety of surface-mounted devices (aka electronics) and connectors. The SMDs are mounted on flexible substrates, while the connectors are on rigid surfaces. This configuration ensures a stable connection and minimizes damage from repeated usage. Because the connection terminations are on the rigid surface, the rigid-flex design allows for greater capacity in SMDs.

HDI PCBs have a higher wiring density than their rigid counterparts. They also provide more refined lines and spaces, smaller vias and capture pad sizes, and a higher density of connection pads. Flexible substrates can benefit from these PCBs as they can be designed with thinner layers or, in some instances, eliminate the need for multiple layers.

The number of layers can help to classify FCBs. A single-layer board has a single layer of polyimide substrate and a thin copper layer. This can be accessed via one side. You can also find single-sided FCBs, with dual access and double-sided FCBs, that have electrically-conductive copper on both the substrate and multi-layered FCBs, with single- and multi-sided access.

The flexible design of PCBs

When designing a flexible PCB, the most critical question is how flexible the PCB will bend. Its ability to bend will determine whether the board is static or dynamic.

Static boards are considered bend-to-install and can flex less than 100 times over their life. Because dynamic boards will have to endure a lot of flexing, their design must be more robust. The bend radius, the minimum curvature in the flex area, must also be considered and identified early in the design process. This will ensure that the design can withstand bends without causing damage to the copper.

When designing, it is crucial to consider solder masks or other materials to cover the FCBs' conductive layers. The stiffener will increase the rigidity of your design and decrease the flexibility in particular or all areas.

Like traditional PCBs with layered designs, FCBs also have a layered structure. Therefore, the engineer or PCB designer must consider the choice of material in the design phase. A film layer, for example, supplies the conductor carrier and acts as insulation in the circuit. However, it must also be flexible. These substrates are often made from polyimide or PET (polyester), which work well as insulators. Aramid, PTFE, and PEN are also used.

Flexible cores made of polyimide also work well in FCB designs. These cores are cladded with electrodeposited or rolled-annealed copper. This is extremely thin and can be used for static and dynamic applications.

Two types of materials are available for FCBs: adhesive-based, where copper is bonded to polyimide using an acrylic adhesive, and adhesive-less, which casts copper directly onto the polyimide substrate. Sticky materials can cause cracks when heated. They can also make copper laminate thicker and absorb moisture, making them unsuitable for specific environments.

These are where adhesive-less materials can come in handy. These materials can withstand harsh environments and offer other benefits such as reduced flex thickness, better flexibility, and higher temperature ratings.

911EDA has expert PCB designers to support your flex design requirements.

Flexible PCB Stacks

PCB stacks refer to the arrangement of layers made up of copper and insulators before the final layout is designed (Fig. 3). A stack-up level allows multiple electronic circuits to be assembled on one board using different layers. However, the stack-up PCB design structure offers many other benefits, such as mitigating external noise and improving electromagnetic compatibility. Multiple layers increase the board's ability to distribute energy, reduce cross-interference and eliminate electromagnetic interference. They also support high-speed signals.

Like traditional PCBs, they can be made with one or more layers. This denotes the number of copper traces, adhesive, laminates, polyimide material, and copper traces. The number of layers will affect the numbers. A four-layer circuit, for example, will include four copper traces and four layers of adhesives or non-adhesives, laminates, and the corresponding substrate materials. Many manufacturers place flexible material in the middle of the stack-up to maintain flexibility and minimize damage.

Before deciding on the number of layers to use, you must consider several factors. These include operating frequencies, routing frequency, shielding requirements, and additional shielding. These factors will allow you to determine the number of layers required and ensure that the FCB functions appropriately.

Conclusion

As new technologies are developed, flexible printed circuit boards will keep evolving. Imagine being able to wash a medical monitor device in the washing machine or wear a biometric patch underwater without worrying about corrosion.

Flexible electronics are already able to grab data from the human body. It will be exciting to see where this technology is implemented. This is a brief introduction to flexible PCB design. It will only give an overview of the processes and materials used in their creation.