Serpentine Routing in flex PCB design

What is Serpentine Routing?

Serpentine routing is like making a detour on a road to ensure all travelers arrive at the same time.

Imagine you're organizing a race where multiple runners start from different locations but need to reach the finish line simultaneously. If one runner has a shorter route, you make them take extra loops to match the longer paths of others.

In PCB design, serpentine routing does the same for high-speed signals. It adds zigzag patterns to shorter traces so that all signals travel the same electrical distance and arrive in sync, preventing timing issues in circuits like DDR memory or high-speed communication buses.

Serpentine routing is a design method used to adjust the signal path length, which is particularly crucial in high-frequency and high-speed circuit designs. Serpentine routing (also known as switchback routing) is commonly used in various applications, especially for delay compensation (such as timing equalization). This design involves controlling the trace length, trace width, and trace spacing and is typically referred to as serpentine routing.

Functions of Serpentine Routing

The most widely used is delay compensation (equal timing). In high-speed flex PCB design, delay compensation keeps the delays of each signal in a special signal group within the same range, ensuring the validity and correctness of the data read by the system in the same cycle (if the delay difference exceeds one clock cycle, the data of the next cycle will be misread).

Delay Adjustment: In high-speed signal transmission, serpentine routing helps maintain signal synchronization by equalizing the length of signal traces.

Minimize Crosstalk: Proper spacing in serpentine routing reduces interference between signals, enhancing signal quality.

Improve EMC: Optimized routing reduces EMI, improving the rigid-flex PCB’s EMC performance.

Used as a board-mounted antenna, many WIFI modules and Bluetooth modules use serpentine traces as board-mounted antennas, and board antennas have significant cost advantages.

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Design Principles of Serpentine Routing

Avoid Excessive Bends: Excessive bending can cause impedance mismatches, affecting signal integrity.

Maintain Proper Spacing: Proper spacing prevents excessive coupling effects between adjacent traces.

Control Routing Length: Excessive routing length increases transmission delay, affecting overall signal synchronization.

Use Meandering Traces: Meandering traces reduce signal reflection, enhancing signal quality compared to right-angle bends.

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Example: Serpentine routing design of DDR SDRAM clock signal 

In high-speed digital circuit design, DDR SDRAM is widely used in computer motherboards, graphics cards, and embedded systems. DDR SDRAM relies on CLK to synchronize data transmission, and the delay matching of DQS and clock signals is crucial, otherwise, it will cause data sampling errors and affect system stability.

Due to the uneven routing length in PCB design, the clock signal and data line may have different propagation delays. If the clock signal reaches the DDR SDRAM faster or slower than the data line, it will affect the read-and-write timing of the data, causing the signal to be unable to align correctly, and affecting system performance.

When designing DDR SDRAM lines, engineers use Serpentine Routing to adjust the delay of the clock signal to keep it synchronized with the data line. The specific approach is as follows:

• Calculate the longest path of the data line and determine the target matching length of the clock signal.

• Add serpentine routing to the shorter clock signal line to achieve the same propagation delay as the data line.

Design rules:

• Equal length principle: Ensure that the length of the clock signal after the serpentine routing matches the longest data line.

• Spacing control: The spacing between serpentine lines is usually required to be ≥2 times the line width to reduce crosstalk.

• Avoid excessive winding: Too many serpentine lines will increase distributed capacitance and distributed inductance, which will affect signal integrity.

Through serpentine routing, the delay difference between the clock signal and the data line of DDR SDRAM can be controlled within a reasonable range (generally not more than 1/4 clock cycle), ensuring that data can be sampled at the correct clock edge, thereby improving signal integrity and system stability.

Conclusion

Serpentine routing is a critical technique in flex PCB design, improving signal synchronization, minimizing crosstalk, and enhancing EMC. Whether for rigid flex PCB or fully flexible PCBs, a well-designed serpentine trace enhances circuit performance. For high-quality PCBs, choose a professional flex PCB manufacturer.