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Engineering Aspects of Electromagnetic Shielding Dr. Sergiu Radu Principal Engineer, EMC Design Sun Microsystems, Inc.

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Page 1: Engineering Aspects of Electromagnetic Shielding Engineering Aspects of Electromagnetic Shielding Dr. Sergiu Radu Principal Engineer, EMC Design Sun Microsystems, Inc

Engineering Aspects of Electromagnetic Shielding

Dr. Sergiu RaduPrincipal Engineer, EMC Design Sun Microsystems, Inc.

Page 2: Engineering Aspects of Electromagnetic Shielding Engineering Aspects of Electromagnetic Shielding Dr. Sergiu Radu Principal Engineer, EMC Design Sun Microsystems, Inc

2

Outline

● Introduction: Electric, Magnetic and Electromagnetic shielding● Basics approaches to shielding: field theory (Kaden) and circuit theory (Schelkunoff)● Limits of the theoretical approaches: numerical simulations● Practical aspects of shielding, typical requirements, grounding scheme● Shield material: metal, plastic, typical coatings● Shield construction: rivets, joints, seams, apertures● Shielding and thermal issues: holes, perf patterns, honeycomb● Shield integrity and gaskets● Internal compartmentalization of a chassis, resonances● Practical aspects of source - aperture coupling● Shielding for radiated emissions, radiated immunity and ESD - reciprocity aspects and limits● Evaluation of shielding effectiveness

Page 3: Engineering Aspects of Electromagnetic Shielding Engineering Aspects of Electromagnetic Shielding Dr. Sergiu Radu Principal Engineer, EMC Design Sun Microsystems, Inc

3

Introduction

● In order to have an EMI problem we must have:

One or multiple noise sources (typically the CPUs, ASICs, DC/DC, other ICs, etc.)

One or multiple coupling paths (capacitive, inductive, galvanic, field coupling)

One or multiple antennas (cables, apertures, holes, seams, etc.)

● The best way is to control EMI at the source whenever possible and to add containment measures at every other level (coupling paths, antennas).

● The noise source, the coupling path and the antennas are all frequency dependent, and this increases very much the complexity of the problem.

● All the EMI problems deal with parasitics. The characterization of all the EMI aspects is never part of the data sheet, making all modeling attempts very difficult.

Page 4: Engineering Aspects of Electromagnetic Shielding Engineering Aspects of Electromagnetic Shielding Dr. Sergiu Radu Principal Engineer, EMC Design Sun Microsystems, Inc

4

EMI Mechanism

Page 5: Engineering Aspects of Electromagnetic Shielding Engineering Aspects of Electromagnetic Shielding Dr. Sergiu Radu Principal Engineer, EMC Design Sun Microsystems, Inc

5

Shielding Effectiveness

S. E. = 20 log ∣ E eE i ∣ [ dB ] S. E. = 20 log ∣ H e

H i ∣ [ dB ] S. E. = 10 log ∣ PePi ∣ [ dB ]

E e = Electric field exteriorE i = Electric field interior

H e = Magnetic field exterior H i = Magnetic field interior

P e = Power exterior P i = Power interior

For external noise source (immunity):

Page 6: Engineering Aspects of Electromagnetic Shielding Engineering Aspects of Electromagnetic Shielding Dr. Sergiu Radu Principal Engineer, EMC Design Sun Microsystems, Inc

6

Shielding Effectiveness

Source parameters that may influence the preceived shielding:

• Source polarization

• Source - aperture distance

• Beam-pattern vs. receive antenna position

• Source impedance

• Antenna- enclosure coupling strength

Page 7: Engineering Aspects of Electromagnetic Shielding Engineering Aspects of Electromagnetic Shielding Dr. Sergiu Radu Principal Engineer, EMC Design Sun Microsystems, Inc

7

Near Field vs. Far Field for Point Sources

ZW m =dEdH

=

jr2 −

2

v rjv r 2

1r3 −

2

v2 r

ZW e =dEdH

= 1

jv2 r 1v r 2

1jr3

jv r 1r2

ZW m =d E d H

j ω μ r ZW e =d Ed H

1j ω r

ZW =dE dH

= 1 v= 1

1 μ

= μ μ0

0= 120 π = 377

Small electric dipole Small electric dipole Small electric dipole Small current loop

Far Field (Radiation Zone, Fraunhoffer Zone):

Near Field (Reactive Field or Fresnel Zone)

Page 8: Engineering Aspects of Electromagnetic Shielding Engineering Aspects of Electromagnetic Shielding Dr. Sergiu Radu Principal Engineer, EMC Design Sun Microsystems, Inc

8

Near Field vs. Far Field Impedance for Point Sources

. . . 5 : : : : 1 0 .................... ~......... . ....... ; ........................... : ................... .

