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Electrical and Mechanical Requirements in Flexible Circuit Designs

Zachary Walker
Written by Zachary Walker
Posted on July 2, 2024 at 8:56 AM

In the dynamic world of flex circuit design, a recurring challenge presents itself: balancing electrical requirements with mechanical demands. These two aspects often clash, creating a complex scenario for designers.

This dilemma arises primarily due to the nature of electrical and mechanical requirements being almost mutually exclusive. The need for a thicker flex circuit to meet electrical needs contradicts the mechanical requirement for thinness to ensure flexibility.

A common ambition in design is achieving high levels of integration, which, while noble, often leads to significant design challenges. High integration typically necessitates a multitude of interconnects and compact real estate, resulting in an increased layer count. This inevitably thickens the flex circuit, compromising its bendability. Moreover, current technologies demand stringent impedance control and efficient power distribution, further complicating the design.

Innovative Design Solutions: The Multi-Part Configuration

To navigate these challenges, a shift towards a multi-part configuration has proven effective. This approach involves using separate flex parts for specific requirements, like controlled impedance or high current signals. This strategy not only reduces the overall flex thickness, enhancing bend capabilities, but also simplifies each component, potentially reducing costs.

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Gerber Layout Considerations

In flex circuit layout, preferences lean towards thinner copper for a slimmer profile and tighter bend radius. Thinner copper allows for reduced line widths and spacings, which can free up space and potentially reduce the layer count. For example, using 1/2-ounce copper can achieve finer line and space measurements compared to 1-ounce copper.

In the nuance realm of flex circuit layout, meticulous attention to material selection and layout intricacies is paramount. A crucial aspect of this is the choice of copper thickness, which significantly influences the circuit's mechanical and electrical performance.

Flexible PCB design with a signal layer

A flexible PCB design with a signal layer of numerous traces running from one end of the board to the other.

Emphasis on Thinner Copper

The preference for thinner copper is not merely a matter of convenience but a critical design choice. Thinner copper, such as 1/2 ounce as opposed to the thicker 1-ounce variety, offers numerous advantages.

  • Slimmer Profile for Enhanced Flexibility: Thinner copper directly translates to a more flexible circuit. This flexibility is vital for applications where the circuit must conform to irregular surfaces or undergo frequent bending. The slim profile achieved with thinner copper thus becomes a cornerstone in designs where mechanical agility is as crucial as electrical functionality.
  • Tighter Bend Radius: The bend radius of a flex circuit is a critical parameter, especially in compact or densely packed devices. Thinner copper allows for a tighter bend radius without compromising the integrity of the circuit. This means that designers can navigate complex geometry and tight spaces without the risk of circuit damage.
  • Reduced Line Widths and Spacings: Thinner copper facilitates finer line widths and spacings. This capability is crucial in high-density designs where every millimeter of space counts. By enabling closer line and space tolerances, thinner copper allows for more complex circuitry in a smaller footprint. This is particularly beneficial in advanced electronics where space is at a premium, such as in wearable technology, medical devices, and aerospace applications.
  • Potential Reduction in Layer Count: By allowing for tighter line and space configurations, thinner copper can lead to a decrease in the overall layer count of the flex circuit. This reduction not only simplifies the design but can also lead to cost savings in terms of material use and manufacturing complexity.

Offsetting Circuits and Layer Count Strategies

A unique tactic in flex circuits is offsetting circuits on adjacent layers, utilizing the flexibility of the polyimide core. This allows for tighter packing, especially beneficial when bending. Additionally, flex and rigid-flex designs can accommodate odd layer counts, unlike rigid boards which typically use even layer counts. This flexibility in design can be crucial in achieving the desired mechanical properties.

Advanced Construction Techniques and Material Selection

The 'air gap construction' technique represents a notable advancement in the realm of flex circuit design. This method involves splitting the flex layers into pairs, strategically arranged to allow them to bend independently of each other. This independence is crucial in significantly reducing the minimum bend radius of the circuit, thereby greatly enhancing its flexibility.

Such a construction is particularly beneficial in applications where the flex circuit must navigate through tight spaces or conform to intricate shapes. By employing air gap construction, designers can achieve a level of mechanical versatility that traditional layered approaches may not offer, making it a go-to solution for complex, high-density electronic assemblies.

Material Choices: Traditional Versus Innovative Approaches

In the sphere of material selection for flex circuits, there's a clear distinction between traditional and modern methodologies. The conventional choice has been polyimide with adhesive, a tried-and-tested option that offers reliable performance. However, this method does come with the added thickness of the adhesive layers, which can impact the flexibility of the circuit.

On the other side of the spectrum is the adhesiveless construction. This newer approach involves directly attaching copper to the polyimide core, eliminating the need for adhesive layers. This direct attachment method inherently reduces the overall thickness of the material, which is a significant step forward in improving bend capabilities. The adhesiveless option is gaining popularity, especially in applications where space is at a premium and enhanced flexibility is a necessity.

Moreover, when the design calls for particularly stringent bend requirements, rolled annealed copper emerges as the preferred choice. Unlike standard electro-deposited copper, rolled annealed copper boasts a grain structure that is elongated and horizontally oriented. This orientation is more amenable to bending, making it an ideal choice for circuits that need to endure frequent or extreme bending. This selection of copper type is a crucial decision in ensuring the longevity and reliability of the flex circuit, especially in demanding operational environments.


Flex circuit design is a balancing act between electrical and mechanical demands. By employing strategies like multi-part configurations, careful material selection, and innovative layout techniques, designers can effectively navigate these challenges. The key is to maintain a flexible approach, adapting to the unique requirements of each design.

This adaptability not only ensures the optimal performance of the flex circuit but also enhances its durability and suitability for a wide range of applications. In an ever-evolving technological landscape, such versatility in design methodology is essential for staying ahead in the field of electronics, ensuring that flex circuits continue to meet the increasing demands for compactness, efficiency, and reliability.

Topics: Flex & Rigid-Flex PCB's, Product Design

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