EMBEDDED CAPACITANCE

Presented by

Scott Piper, General Motors with special acknowledgement to Gentex corporation

Case for Embedded Capacitance2

Capacitor Equivalent Circuit

Capacitance

(Large)

Parasitic

Inductance

(Small)Parasitic

Resistance

(Small)Capacitance becomes a

short circuit at high

frequency

Inductance

becomes an open

circuit at high

frequency

3

Impedance vs. Frequency

1µF 0603 Capacitor

Self Resonant

Frequency

4

Real Estate

As designs become

more complex and

microprocessors are

required to do more,

space near an IC is at

a premium

To make things worse,

microprocessor speeds are

increasing requiring low impedance

PDN at high frequency

5

Concept of embedded capacitance6

Parallel Plates Separated by Dielectric

+ + + + + + + + + + + + + + + + + + + + + + + + + + + +

- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -

Permittivity

Of Dielectric Plate Area

Plate

Separation

d

Capacitance

7

Multi-layer PCBs

Most PCBs are a

standard thickness

More layers in a PCB

require the layers to

be closer together

This results in more

capacitance between

layers

8

Methodology9

Experimental PCB

PCBs were designed and manufactured for embedded capacitance research

Various Dimensions

4 Layer (two planes and two signal layers)

Most are FR4 cores

Various spacing between planes

Measurement Methodology11

Network Analyzer

Port 1

Z11 was measured

to determine power

distribution network

(PDN) Impedance

PCB

A PCB mounted

coaxial connector was

used to connect the

planes to the network

analyzer

Z11 Measurement Results12

The expectation was to see the narrow plane spacing (76µm) perform better at high frequency

The measurement did not show this

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0

20

40

60

80

0.1 1 10 100 1000 10000

Imp

edan

ce (

dBΩ

)

Frequency (MHz)

Plane Separation Comparison (Z11)

76µm 254µm

Ideal 76µm Ideal 254µm

Single port measurement and

simulation13

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0

10

20

30

40

50

10 100 1000

Z1

1 (

dB

)

Frequency (MHz)

Single Port Impedance vs. Connector Height

Simulation Data

2mm height

1.1mm height

0.2mm height

• Simulations were

performed using the

Finite Integration

Technique

• It was discovered that

the high frequency

impedance was

dependent upon the

height of the coax

connector

• 0.2mm height was

represented by a port

existing between PCB

planes

New Measurement Methodology14

Network Analyzer

Port 1

PCB

Port 2

To port 1

To port 2

Plane 1

Plane 2

S21 Test Setup

50ΩSource

50ΩLoad

Distributed

Embedded

Capacitance

Path 1 Path 2

15

EM Simulation16

Technique:

Full Wave 3D Solver

Finite Integration Method

EM Simulation vs. Measurement17

6” x 2” PCB with

3 Mils Plane Separation (FR4 Core)

Source and 50 ΩLoad at Center

18

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0

10

1 10 100 1000 10000

Rela

tive Im

pedance

(dBΩ

)

Frequency (MHz)

Calculated vs. Simulated Plane Pair Impedance

Z Simulated

Z Calculated

Embedded capacitance works as a parallel plate

capacitor at low frequencies but at higher frequencies

other factors become dominant

19

Impedance of

Capacitors

Energy is stored in a capacitor as an electric field

In the following field plots

A strong electric field indicates the applied energy was

used to charge the planes

A weak electric field indicates that embedded

capacitance was utilized thus generating a magnetic

field.

