Bending the Rules: Designing Flexible Circuits for Dynamic Applications

Flexible circuits enable compact, lightweight, and adaptable designs that rigid PCBs cannot achieve. From wearable devices to robotic systems, flexible PCBs excel in applications requiring constant movement. However, these dynamic environments pose unique challenges for circuit design, demanding a blend of technical expertise and strategic planning.

In this blog, we’ll look into designing flexible circuits for dynamic applications. From understanding material science to tackling common challenges, this blog will equip PCB designers with the insights they need to create durable and reliable flexible circuit boards.

What Makes Flexible Circuits Essential for Dynamic Applications?

Flexible PCBs stand out due to their ability to conform to tight spaces and endure repeated bending and twisting. This makes them useful in applications where rigid PCBs would fail, such as:

• Wearable electronics like fitness trackers and smartwatches.
• Robotics, where circuits must handle continuous motion in robotic arms and joints.
• Automotive systems, including airbags, sensors, and interior lighting.
• Foldable and rollable displays in cutting-edge smartphones and portable devices.

These dynamic applications require designs that can withstand mechanical stress and repetitive motion without compromising performance. 

Material Science

The materials you select for a flexible circuit significantly impact its performance in dynamic applications. Let’s break down the essential materials and their roles:

Base Materials

• Polyimide (PI): The most commonly used material for flexible circuits due to its excellent mechanical strength, flexibility, and heat resistance.
• Liquid Crystal Polymer (LCP): Ideal for applications requiring low moisture absorption and superior high-frequency performance.

Copper Types

• Rolled Annealed (RA) Copper: Preferred for dynamic applications because of its smooth surface and high ductility. RA copper is less prone to cracking under repeated stress compared to electro-deposited (ED) copper.
• Adhesiveless Laminates:  Adhesiveless constructions eliminate potential weak points, such as delamination, and offer reduced material thickness, improving the circuit's ability to withstand repeated bending.

An example rigid-flex stackup with bookbinder construction is shown below. Read more about the approach to designing these stackups in this article.

Testing for Success: Dynamic Bend and Flex Cycle Testing

Designing for dynamic environments requires rigorous testing to validate a circuit’s durability. Testing simulates real-world conditions to predict the lifespan and reliability of the flexible PCB.

Key Testing Methods

• Flex Cycle Testing: Determines how many bends a circuit can withstand before conductor fatigue sets in.
• Twist Testing: Evaluates the circuit's ability to handle torsional stress, a critical factor in robotic and automotive applications.
• Environmental Stress Testing: Includes thermal cycling and moisture resistance tests to simulate harsh operating conditions.

Design Tips for Flexible Circuits in Dynamic Applications

When designing flexible PCBs for dynamic use, every detail matters. A small oversight can lead to conductor cracking, delamination, or premature failure. Here are some essential tips:

Optimize Trace Routing

• Smooth Curves Over Sharp Corners: Sharp corners concentrate stress and can lead to cracks. If traces must be routed over a bending region, use curved traces to distribute stress more evenly rather than placing a 45° corner in the bending region.
• Parallel Trace Orientation: When routing near a bending region, consider routing traces parallel to the bending axis to reduce mechanical stress during flexing. Put enough space between traces so that they do not fall too close to the bend area.

Stagger Vias to Reduce Stress Concentration

• Avoid aligning vias in a straight row, as this creates stress concentration across the row of vias. Instead, stagger vias to evenly distribute stress across the circuit.

Use Teardrop on Pads and Vias

• Teardrop-shaped pads reduce the stress concentration at trace-to-pad junctions, preventing mechanical failure during bending. Learn more about applying teardrops on pads and vias.

Reinforce High-Stress Areas

Connector regions and solder joints are especially prone to stress. If there is excessive bending or repeated bending in these areas, the copper pads could delaminate or solder joints could crack. Consider reinforcing these areas with polyimide stiffeners, thicker copper layers, or add mounting holes to fixate the flex circuit to a housing.

Choose a Flexible Solder Mask

A standard solder mask can crack under repeated bending. However, there are flexible solder mask options (known as polyimide coverlay) to protect copper traces while still allowing for dynamic motion.

Addressing Common Challenges in Dynamic Flexible Circuits

Dynamic applications present unique challenges, but proactive design strategies can mitigate these risks.

Conductor Fatigue

Repeated bending can lead to micro-cracks in the copper conductors.
Solution: Use rolled annealed copper and design with an adequate bend radius to reduce stress.

Delamination

Flexible circuits with adhesive layers can delaminate under stress or high humidity.
Solution: Select adhesiveless laminates or high-bond-strength adhesives.

Thermal Expansion Issues

Differing expansion rates between materials can cause mechanical stress in environments with temperature fluctuations.
Solution: Choose materials with compatible coefficients of thermal expansion (CTE) to minimize this risk.

Vibration-Induced Failures

In automotive and aerospace applications, constant vibration can degrade circuit integrity.
Solution: Use reinforced stiffeners and vibration-damping materials.

Exciting Applications of Dynamic Flexible Circuits

Flexible circuits are powering some of the most innovative technologies of our time. Here are a few examples:

Wearable Electronics

Smartwatches and fitness trackers rely on flexible circuits to maintain functionality despite constant movement and close contact with the body.

Foldable Devices

The latest smartphones and tablets feature foldable designs, made possible by ultra-thin flexible circuits that can bend repeatedly without breaking.

Robotic Systems

Flex circuits are used in some robotic arms, where they can endure twisting and bending to deliver power and data to moving parts.

Automotive

Some lighting systems in automobiles demand flexible PCBs that can withstand vibration and temperature extremes.

Emerging Trends in Flexible PCB Design

The future of flexible circuits is exciting, with new materials and technologies pushing the boundaries of what’s possible. New materials and more advanced commercial materials that are currently available offer a new class of electronic devices on flexible platforms.

Stretchable Circuits

Stretchable electronics can bend and stretch like rubber, enabling new uses in medical devices, e-textiles, and wearable technology.

High-Frequency Applications

Certain designs that implement high-frequency/RF components or circuits on flexible designs can use materials like LCP, which offers lower loss tangent than polyimide blends.

Some flexible PCB materials, specifically polyethylene (PET), are transparent to all colors of visible light. Contrast this with traditional polyimide, which is only transparent to orange light.

E-Textiles

Smart fabrics integrate flexible circuits directly into textiles, enabling applications like health monitoring, augmented reality clothing, and more.

Conclusion: Mastering the Art of Dynamic Flex Circuit Design

Designing flexible circuits for dynamic applications is both an art and a science. Success depends on selecting the right materials, rigorously testing for durability, and employing best practices to mitigate mechanical stress.

From wearables and robotics to automotive and foldable devices, flexible PCBs are at the forefront of modern innovation. By addressing challenges proactively and leveraging emerging trends, PCB designers can create robust circuits that meet the demands of even the most dynamic environments.