PCB Trace Width Variation for Impedance Matching
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Solution Overview
Problem
Impedance mismatches between printed circuit boards (PCBs) in multi-board assemblies lead to signal reflection and instability, exacerbated by manufacturing tolerances in trace widths.
Innovation Solution
The method involves determining trace design widths and tolerances for traces on multiple PCBs, forming traces with wider ends than specified by the design width, and optionally using thicker conductors and a conductive silk screen to reduce impedance sensitivity and improve matching.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Manufacturing precision
If standard trace width design is used for PCB traces, then manufacturing is simpler and faster, but impedance mismatches occur between connected PCBs due to tolerance variations
Solution Approach 1:
The trace geometry is made non-uniform along its length, with different widths at different positions. The trace has a first width at the first end, a second width at the second end, and an intermediate width in between, allowing each section to compensate for impedance variations and achieve better overall impedance matching.
Solution Approach 2:
The trace width parameter is varied continuously or in steps along the length of the trace. By changing the width parameter from the first end to the second end, the impedance characteristics are adjusted to compensate for manufacturing tolerances and achieve better impedance matching between connected PCBs.
2Reliability
If trace width is increased at the ends of PCB traces, then impedance matching improves and reflection coefficient decreases, but manufacturing precision requirements increase
Solution Approach 1:
The trace is designed with different widths at different locations - wider at the ends and narrower in the intermediate section. This local variation in geometry allows the trace to be more tolerant of manufacturing variations in the critical end regions while maintaining good impedance matching.
Solution Approach 2:
The trace geometry is designed in advance to compensate for expected manufacturing tolerances. By making the trace wider at the ends, the design creates a buffer or cushion that absorbs the impact of width variations, ensuring reliable impedance matching even when manufacturing precision varies.
3Ease of manufacture
If PCB traces are designed with uniform width, then manufacturing is easier, but signal reflection increases due to impedance mismatches at connection points
Solution Approach 1:
Instead of uniform width, the trace has varying width with larger dimensions at the connection points (ends) and smaller dimensions in the intermediate section. This local variation reduces impedance mismatches at connection points, thereby reducing signal reflection while remaining manufacturable.
Solution Approach 2:
Rather than making the trace wider in the middle and narrower at ends (which would be simpler to manufacture but cause reflection), the design inverts this by making it narrower in the middle and wider at ends, where the wider sections are needed for impedance matching at connection points.
Data Source
AI summary
In one or more embodiments, a system and method for impedance matching across subassemblies comprises forming a PCB with wider traces, wherein a trace is formed based on a width corresponding to a trace design and a tolerance. The larger width reduces a reflection coefficient to reduce impedance differences. A Surface mount technology (SMT) breakout area may include a thicker microstrip and a conductive silk screen layer may be added to further improve impedance matching without increasing the total height of a PCB.