Electric Field at 10 MHz

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-10

0

10

1 10 100 1000 10000

Z Simulated

Z Calculated

Electric Field at 100 MHz

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0

10

1 10 100 1000 10000

Z Simulated

Z Calculated

Electric Field at 170 MHz-70

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0

10

1 10 100 1000 10000

Z Simulated

Z Calculated

Electric Field at 250 MHz

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-60

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0

10

1 10 100 1000 10000

Z Simulated

Z Calculated

Electric Field at 800 MHz

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0

10

1 10 100 1000 10000

Z Simulated

Z Calculated

Scale

Changed

Surface Current

at 800 MHz-70

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0

10

1 10 100 1000 10000

Z Simulated

Z Calculated

Scale

Changed

Concept of Embedded Capacitance

Below 100 MHz, PCB capacitance is utilized

throughout the entire PCB and behaves as an ideal

parallel plate capacitor

Above 100 MHz other factors cause impedance to

increase making the PCB not an ideal parallel plate

capacitor

PCBs of various dimensions with

discrete capacitorsDemonstration

Example

4x4 inch PCB with 2 adjacent

plane layers and capacitor pads

Will compare

- 3 mil plane spacing

- 30 mil plane spacing

Each comparison will involve

capacitors located

-1 inch away from the source

- 2 inches away from the source

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0

1 10 100 1000

S21 (

dB)

Frequency (MHz)

Decoupling Capacitor Placement

4x4 3 mil close

4x4 3 mil far

4x4 30 mil close

4x4 30 mil far

Capacitor Location

As plane distance decreases, distance between the

capacitors and the excitation source (or load)

becomes less important

PCBs of various dielectric thicknesses

and the same dimensions32

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0

1 10 100 1000

S21 (

dB)

Frequency (MHz)

Measurement of PCBs with Varying Dielectric Thickness2x8 Inch Dimension

3 Mil

30 Mil

3M C0614(0.55 mil not FR4)

33

Dielectric Thickness

Closer plane spacing results in lower plane

impedance at low frequencies as well as high

frequencies

Lower Q factor is also achieved by closer plane

spacing

34

PCBs of various dimensions and the

same dielectric thickness35

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0

1 10 100 1000

S21 (

dB)

Frequency (MHz)

Measurement of PCBs with Varying Dimensions3 mil Dielectric Thickness

2x2"

2x6"

36

37

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0

1 10 100 1000 10000

S21 (

dB)

Frequency (MHz)

2x8 Inch 3 Mil PCB vs. 4x4 Inch 3 Mil PCB

2x8 3 Mil

4x4 3 Mil

Plane Area

Increasing PCB size causes lower impedance at low

frequencies, but does not affect impedance at high

frequencies

PCBs with identical plane area but different length

and width dimensions can have different

impedances at high frequency

38

Impact of Dielectric Material39

Ceramic Core Dielectric <3 mil40

Simulation results

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0

1 10 100 1000

S2

1 (

dB

)

Frequency (MHz)

FR4 Core vs Ceramic Core Material

12µm Plane Separation

Ceramic Core FR4 Core

Dielectric Material41

Higher permittivity materials definitely have their

advantages at lower frequency

At higher frequency (in this case around 200MHz

and up) the core material does not provide lower

impedance

Remember: closer plane spacing DOES impact high

frequency impedance

New materials can make it easier (cost effective) to

manufacture PCBs with very close layers

PCBs of various dimensions with

discrete capacitors42

12 x 0.1µF capacitors added43

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0

1 10 100

S21 (

dB)

Frequency (MHz)

Measurements of PCBs with and without Capacitors2x8" 3 mil Dielectric

12x0.1µF Caps

No Caps

44

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0

1 10 100

S21

(dB)

Frequency (MHz)

Measurements of PCBs with and without Capacitors2x8" with 3 mil Dielectric & 3M Dielectric

12x0.1µF Caps

No Caps

No Caps 3 M Material

45

The Concept

Discrete chip decoupling capacitors

provide low impedance at low

frequencies (<100 MHz) but not at

higher frequencies due to ESL

Embedded capacitance provides low

impedance at high frequencies but

typically do not perform as well as

discrete capacitors in their usable

range

Why not use

both?

46

47

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0

1 10 100 1000

S21 (

dB)

Frequency (MHz)

6x23cm PCBs with and without Capacitors76µm Dielectric

12x0.1µF Caps No Caps

PCB DominatesCapacitors Dominate

Radiated Emissions caused by plane

resonance48

Radiated Emissions Setup

Network

AnalyzerPort 1 Port 2

PCB

Antenna

Load

2 m longWires

Anechoic Chamber

1m

Metal Table

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0

20

40

60

80

0 100 200 300 400 500 600 700 800 900 1000

S21 (

dB)

and E

lect

ric

Fie

ld (

dBµ

V/m

)

Frequency (MHz)

