Shielding Tutorial
Gentex EME Lab
Shielding Course Outline:
I.
II.
Why do we need shields?
Introduction to the Basic Shield Design Process
Apertures
B. Materials
Corrosion
Summations and Conclusions
Demonstrations
A. Measuring Shielding effectiveness of various materials in a Near‐Field Magnetic Field
B. Aperture Measurement Program
Questions
References
A.
III.
IV.
V.
VI.
VII.
EME Lab Shield Tutorial
2
The Question: Why do we need Shields?
EME Lab Shield Tutorial
3
Why do we need shields?
•
Immunity
–
•
Emissions
–
•
Prevent external energy from interfering with sensitive circuits Prevent noisy circuits and devices from interfering with neighboring devices
Self Compatibility
–
Prevent a device from interfering with itself
EME Lab Shield Tutorial
4
Shields: Definition and Misconception
Definition: A Shield is a conductive barrier enveloping an electrical circuit to prevent time varying Electromagnetic fields from coupling or radiating from the circuit.
Misconception: Most engineers take it as an almost unshakable axiom of engineering faith that a conductive surrounding will provide adequate shielding protection in all cases.
While shields can be very effective, designers will get the most performance when some key issues are kept in mind.
EME Lab Shield Tutorial
5
What makes a good shield?
•
It depends….
–
–
Frequency of interference
Type of interference:
•
•
–
Location of Field:
•
•
–
–
–
–
Magnetic‐Field
Electric‐Field
Near‐Field
Far‐Field
Number of openings and size of openings (Apertures)
Type of shielding material (Conductivity/Permeability)
Thickness of shielding material
Available mating surface (printed circuit board)
EME Lab Shield Tutorial
6
Introduction to the Basic Shield Design Process
EME Lab Shield Tutorial
7
Aperture Considerations
EME Lab Shield Tutorial
8
Aperture Design:
1)
2)
3)
Holes and slots act as windows for EM radiation to penetrate or escape a shield
Many small apertures allows less leakage than a single large aperture of the same area
Models show that in general, the aperture length should not exceed /50 for the highest frequency to be shielded (Wavelength)
EME Lab Shield Tutorial
9
Aperture Design:
1)
2)
3)
The main item determining the leakage from a slot is the maximum linear dimension (not area) of the opening.
Remember to take into account the highest frequency harmonic present.
Multiple apertures farther reduces the shielding effectiveness. The amount of reduction depends on:
a)
b)
c)
The spacing between the apertures
The frequency
The number of apertures
EME Lab Shield Tutorial
10
Aperture Equations:
SEdB = 20Log10(/(2L)), where L< /2
Where:
SEdB = shielding effectiveness
 = wavelength
L = aperture length, longest dimension


This is applicable for slots with a dimension equal or less than /2 wavelength. The equation illustrates: 