~ I Z wl e ~ ~ ~ ~ ~ (1) . . . . . . . . . . . . . 3 : : : : a 1 0 ·····················r·····················~··········· ·:··············· ......... ~0 = 3770 . . . . . . . . .......... . . . .

$ IZwlm : N - . . - 1 0 1 ·····················~········· ···········~···············~····················· (2)

~Near Field Far Field . . . . . . -1 . .

10 0.001 : : 0.01 0.1 1 10 100 r I (A/2rr)

Page 9: Engineering Aspects of Electromagnetic Shielding Engineering Aspects of Electromagnetic Shielding Dr. Sergiu Radu Principal Engineer, EMC Design Sun Microsystems, Inc

9

Impedance Method (TL, S.A. Schelkunoff)

= j j = R jLG jC

Z ℑ = j jZ c = R j L

G jC

(propagation constant)

(medium intrinsic impedance)

Page 10: Engineering Aspects of Electromagnetic Shielding Engineering Aspects of Electromagnetic Shielding Dr. Sergiu Radu Principal Engineer, EMC Design Sun Microsystems, Inc

10

Impedance Method – Reflected & Transmitted Waves

Z1 = μ0

0

= 120 π = 377 []

openspace impedance

Z 2 = j j

[]

shield intrinsic impedance

R = 20 log10 ∣ Z2 Z 12

4Z2 Z 1∣ [dB]

R=Reflection losses

Open space impedance

z 1

ELECTRIC FIELD

MAGNETIC FIELD

Eo ... E r 1-----.. -

Ho

H r 1 ____ .. _

Shield Open impedance z 2 space z1

. ;; ....... impedance

... Et=

2 z 1 E 1 z 1 + z 2

mi .. w 2 z 2

z 1 + z 2

SHIELD

Page 11: Engineering Aspects of Electromagnetic Shielding Engineering Aspects of Electromagnetic Shielding Dr. Sergiu Radu Principal Engineer, EMC Design Sun Microsystems, Inc

11

Absorption Losses & Skin Effect

A = 8.69 d [dB] Absorption Losses

B = 20 log 1 − e− 2d [dB] Correction for thin shields

S. E. = A R B [dB ] Shielding Effectiveness

= 2

= 1 f

Where:δ = skin depth (mm)ω = angular (radian) frequency

μ = material permeability (4π x 10-7 H/m

σ = material conductivity (5.82 x 107 mho/m for Cu)f = frequency (hertz)

d = thickness of the shield (mm)

Page 12: Engineering Aspects of Electromagnetic Shielding Engineering Aspects of Electromagnetic Shielding Dr. Sergiu Radu Principal Engineer, EMC Design Sun Microsystems, Inc

12

Magnetic Behavior of Materials

~ r initial ( x 1 0 3

) 8 = 40 Gauss

60~---. . 50 ........... :·· ..... ... ; ........... -:·· .. .... ... . ~ .... ..... . ... : . . . ' . . .

40 .......... .. : .......... : ...... . Muni.etal ...... : ........ .... .. : . . ' ' '

: (0.389 mm) : : 3 0 .. .. . .. . .. . : .. .. . . . . . . . ~ . .. . .. . . . ·:.. . . . .. . .. . ·:··.. . . . . . . . . . : . . I 0 I +

20 .. M~Shretd .. High ·P·erm · · ·· ··· : ...... ··· ··· ............... ;

10 (0.35q mm) f [kHz]

0.001 0.01 0.1 1 10 100

Page 13: Engineering Aspects of Electromagnetic Shielding Engineering Aspects of Electromagnetic Shielding Dr. Sergiu Radu Principal Engineer, EMC Design Sun Microsystems, Inc

13

Impedance Method for Multilayer Shields

Layer1

1 , 1

Z(O) Z(d 1)

X=O

Layer 2 ( air )

Layer3 X ( \ • I I))()()( ( ( Y . • \ ( ttt ~, '"'" ( ( t X<<+++>>XXX((tt)))(lt (ttt)))()()(((t

+>>XXX<< tH >>lOt ( < tH XU ( ( Ht)) X +>>lOOt<< H+ >>XX< ( H+) XU ( ( tH)) lt +> >ltltlt<<+++>>XX<<+++>>ltltlt((Ut))lt +> ) XXX< <++t)))()(((ttt))lCltlC((ttt)))(

