PCB Reference Plane Void for Impedance Matching
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Solution Overview
Problem
In printed circuit board (PCB) designs, high-speed serial signal lines connected to passive elements via pads experience poor signal integrity due to sharp changes in characteristic impedance from the signal transmission line to the pad, primarily because the pad width is greater than the signal transmission line width, causing signal reflections and degradation.
Innovation Solution
The PCB design incorporates an elliptic-shaped void in the reference plane corresponding to the passive element, ensuring a smooth return path and matching impedance between the signal transmission line and the pad, which is calculated using specific equations to maintain consistent impedance and improve signal integrity.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Ease of manufacture
If the pad width is increased to accommodate passive elements, then the passive element mounting capability is improved, but the characteristic impedance changes sharply causing poor signal integrity
Solution Approach 1:
The patent applies curvature by replacing the conventional rectangular pad shape with a circular pad shape. This circular configuration smooths the impedance transition between the signal transmission line and the pad, reducing signal reflections and improving signal integrity while still providing adequate mounting area for passive elements.
Solution Approach 2:
The patent modifies the local geometry of the pad structure by introducing a circular shape with specific dimensional relationships (pad diameter, annular ring width, via diameter) to optimize impedance matching in the critical transition zone between transmission line and pad, while maintaining standard mounting capabilities.
2Ease of operation
If the pad width is greater than the signal transmission line width, then passive element mounting is facilitated, but signal reflections increase due to impedance mismatch
Solution Approach 1:
The circular pad geometry replaces the sharp rectangular corners and edges with continuous curved boundaries, creating a gradual impedance transition that reduces signal reflections. The circular shape with optimized diameter maintains adequate mounting area while minimizing harmful impedance discontinuities.
Solution Approach 2:
The patent changes the geometric parameters of the pad structure by defining specific dimensional relationships (pad diameter D, annular ring width W, via diameter d) that optimize impedance matching. These parameter adjustments ensure smooth impedance transition while preserving passive element mounting functionality.
Data Source
AI summary
A printed circuit board includes a signal plane and a reference plane. The signal plane includes a pad, a passive element mounted on the pad, and a signal transmission line electrically connected to the passive element via the pad. The reference plane provides a return path for a signal transmitted through the passive element and the signal transmission line. A void is defined in the reference plane corresponding to the passive element, to increase a length of the return path. A length of a first axis, perpendicular to the signal transmission line, of the void satisfies a following equation:W1≈8Wpad+10T0.8Wtrace+T,wherein Wpad is a width of the pad, Wtrace is a width of the transmission line, T is the height of the pad.