The shielding effectiveness is 0 dB when the slot is /2 long and Increases 20 dB/decade as the length L is decreased. Reducing the slot length by ½ increases the shielding by 6 dB.
EME Lab Shield Tutorial
11
Effect of Aperture Length on Shield Attenuation:
SEdB = 20Log10(/(2L)), where L< /2
EME Lab Shield Tutorial
12
Effect of Aperture Length
EME Lab Shield Tutorial
13
Aperture Length vs. Frequency for Various Attenuations:
EME Lab Shield Tutorial
14
Shield Attenuation with Multiple Apertures and fixed  and Aperture Length
RdB = 20log10(/2L) – 20log10(n1/2)
EME Lab Shield Tutorial
15
Affects of Apertures on Shield Currents
EME Lab Shield Tutorial
16
Aperture Design and Babinet’s Principle:
1.
2.
3.
The theory behind magnetic‐field shielding provided by induced currents presumes that currents will flow as long as there are no obstacle in their path.
It is essential that any and all apertures be arranged in such a way as to minimize their effect on the currents.
Apertures have HF resonances, so an induced HF current flowing on the shield can cause the aperture to act as a transmitting antenna (Babinet Principle or Effect).
EME Lab Shield Tutorial
17
Babinet’s Principle: The Potential difference. Length is the issue.
EME Lab Shield Tutorial
18
3‐D Simulation Results: (Scott Piper)
EME Lab Shield Tutorial
19
Simple Radiation Pattern
EME Lab Shield Tutorial
20
Simple Slot Graph
EME Lab Shield Tutorial
21
Base Line Shield
EME Lab Shield Tutorial
22
Real Shield with Slot
EME Lab Shield Tutorial
23
Slot Broken in Two
EME Lab Shield Tutorial
24
Slot Broken in Four
EME Lab Shield Tutorial
25
Summary Graph
EME Lab Shield Tutorial
26
CST Microwave Studio Simulation of a RCD Shield with various lifted terminations:
EME Lab Shield Tutorial
27
Shield Materials
EME Lab Shield Tutorial
28
Key Issues Determining Shield Performance: Shield Materials
Conductivity (
(The measure of the ability of a material to conduct an electric current.)
Permeability (
The measure of the ability of a material to support the formation of a magnetic field within itself.)
EME Lab Shield Tutorial
29
Required Material Size for Equivalent Conductivity
These squares are different metals sized for constant conductivity:
=l/(RA)
Where:
R is the electrical resistance of a uniform specimen
l is the length
A is the area EME Lab Shield Tutorial
30
Key Issues Determining Shield Performance: Shield Geometry
1.
2.
Continuity of the shield and connections.
Thickness (important for low‐frequency magnetic field applications).
3.
Apertures (which always impact negatively Shielding Effectiveness).
4.
Near‐Field or Far‐Field Emissions (where is the source of emissions?).
EME Lab Shield Tutorial
31
Shielding in a Nutshell:
How do Shields Work?
Reflection at the boundary surfaces (Low Frequencies)
Absorption as fields attempt to transverse the shield (High Frequencies)
Magnetic Field Shunting (Very Low Frequencies)
EME Lab Shield Tutorial
32
How Do Shields Work? Reflection and Absorption in a Near or Far‐Field
EME Lab Shield Tutorial
33
Notes on Near and Far Field Emissions
•
99% of the Emissions Under the Shield will be Near‐Field
–
–
Magnetic (Switched‐Mode‐Power‐Supplies)
Electric (DDR RAM, Micro)
•
85‐90% of the Emissions Outside of the Shield will be Far‐
Field
•
The External Near‐Field Exception: –
Handheld Antenna Testing:
EME Lab Shield Tutorial
34
Basic Shield Effectiveness Formulas:
SEdB = 20 log10(Et/Ei) (Electric Field)
SEdB = 20 log10(Ht/Hi) (Magnetic Field)
Where:
SEdB is the Shielding Effectiveness
Ei is the Incident Electric Wave
Et is the Transmitted Electric Wave
Hi is the Incident Magnetic Wave
Ht is the Transmitted Magnetic Wave
EME Lab Shield Tutorial
35
S. A. Schelkunoff Shield Effectiveness Equation:
SEdB = RdB + AdB + MdB
Where:
RdB is Reflected losses at the outer and inner shield surfaces
AdB is the Absorption loss through the material
MdB is the additional losses of Multiple reflections and transmissions within the shield*
*MdB can be disregarded for shield thicknesses that are much greater than a skin depth.
EME Lab Shield Tutorial
36
Shielding Effectiveness Equations Change
if the emissions are Far‐Field or Near‐Field
•
•
The boundary between the Far and Near‐Field is approximately o/2. Far and Near‐Field sources have differing source characteristics and E/H ratios.
EME Lab Shield Tutorial
37
Reflection Loss (RdB):
•
•
•
•
Occurs at a Boundary
Where the is a difference in the Conductivity (and Permeability (µ) of Two Materials (Air and Shield)
The Greater the Difference, the Greater the Reflection Loss
Low Frequency Dominant
(Switch Mode Power Supplies)
EME Lab Shield Tutorial
38
Reflection Loss (RdB) General Formula for Far‐Fields:
RdB=168 + 10 log10(r/rf)
Where:
r = Conductivity relative to Copper
r = Relative permeability relative to free space
f = Frequency
Note, Reflection loss is greatest for:
Low Frequency (f)
High Conductivity (r)
Low Permeability (r)
The larger the RdB the better the Shield.
EME Lab Shield Tutorial
39
Far‐Field Reflection Loss (RdB) Example:
Material
r
r
Copper Nickel – Silver
Steel
1.0 1.0 138 dB
1.0 0.06 126 dB
1000 0.1
98 dB
RdB@1kHz
RdB@10MHz
98 dB
86 dB
58 dB
RdB=168 + 10 log10(r/rf)
EME Lab Shield Tutorial
40
Absorption Loss (AdB):
Absorption Loss is the exponential decay of energy due to ohmic and heating of the material which occurs when an electromagnetic wave passes through a medium.
High Frequency Dominant
(Video Data, DDR, Micro Data Communications)
EME Lab Shield Tutorial
41
A little aside, Skin Depth ()
To understand Absorption Losses, there is a need to understand the term Skin Depth.
What is Skin Depth? The distance required for the wave to be attenuated to 1/e or 37% of its original value is defined as the Skin Depth () which is:
= (2/)0.5 (meters) Or:
 = 2.6/(frr)0.5 (inches)
Remember this term:  (Skin Depth) – it is important.
EME Lab Shield Tutorial
42
Back to Absorption Loss (AdB):
A = et/
Or in dB:
AdB = 20 log10 et/
Where:
t = thickness
Skin Depth
Note, Absorption Loss is greatest for:


