Multi-layer Substrate Via Transition Impedance Control
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Solution Overview
Problem
High-speed multi-layer substrates face challenges in maintaining consistent characteristic impedance across interconnection circuits, leading to reflection losses and mode transformations, particularly in differential signaling, due to impedance mismatching at signal via transitions to planar transmission lines.
Innovation Solution
A multi-tier transition structure is introduced, featuring dummy pads in the clearance hole area to control impedance, with a matching section connecting signal via pads to planar transmission lines, and a specific ground system to reduce mode transformations, ensuring impedance matching and minimizing leakage losses.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If a large clearance hole is formed in the conductor layer to prevent electrical contact between signal via and other conductive surfaces, then electrical isolation is improved, but characteristic impedance control deteriorates due to impedance mismatching at the via-to-transmission line transition
Solution Approach 1:
An intermediary structure (transition structure with tapered or curved sides, and/or dummy pads) is introduced between the signal via and the planar transmission line to gradually transform the impedance. This intermediary element bridges the gap between the via structure and the transmission line, preventing abrupt impedance changes while maintaining the clearance hole for electrical isolation.
Solution Approach 2:
The geometry parameters of the via structure (tapered sides, curved sides, or added dummy pads) are modified to control the impedance transformation. By changing the shape and dimensions of the via structure, the impedance transition is smoothed, allowing the clearance hole to maintain isolation without causing severe impedance mismatching.
2Manufacturing precision
If ground vias are added around the signal via structure to improve characteristic impedance control and reduce leakage loss, then impedance control is improved, but device complexity increases
Solution Approach 1:
The signal via structure is designed to serve multiple functions: it provides signal transmission, impedance control through its modified geometry (tapered/curved sides), and structural support. The dummy pads, when used, also serve dual purposes of impedance matching and providing mechanical support, reducing the need for additional ground vias in some configurations.
Solution Approach 2:
Instead of uniformly adding ground vias around the entire signal via structure, the patent applies local modifications only where needed - such as tapered sides at specific portions of the via or dummy pads positioned strategically in the clearance hole area. This localized approach achieves impedance control without unnecessarily increasing overall device complexity.
3Device complexity
If a simple via structure is used to connect signal terminal and planar transmission line, then device complexity is reduced, but reflection loss increases due to characteristic impedance mismatching
Solution Approach 1:
The patent employs curved or tapered side profiles for the via structure instead of straight cylindrical walls. This curvature gradually transforms the impedance from the via structure to the planar transmission line, reducing abrupt discontinuities and minimizing reflection losses. The curved geometry acts as an impedance matching transformer.
Solution Approach 2:
The via structure is extended into the radial dimension by adding dummy pads or tapered/curved sides, transforming it from a simple cylindrical hole into a multi-dimensional structure. This dimensional expansion allows for gradual impedance transformation and better matching between the via and transmission line, reducing reflections.
Data Source
AI summary
A multi-layer substrate includes a planar transmission line structure and a signal via, which are connected by a multi-tier transition. The multi-tier transition includes a signal via pad configured to serve for a full-value connection of the signal via and the planar transmission line; and a dummy pad connected to the signal via, formed in an area of a clearance hole in a conductor layer disposed between a signal terminal of the signal via and the planar transmission line, and isolated from the conductor layer.