1. There are several common challenges shared by both flexible circuit design and rigid PCB design, but there are also key differences. The fundamental property of a flexible circuit—its ability to bend and flex—means it behaves more like a mechanical device than an electrical one. As a result, flexible circuits come with a unique set of requirements. Understanding how these requirements interact is crucial for PCB designers to create reliable and cost-effective flexible circuit solutions, balancing both electrical and mechanical functions.

2. Assess the stress concentration characteristics of the design. Stress concentration is the primary cause of mechanical failure in flexible circuits (e.g., conductor cracking, insulation material tearing, etc.). To avoid stress concentration points, the circuit structure should remain consistent in the bending area and its immediate surroundings. In the bending zone, there should be no changes in conductor width, thickness, or orientation. Additionally, there should be no electroplating layers or coatings, and the covering layer or external insulating material must remain intact with no openings. No holes of any kind should be present in the bending area.

3. **Bending Ratio**

Define and evaluate the minimum bending ratio for the design. The bending ratio is the best indicator of whether a flexible circuit will face issues during use. It is determined by the ratio of the bending radius to the circuit thickness.

4. **Optimal Bending Ratio of Structure**

The optimal bending ratio ensures that the circuit can withstand repeated bending without damage, contributing to its longevity and performance.

5. **Conductor Routing**

Conductors should ideally pass through the bending area perpendicular to the bending surface (Figure 1). This orientation minimizes the stress exerted on the conductor during bending, thereby extending the circuit’s service life. Curved bends should always be used instead of sharp angles when changing the direction of the conductor. If a curved bend is not feasible, it’s preferable to use two 45° angles to change the conductor direction, rather than a single 90° angle. This helps to distribute stress more evenly and avoids concentrating it at a sharp corner.

6. When it is not possible to use a smooth curve to change the direction of a conductor, two 45° bends are a better option than a single 90° angle.

1. It is recommended to place small conductors inside the bending area. Small conductors (<0.007") are more resistant to extrusion than to tension. Positioning such conductors on the inside of the bend can minimize or prevent tension. Avoid stacking conductors in multilayer structures to prevent the I-beam effect. Stacking conductors inevitably increases the overall thickness of the circuit, reducing both flexibility and the ability of the circuit to bend reliably.
2. **Conductor**

The flexible circuit conductor is produced using a photoetching process, starting with a full sheet of copper. A mask is applied to define the ideal conductive path, and then a chemical process removes the excess copper, leaving the desired circuit pattern to form the conductor. The etchant dissolves the unmasked copper and also etches the edges of the conductor, leading to “side etching.”

As the thickness of the copper foil increases, side etching becomes more pronounced. This makes it difficult for flexible circuit manufacturers to produce very fine conductors with thick copper foils. Additionally, the etching process can vary, with the strength of the etchant changing depending on the copper content in the solution. Designers must account for these processing tolerances in trace width (and line spacing). For optimal etching results, the width of the conductor should be at least five times the copper foil thickness.

It is advisable to set the conductor width as wide as possible. For instance, if a conductor width of 0.005″ is required between pads in an isolated area, the conductor width should increase to 0.010″ to 0.012″ once it leaves the isolated area. This helps improve the etching yield, thereby lowering the total cost of the circuit.

If it is necessary to reduce the conductor width between the pads in the isolated area, it should revert to the original width once the conductor exits the isolated area.

3. **Land Fillet**

It is recommended to add a fillet at every point where the conductor enters the pad. Pad fillets help reduce or eliminate potential stress concentration points.

Avoid relying on copper tear-stop blocks to prevent tearing, as these have been shown to be ineffective in stopping tears or preventing cracks from spreading.

4. **Design Solutions to Reduce Tearing**

To reduce tearing, consider optimizing design elements such as the conductor layout, copper thickness, and pad sizes to minimize stress during bending and flexing.

5. **Via**

Vias are used to connect multiple layers at specific locations on the PCB. Blind vias connect the outer layer to adjacent internal layers but do not pass through the entire circuit. Buried vias connect inner layers but do not extend to the outer layer. Both blind and buried vias increase manufacturing costs but provide more usable space on undrilled layers of the PCB.

For SMT clearance openings, the two most common cover materials are polyimide film and flexible solder mask. The methods of creating clearance openings in these materials differ, so design requirements vary as well. A clearance opening in polyimide film can be created through drilling, milling, or punching. However, the shape and size of the opening are constrained by the circular drill or tool used, so the resulting openings are typically circular or elliptical. A common design practice in flexible circuits is to use a set of clearance openings for multiple SMT pads.

Flexible solder masks, like traditional PCB solder masks, are created using photosensitive imaging, which allows for any shape of opening. The clearance opening for the solder mask should be slightly larger than the SMT pad to account for any alignment deviation during the printing process, ensuring the solder mask does not interfere with the pad.

6. **Controlled Impedance and Signal Integrity**

As the operating speed of electronic equipment continues to rise, it is crucial for the characteristic impedance of all components in an electronic assembly—whether flexible PCB or rigid PCB—to match. Impedance mismatches lead to signal reflections and degradation at each mismatch point, resulting in erroneous signals and potentially causing the equipment to malfunction.
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