Greater Thickness (t) Smaller Skin Depth (




Higher Frequency, Greater Conductivity (, Greater Permeability (µ)
The Larger the AdB the better the Shield
EME Lab Shield Tutorial
43
Far‐Field Absorption Loss (AdB) for a 20 mil sheet of Copper, Nickel‐Silver, and Steel
AdB = 20 log10 et/
EME Lab Shield Tutorial
44
Far‐Field Summation of Reflection Loss (RdB) and Absorption Loss (AdB) of a 20 mil Copper, 20 mil Nickel‐Silver, and 20 mil Steel Shield
Schelkunoff Shield Effectiveness Equation: SEdB = RdB + AdB
EME Lab Shield Tutorial
45
Far‐Field Shield Absorption Loss (AdB) with Changing Shield Thicknesses for Copper, Steel, and Nickel‐Silver:
EME Lab Shield Tutorial
46
Near Field Reflection Losses
For Both Magnetic and Electric Fields
Magnetic Field Source:
Rm,dB = 14.57 + 10 log10(fr2r/r)
Electric Field Source:
Re,dB = 322 + 10 log10(r/rf3r2)
Where r = distance from source
EME Lab Shield Tutorial
47
Near‐Field Reflection Losses for Steel, Copper, and Nickel‐Silver:
Reflection Losses of Magnetic Field
Reflection Losses of Electric Field
EME Lab Shield Tutorial
48
Near‐Field Absorption Losses:
A = et/ where t = thickness
Or
t/
AdB = 20 log10 e
This is the same equation as the far‐field.
EME Lab Shield Tutorial
49
Comparison of Near‐Field Reflection Loss (Electric and Magnetic) and Absorption Loss in a 20 mil Steel,
Nickel‐Silver, and Copper Shield:
AdB = 20 log10 et/
Rm,dB = 14.57 + 10 log10(fr2r/r)
Re,dB = 322 + 10 log10(r/rf3r2)
= (2/)1/2 (meters)
EME Lab Shield Tutorial
50
Summary of Fields and Losses:
I.
For far‐field sources:
A.
B.
II.
For near‐field, electric sources:
A.
B.
III.
Reflection loss is predominant at the lower frequencies
Absorption loss is predominant at the higher frequencies. Reflection loss is predominant at the lower frequencies
Absorption loss is predominant at the higher frequencies. For near‐field, magnetic sources:
A.
B.
Absorption loss is the dominant shielding mechanism for all frequencies. However, both reflection and absorption losses are quite small for near‐field, magnetic sources at low frequencies. EME Lab Shield Tutorial
51
Low Frequency Magnetic Shielding in the Near‐Field:
Basic methods for shielding against low‐
frequency sources:
1. Diversion of the magnetic flux with high‐ materials
2. Generation of opposing flux via Faraday’s law commonly known as the shorted turn method.
EME Lab Shield Tutorial
52
Magnetic Shunting ‐ diverting the magnetic flux:
1.
2.
Requires a high‐ material to divert magnetic flux.
Problems with high‐ materials (Mumetal):
1.
2.
High‐ materials are expensive
Permeability () is:
Nonlinear
Decreases with increasing frequency Decreases with increasing magnetic field strength
Changes with Mechanical Handling/Treatment
Dominant at Very‐Low‐Frequencies
(High‐ materials are only effective for magnetic fields below 1 kHz. “This is why shielding enclosures for switching power supplies are constructed from steel rather than Mumetal.” Professor Clayton Paul, Introduction to Electromagnetic Compatibility, Second Edition, Page 743.) EME Lab Shield Tutorial
53
Magnetic Shunting:
No effect at High‐Frequencies: The  is approaching in this high‐material.
EME Lab Shield Tutorial
54
Faraday’s Law or Shorted Turn:
A changing current in one wire causes a changing magnetic field that induces a current in the opposite direction in an adjacent wire.
EME Lab Shield Tutorial
55
Magnetic Shielding:
What is a Magnetic Shield?
•
A shield of high permeability that can “shunt,” “divert,” “attract,” “Channel,” or “guide” (like a duct) a magnetic field.
Because:
•
High‐permeability magnetic materials have low reluctance.
Therefore:
•
A magnetic field follows the path of least reluctance similar to how current follows the path of least resistance.
EME Lab Shield Tutorial
56
Degree of magnetic shielding is determined by:
–
Material
–
Thickness
–
Shape
–
Position relative to the applied Magnetic Field
EME Lab Shield Tutorial
57
Shield Effectiveness (SE) of Flat Sheets of Different Materials in a Magnetic Field 15 mil Thick
500
500
400
300
CUSE
SSSE.1 ( f )
ALSE
NISE
FESE
200
100
0
0
0
100
1100
2200
3300
4400
5500
f
6600
7700
8800
8800
9900
9900
11000
11000
10000
10000
Nickel
Cold‐Rolled Steel (SAE 1045)
Stainless Steel (430)
Copper
Aluminum
EME Lab Shield Tutorial
58
Corrosion
EME Lab Shield Tutorial
59
THEORY OF CORROSION:


