Shielded Three-Layer Patterned Ground Structure for EMI Reduction
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Solution Overview
Problem
Hard disk drives (HDDs) face electromagnetic interference (EMI) issues due to common mode current flowing through conductive traces on printed circuit boards (PCBs), which existing solutions like surface mounted common-mode filters either fail to address effectively or compromise signal integrity.
Innovation Solution
A shielded three-layer patterned ground structure in PCBs, featuring a middle shield laterally spaced by a moat filled with dielectric material, with a top shield having distinct portions over the moat, and conductive traces between these portions, reduces common mode current and EMI noise without impacting differential signal integrity.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Object-affected harmful factors
If a surface mounted common-mode filter is used to correct EMI, then EMI is reduced, but signal integrity is compromised
Solution Approach 1:
The harmful common-mode current is extracted and redirected through the patterned ground shield structure. The PGS provides a dedicated return path for common-mode current that is separated from the differential signal traces, removing the harmful effect without affecting the signal integrity of the differential pair.
Solution Approach 2:
The patterned ground shield acts as an intermediary structure between the differential traces and the reference ground plane. It provides a controlled impedance path that mediates the common-mode current flow, preventing it from coupling into the signal traces while maintaining proper signal reference.
2Object-affected harmful factors
If connection changes are made to address common mode current, then EMI is reduced, but space requirements and signal integrity are impacted
Solution Approach 1:
The solution moves from a two-dimensional trace layout to a three-dimensional shielded structure by adding vertical separation with dielectric layers. The patterned ground shield is positioned in a different vertical plane from the differential traces, allowing EMI reduction without increasing the horizontal footprint of the PCB.
Solution Approach 2:
The patterned ground shield structure is nested within the existing PCB layer structure. The PGS is embedded between the differential traces and the reference ground plane, utilizing the available vertical space within the standard PCB stackup without requiring additional external components or increased PCB area.
3Ease of manufacture
If PCB layer count is reduced to four layers for cost reduction, then manufacturing cost decreases, but EMI control becomes more difficult
Solution Approach 1:
The patterned ground shield layer serves multiple functions simultaneously: it provides a reference plane for differential signals, creates a shield against common-mode noise, defines controlled impedance paths, and acts as a ground reference. This multi-functionality allows effective EMI control in a four-layer design without requiring additional dedicated shielding layers.
Solution Approach 2:
Rather than providing continuous ground coverage, the ground plane is locally patterned only where needed beneath and adjacent to the differential traces. This localized grounding approach provides EMI protection and controlled impedance where required while leaving other areas open for routing or cost reduction, optimizing the balance between performance and manufacturing cost.
Applied Scientific Principles
This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.
Function Achieved in This Case
This solution effectively reduces common mode current and EMI noise in PCBs, lowering costs and maintaining signal integrity, and can be applied to both HDDs and solid-state devices.
Implementation Method 1
the patterned ground shield is laterally spaced from the middle shield by a moat, wherein the moat is filled with dielectric material
Implementation Method 2
A shielded three-layer patterned ground structure in PCBs... reduces common mode current and EMI noise
Data Source
AI summary
The present disclosure generally relates to a shielded three-layer patterned ground structure in a PCB. The PCB may be disposed in a hard disk drive. To reduce costs, PCBs are being made with only four total layers separated by dielectric material. Conductive traces in PCBs can have the problem of common mode current flowing through the traces and thus increasing the magnitude of EMI noise. By providing a shielded three-layer patterned ground structure, not only is the cost reduced, but so is the common mode current and the magnitude of EMI noise, all without any negative impact to the differential signal.