Comparison Between Decoupling Analysis and Radiated Emissions

Impedance

Measurement

Radiated

Emissions

Measurement

50

Radiated Emissions vs. PCB Impedance51

The measurement shows correlation between plane

resonance and emissions

Inadequate PDN impedance may cause excessive

emissions

PCB geometry is an important factor in emissions

results

Changing PCB resonance frequency52

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0

1 10 100 1000

S21 (

dB)

Frequency (MHz)

Measurements of PCBs with and without Capacitors2x8" 3 mil Dielectric

12x0.1µF Caps

No Caps

133 MHz

53

2x8” 3 Mil Simulation Results54

Equivalent Circuit

Short Circuit

at High

Frequency

Still a Capacitor

at High

Frequency

LC Resonance

55

0.1 µF vs. 1 µF Capacitor Values56

Discrete Capacitors and Embedded

Capacitance

Discrete capacitors and embedded capacitance can

interact causing a parallel resonance raising PCB

impedance at some frequencies even as low as 130

MHz for a 2x8” PCB

This resonance is caused by the embedded

capacitance and the ESL of the capacitor

The value of the capacitor is not important

57

Manipulation of the first parallel

resonance58

#1 Change

Plane Spacing59

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0

1 10 100 1000

S21 (

dB)

Frequency (MHz)

Measurements of PCBs with Various Plane Spacing2x8" PCB With 12x0.1µF Capacitors

3 Mil

10 Mil

60

Changing plane spacing changes capacitance

#2 – Change Number of capacitors61

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0

1 10 100 1000

S21 (

dB)

Frequency (MHz)

Measurements of PCBs with Various Number of Capacitors2x8" PCB With 3 Mil Plane Spacing

12x0.1µF Caps

4x0.1µF Caps

62

Changing the number of capacitors changes inductance

#3 Change PCB Dimensions63

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0

1 10 100 1000

S21 (

dB)

Frequency (MHz)

Measurement of PCBs with Varying Dimensions3 mil Dielectric Thickness with 12x0.1µF Capacitors

2x8"

2x6"

2x2"

64

PCB

dimensions

affect

capacitance

One Plane Split65

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0

1 10 100 1000

S21 (

dB)

Frequency (MHz)

Measurement of PCBs with Varying Dimensions3 mil Dielectric Thickness with 12x0.1µF Capacitors

2x8" Solid

Split Plane2x2" Section

2x2"

66

Splitting the plane will decrease the plane area causing a lower capacitance value

#4 - High ESR Capacitors

0603 Size capacitors made by TDK

Center terminal is not connected to anything

Contains up to 1.2 Ohms of series resistance

More expensive than standard capacitors but

benefit can be seen by replacing some standard

capacitors with high ESR capacitors

67

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0

1 10 100 1000

Axis

Title

Axis Title

Measurement of PCBs with Varying Capacitor Types2x8" 3 mil Dielectric Thickness

12x0.1µF

8x0.1µF &4xHigh ESR

68

Resistance in series with the capacitor will create loss at the resonant frequency

Parallel Resonance due to Embedded

Capacitance

Further plane spacing = higher frequency

resonance

More capacitors = higher frequency resonance

Smaller planes = higher frequency resonance

More Series Resistance = less resonance

Is a higher frequency resonance better

than a lower frequency resonance?

….. It depends!

69

Summary70

Plane impedance can be determined using EM simulation and measuring equipment but plane connections may cause error

PCB Power/Ground Plane Separation

Below resonant frequency, the PCB planes behave as a parallel plate capacitor and the capacitance can be easily calculated

Above resonant frequency, plane impedance is more complicated and it depends on several factors including PCB geometry

Dielectric Material

Higher permittivity materials cause higher capacitance below the PCB resonance which can be useful in PDN design

Higher permittivity materials do not make a significant difference in PDN impedance at high frequency but the close plane spacing which generally accompanies high permittivity materials make a great difference

Adding Discrete capacitors

PCB embedded capacitance for most devices still can’t provide low impedance PDN compared to discrete capacitor components on PCBs

Using embedded PCB capacitance along with discrete capacitors can be a good solutionbut will cause a resonance at a frequency depending on the number of capacitors and the amount of PCB capacitance.