Flex PCB Design Layout and Routing Guidelines

In flex PCB design, many beginners or engineers switching fields often think: "Isn't routing just connecting A to B? As long as it’s connected, it’s fine, right?"However, in reality, the quality of routing directly determines the circuit’s performance, manufacturing yield, and long-term reliability! Careless routing can lead to minor issues like EMI exceeding limits and signal reflections, or major problems like component soldering defects and full board rework.

1. Trace Width in Flex PCB Routing

Under the production capabilities of mainstream FPC manufacturers in China, it is recommended that the trace width be ≥ 0.05mm.

2. Trace Length Control

Controlling trace length is very important in flex PCB design. Traces should be as short as possible to avoid unnecessary interference caused by excessive length. For critical signal traces (e.g., clock signals), ensure that the oscillator is placed very close to the device, unless there are special requirements.

3. Major Routing Taboo: 90° Right-Angle Traces

Avoid messy and irregular sharp angles or right angles in printed circuit routing. It is recommended to use 45° or 135° routing instead. Otherwise, unnecessary radiation may occur, leading to poor manufacturing performance. For example:

• Impedance discontinuity and distortion

• EMI interference from sharp corners: sharp points are high-frequency radiation sources

• Additionally, etching processes can easily introduce problems at sharp or right angles, causing etching deformation, trace thinning, or even breakage.

Thus, if a signal trace can transition at 135°, it must never turn at 90°!

4. Pads on the Same Net Should Not Be Directly Connected

Do you also habitually directly route between adjacent pads on the same net? It seems convenient, but actually creates big risks! Directly connecting two pads can easily cause solder bridging or short-circuiting during manual soldering, because solder naturally "wicks" to fill the gap between them.

Correct method for PCB routing:

Each pad should independently route out a short trace before merging — this way, soldering becomes safer and cleaner.

5. Proper Routing Out from Pads: Symmetry Along Long Axis or Short Axis

When routing traces diagonally from pads, production deviations in the solder mask can cause "irregular pads."Because of uneven solder tension, components like resistors might rotate or flip during placement! This is not an electrical design problem, but a soldering defect caused by improper PCB routing.

Key to solving rotation or shifting issues:

• Symmetrical routing along the long axis: reduces rotation

• Symmetrical routing along the short axis: reduces lateral shifting

Proper fan-out ensures chip components remain stable and evenly stressed after reflow soldering.

6. Connector Routing Must Exit from the Pad Center

Connectors are subject to frequent plugging and unplugging. If traces are routed improperly, they can tear during use!

Correct method:

• Traces must exit from the center of the pad.

• Routing from the edge, at an angle, or with unnecessary bends is strictly prohibited.

This detail is critical in flex PCB applications where mechanical stress is a major concern.

7. Open Loop Detection During Routing

In general, floating or disconnected traces are not allowed in flex PCB, primarily to avoid "antenna effects," which can introduce unwanted radiation and susceptibility to noise, possibly leading to serious consequences.

8. Avoid Self-Loop Routing

During flex PCB design, be careful about forming signal loops between different layers, especially when routing multilayer flex PCB boards. Self-loops can significantly increase radiation emissions.

9. Ground Loop Rule

The minimum loop rule states that the loop area formed by a signal trace and its return path should be as small as possible.

The smaller the loop area:

• The less electromagnetic radiation it emits

• The less external interference it receives.

Effective ground loop management is a vital skill in professional flex PCB routing.

10. Power and Ground Plane Integrity Rule

In regions with dense vias, be cautious to avoid creating voids that cause segmentation of the power or ground planes. Such segmentation compromises plane integrity and increases loop areas in signal routing, degrading overall performance. A solid and continuous ground plane is even more critical in flex PCB structures due to their physical flexibility and complex current return paths.

11. Shielding and Protection

Shielding corresponds to the ground loop rule and also aims to minimize signal loop areas. It is common for important signals, such as the clock and synchronization lines. For very critical high-frequency signals, consider coaxial shielding designs, which involve surrounding signal traces with ground traces on all four sides (top, bottom, left, and right). It is also crucial to carefully consider how to connect the shielding ground to the actual ground plane effectively.