PCB Non-Functional Slivers Reduce Parasitic Capacitance
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Solution Overview
Problem
Existing printed circuit boards (PCBs) face challenges with thermal stress and signal integrity due to unused via stubs and parasitic capacitances caused by non-functional pads, which increase manufacturing costs and affect performance.
Innovation Solution
Replacing non-functional pads with slivers that reduce parasitic capacitances and enhance thermal robustness, allowing for backdrilled holes to be unfilled, thus eliminating the need for epoxy filling and minimizing thermal stress.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Strength
If non-functional pads are used to strengthen the structure and reduce thermal stress, then PCB robustness is improved, but parasitic capacitances increase which deteriorate signal integrity
Solution Approach 1:
The non-functional pad is segmented into multiple non-functional features (NFFs) arranged around the via. Instead of a single large pad, multiple smaller conductive features are distributed around the via, providing structural support while reducing the total parasitic capacitance. This segmentation allows the structure to maintain strength through distributed support points while minimizing the harmful capacitive effects of any single feature.
Solution Approach 2:
The conductive features are strategically positioned at specific locations around the via rather than forming a continuous pad. This local placement provides structural reinforcement exactly where needed around the via interface, while leaving other areas empty to minimize parasitic capacitance. The local quality approach ensures that structural support is concentrated at critical stress points without the penalty of a large continuous conductive area.
2Productivity
If through vias are used for manufacturing efficiency, then productivity is improved, but unused via stubs create resonance which deteriorates signal integrity
Solution Approach 1:
The via stub, which would normally be a harmful resonance source, is converted into a beneficial structural support feature. The non-functional conductive features are positioned to utilize the via stub structure, transforming what would be a signal integrity problem into a mechanical reinforcement element. This approach maintains the manufacturing efficiency of through vias while eliminating the harmful resonant effects by repurposing the stub structure for structural support rather than electrical connection.
3Reliability
If backdrilled holes are left unfilled to eliminate via stubs, then signal integrity is improved, but thermal stress causes pads to rotate and micro-cracks to develop
Solution Approach 1:
Non-functional conductive features are placed around the via holes before the holes are backdrilled and left unfilled. This preliminary structural reinforcement is in place before the backdrilling operation removes the via stub material, providing immediate support to the pad structure. The preliminary action of adding these support features ensures that when the via stubs are later removed to improve signal integrity, the pads have already been strengthened and are resistant to thermal stress-induced rotation and micro-crack formation.
Data Source
AI summary
A multi-layer PCB has conductive vias (134) passing through multiple layers. A layer may have a conductive non-functional feature (710) physically contacting a via but not surrounding the via, to make the PCB more resistant to thermal stresses while, at the same time, reducing the parasitic capacitance compared to a prior art non-functional pad (310n).