Galvanic and electrolytic corrosion are two types of corrosion suspected in shielding degradation. In both cases the anode metal gets corroded. EME Lab Shield Tutorial
60
Galvanic corrosion: A natural phenomenon produced:
1. When two dissimilar metals are brought in contact with each other 2. In the presence of acidic atmospheric moisture.  An electrochemical process where the metal with higher anodic index voltage corrodes and an external electric current is produced by an internal chemical reaction. 
EME Lab Shield Tutorial
61
Electrolytic Corrosion
•
An electrochemical process:
–
–
Where the corrosive internal chemical reaction is induced by an externally applied electric potential Although the metals may be similar as opposed to galvanic corrosion (dissimilar metals)
EME Lab Shield Tutorial
62
Material Electrochemical Potentials
EME Lab Shield Tutorial
63
Galvanic Corrosion Risk
EME Lab Shield Tutorial
64
Summations and Conclusions:
EME Lab Shield Tutorial
65
Summations and Conclusions: Shields
1.
2.
3.
Shield the Whole active circuit
Use THICK Steel for Magnetic Shielding‐
Switch‐Mode‐Power Supplies
Know what type of field (Near/Far) is the threat
(If you want to keep the field on the board then the field is near. If you want to keep the field off of the board then the field is usually far.)
EME Lab Shield Tutorial
66
Summations and Conclusions: Shields 1.
2.
3.
4.
5.
Reflection loss is large for electric fields.
Reflection loss in normally small for low‐frequency magnetic fields.
Magnetic fields are harder to shield against than electric fields.
Use a good conductor to shield against electric fields and high‐frequency magnetic fields.
Use a magnetic material to shield against low‐frequency magnetic fields.
EME Lab Shield Tutorial
67
Summations and Conclusions: Apertures
Keep Apertures’ dimensions minimal
Keep number of Apertures few
EME Lab Shield Tutorial
68
Demonstrations:

Measuring Shielding effectiveness of various materials in a Near‐Field Magnetic Field

Aperture Measurement Program

Palantir Shield Simulations
EME Lab Shield Tutorial
69
Plane Wave
Setup overview
EME Lab Shield Tutorial
70
Overview of Plane Wave
Cross Section of Camera
EME Lab Shield Tutorial
71
Field Comparison inside of Camera
H‐field probe
EME Lab Shield Tutorial
72
That’s all…
•
Question and Answer Time
•
Thank you for attending.
EME Lab Shield Tutorial
73
Questions?
EME Lab Shield Tutorial
74
References
EME Lab Shield Tutorial
75
References:
•
•
•
•
•
•
Henry W. Ott, Electromagnetic Compatibility Engineering, Wiley, 2009
Clayton, R. Paul, Introduction to Electromagnetic Compatibility, Wiley Interscience, 2nd. Ed, 2006
Ralph Morrison, Grounding and Shielding Circuits and Interference, Wiley Interscience, 5th. Ed, 2007
Tom van Doren, Grounding and Shielding Electronic Systems, T. Van Doren, 1997
Gary Fenical, The Basic Principles of Shielding, In Compliance Magazine, June 2010
Scott Piper (Gentex Corp.) CST Microwave Studio Simulations
EME Lab Shield Tutorial
76