EMC Issues on Printed Circuit Boards - CST

EMC Issues on Printed Circuit Boards Matthias Tröscher

www.cst.com | May-11

Facts  Electrons do not read schematics!  Currents must always return to their source!  Currents don’t have to return along the path dedicated by the schematic!  Currents will return to their source along the path of the least impedance!


More Facts  All conductors have got a certain amount of impedance, therefore the conventional notion of “ground” as a zero-impedance, equipotential surface is WRONG!

 Bruce Archambeault from IBM says: “Ground is a place where potatoes and carrots thrive!” (that’s a fact!)  Always think in terms of (frequency dependent) current return paths when talking about “ground”  Low frequency  lowest resistance  High frequency  lowest inductance  Change over ~ 100 kHz www.cst.com | May-11

PCB Layout - Potential Problems  Intentional Signals    

Clock I/O Video …

 Unintentional Signals    

Common mode currents Crosstalk coupling Power plane bounce …

 Most EMC problems come from common mode currents  All common mode currents are caused by intentional signals  Reduction of high frequency harmonics on the current lowers EMC common mode currents  So – why fight an emission problem which is due to a current that is not required? www.cst.com | May-11

CST BOARDCHECK (reading the hidden schematic)  EMC rule checking based on IBM technology


EMC Issues on Printed Circuit Boards Example 1: Splits in reference plane Example 2: Nets changing reference plane


Splits in Reference Plane  Power layers often have splits  There are multiple power islands on one layer  Return current path is interrupted  Consider spectrum of clock signal  Consider stitching capacitor impedance  High frequency harmonics might not return directly www.cst.com | May-11

Split Reference Plane Example

Courtesy of Bruce Archambeault


Split Reference Plane Example

Stitching capacitors allow return currents to cross split ??? Really?



Capacitor Impedance

C, ESR, ESL


Near Field Radiation from Microstrip on Board with Split in Reference Plane Comparison of Maximum Radiated E-Field for Microstrip With and without Split Ground Reference Plane 120

110

Maximum Radiated E-Field (dBuv/m)

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90

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70 No-Split

60

Split 50

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20 10

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Frequency (MHz)



With “Perfect” Stitching Capacitors Across Split Comparison of Maximum Radiated E-Field for Microstrip With and without Split Ground Reference Plane and Stiching Capacitors 120

110


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60 No-Split 50

Split Split w/ one Cap

40

Split w/ Two Caps

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20 10

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Frequency (MHz)



Stitching Caps with Inductance and Via Inductance Comparison of Maximum Radiated E-Field for Microstrip With and without Split Ground Reference Plane and Stiching Capacitors 120

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60 No-Split Split Split w/ one Cap Split w/ Two Caps Split w/One Real Cap Split w/Two Real Caps

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Frequency (MHz)



Estimated Transfer Inductance for Trace Crossing Split Plane Microstrip Configuration (valid to 2 GHz) Estimated Transfer Inductance for Trace Crossing Split Plane Microstrip Configuration (Valid to 2 GHz) 12

Split Width = 20 mil Split Width = 40 mil Split Width = 60 mil

Transfer Inductance (nH)

10

8

d

6

4

2

0 0

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700

Distance to Capacitor (mils)



Are Stitching Capacitors Effective?  YES, at low frequencies  NO, at high freqiencies  Consequences  Need to limit the high frequency current spectrum  Need to avoid split crossings with ALL critical signals  The same is true for so-called “differential” signals!! www.cst.com | May-11

EMC Issues on Printed Circuit Boards Example 1: Splits in reference plane Example 2: Nets changing reference plane


Return Current Across Reference Plane Change What happens to return current in this region?

Critical Net Segment #2 Reference Plane B for Critical Net Segment #2

Reference Plane A for Critical Net Segment #1 Critical Net Segment #1


Via

How can the Return Current Flow When The Signal Line goes through a Via?

 The current can NOT go from one side of the plane to the other through the plane (reason: skin depth)

 The current must go around the plane at the via hole, through a demanded decoupling capacitor, around the second plane at the second via hole  Or… by displacement current spread www.cst.com | May-11

Decoupling capacitors close to Via

Critical Net Segment #2 Reference Plane B for Critical Net Segment #2 Via

Reference Plane A for Critical Net Segment #1 Critical Net Segment #1


ACCEPT

Critical Net Via Options



Via Options for Multi-Layer Boards



EMC Issues on Printed Circuit Boards Online demo of CST BOARDCHECK


CST PCB STUDIO & CST BOARDCHECK


EMC Issues on Printed Circuit Boards Thank you for your attention!
