Flexible Circuit Board Layout for Stable High-Speed Optical Links
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Solution Overview
Problem
Conventional flexible circuit boards in optical modules suffer from insufficient GND referencing around high-speed links, leading to impedance discontinuities, instability, and reduced bandwidth, which affects the overall reliability of the optical module.
Innovation Solution
A flexible circuit board design with a top metal layer featuring a high-speed signal line and an auxiliary ground reference adjacent to the signal line, electrically connected to a ground zone through conductive holes, ensuring sufficient grounding and forming a coplanar waveguide to stabilize impedance and increase bandwidth.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Device complexity
If high-speed signal lines are routed on the top surface of the flexible circuit board with ground reference on the bottom surface, then the structural design is simple, but the GND referencing around the high-speed link is insufficient, causing impedance discontinuities and reduced bandwidth
Solution Approach 1:
The patent transitions from a two-dimensional planar ground reference layout to a three-dimensional stacked configuration. The auxiliary ground reference is positioned on the top surface in vertical alignment with the ground zone on the bottom surface, connected through conductive holes. This dimensional change enables sufficient GND referencing around high-speed signal lines without increasing lateral complexity, resolving the contradiction between structural simplicity and impedance stability.
Solution Approach 2:
The patent applies local quality by creating an auxiliary ground reference specifically around the high-speed signal line area where it is most needed. Rather than uniformly distributing ground references across the entire board, the auxiliary ground reference is localized to the vicinity of high-speed signals, providing targeted impedance control where required while maintaining overall design simplicity.
2Ease of manufacture
If the high-speed pins of the TO-CAN are soldered to the high-speed signal via pads, then the connection is established, but the high-speed pins and signal lines are far away from the GND reference, causing impedance discontinuities and instability
Solution Approach 1:
The auxiliary ground reference acts as an intermediary element between the high-speed signal lines and the ground zone. By positioning the auxiliary ground reference on the top surface adjacent to high-speed signal lines and connecting it to the bottom surface ground zone through conductive holes, it provides a local GND reference that mediates the impedance control requirement, enabling stable impedance throughout the signal path from pins to signal lines.
3Productivity
If conventional flexible circuit board design is used, then the manufacturing process is straightforward, but the bandwidth is reduced due to insufficient grounding around the high-speed link
Solution Approach 1:
The patent segments the ground reference function into two distinct parts: the ground zone on the bottom surface and the auxiliary ground reference on the top surface. This segmentation allows each part to fulfill its specific function optimally - the bottom ground zone provides overall grounding while the top auxiliary ground reference provides local GND referencing for high-speed signals, thereby maintaining manufacturing simplicity while enhancing bandwidth performance.
Data Source
AI summary
A flexible circuit board, a light receiving/transmitting assembly, an optical module, and a connection method. The flexible circuit board includes a top metal layer having a high-speed signal line formed therein, a substrate, and a bottom metal layer having a ground zone formed therein which are sequentially stacked in a thickness direction. The high-speed signal line has a high-speed-signal via a pad located at one end of the flexible circuit board and electrically connected to an external high-speed pin. The top metal layer has an auxiliary ground reference formed therein. The auxiliary ground reference is disposed neighboring the high-speed signal line and stacked with the ground zone. The auxiliary ground reference is electrically connected to the ground zone by means of an auxiliary conductive hole, and the auxiliary ground reference and/or the ground zone is provided with a ground via pad electrically connected to an external ground pin.