"''"""'''""'''""''"""''"""''"""''" "''"""''"""''""''"'~'~''"""''"""''" X<<+++>>lOtlt<<H)))tltlt((tU)))tltlt((t X<<+++>> XXX<<++)> ltltlt ( ( +++)) ltltlt ( ( t X<<+++>>XU<<++>>ltltlt((tU)))tltlt((t

~~~===~ ~z ...... · · · · · · · ... ·r· · · ~!!~~~~~! +»Xltlt« HUH))X

+»XXX« 3 J 3 tH)))( +>>XXX<UH> +++>>X +>> XXX<<+++> ttt)))t

X<<+++>> XXX<<++)) Ull ( ( Ht)) XU< ( t X<<+++>> XU:<< H >)XXX<< X<U++>>XXX«H»UXH UUlllt + X<<+++>>XU <<tt)))IXJI(( t))XJI)I((t

"''"""''"""''""''"""'' 1o\\¥¥¥>11ololo\\¥WIIIoAA\\

+))lllllt.~H+))Ut.t+U))

+))liUtt+++UU~t+++U t))lllllltU+U)lllttHt)) t))lllltt+tU)JUtt+tU) .,,, .......... ,, .... , ...... ,, .,,, •••• ~ ..... ,1 ... ~~···11

liHHUU lHHUU liCUUUl lH+++Ull

lttHUUUU+UUIIH UUUH+ IUHtUIU~UUUUH UUUH+ ltt.tttU:UIH+•''"Sitt tUIII"+ .,,..

0 X= d X 2 0

H

.... . X

L ow H and high permeability laye

0 ... .. ... .. ... ... .. ... .. ..

H 1

High Hand low permeability layer

H 2

...

..

Page 14: Engineering Aspects of Electromagnetic Shielding Engineering Aspects of Electromagnetic Shielding Dr. Sergiu Radu Principal Engineer, EMC Design Sun Microsystems, Inc

14

Circuit Method – H. Kaden

Needs continuity of the shield for the circulation of the Eddy currents (Foucault currents)

Incident magnetic field

Opposite field created by the eddy currents

Eddy currents induced in the shield's wall

Page 15: Engineering Aspects of Electromagnetic Shielding Engineering Aspects of Electromagnetic Shielding Dr. Sergiu Radu Principal Engineer, EMC Design Sun Microsystems, Inc

15

Mathematical Approach for Circuit Method

E = 2 E H = 2 H

2 = j ω μ σ j ω

= j ω μ σ = 1 jδ

[m−1] δ = 2ω σ μ

[ m ]

= j ω μ = j 0 = j ωv= j 2 π

λ

● Consider a homogeneous, isotropic, non-polarized medium without electric charges, and from Maxwell's equations obtain the Helmholtz's equations (propagation equations):

● Propagation constant for harmonic field:

● In metal:

● In dielectric:

● Solve the Helmholtz's equation in each of the three regions: exterior space (air), interior space (air), and in the wall of the shield (metal). ● Use boundary conditions, continuity and conservation of the tangent component of the field at each of the shield's surfaces to find the integration constants and obtain the unique solution, and the S.E.

(source of the field placed at infinity)

● Use the separation of variables method to solve Helmholtz's equation. This is possibleonly for 11 geometries with orthogonal coordinates (plan-parallel, spheric, cylindrical, elliptic, parabolic, toroidal, etc.)

Page 16: Engineering Aspects of Electromagnetic Shielding Engineering Aspects of Electromagnetic Shielding Dr. Sergiu Radu Principal Engineer, EMC Design Sun Microsystems, Inc

16

Circuit Method Applied for the Plan-Parallel Shield

S. E. [ dB ] = 10 log [ 12 cosh 2 d

cos 2 d

0 x0

sinh 2 d− sin 2 d

0 x0

2

cosh 2 d− cos 2 d

]

Page 17: Engineering Aspects of Electromagnetic Shielding Engineering Aspects of Electromagnetic Shielding Dr. Sergiu Radu Principal Engineer, EMC Design Sun Microsystems, Inc

17

Circuit Method Applied for the Spherical Shield

S.E. = 10 log [ 12 cosh 2 d

δ cos 2 d

δ 5 A2 2 B2− 2 A B 12 cosh 2 d

δ− cos 2 d

δ ...][... A sinh 2 d

δ 2 sin 2 d

δ B sinh 2 dδ− sin 2 d

δ ]A = μ δ

3 μo r oB =

μo r o3 μ δ

δ = 2ω σ μ

Page 18: Engineering Aspects of Electromagnetic Shielding Engineering Aspects of Electromagnetic Shielding Dr. Sergiu Radu Principal Engineer, EMC Design Sun Microsystems, Inc

18

Example of Field Behavior (Spherical Shield)

LF – Magnetic Material (no tangential field)(No shielding effect for non-magnetic)

HF – Magnetic or Non-Magnetic Material(no radial field component on the exterior)

t J

Page 19: Engineering Aspects of Electromagnetic Shielding Engineering Aspects of Electromagnetic Shielding Dr. Sergiu Radu Principal Engineer, EMC Design Sun Microsystems, Inc

19

Circuit Method and Multilayer Shields

● Double shield from materials with the same magnetic behavior (Cu + Al) is worse than a single shield with equivalent thickness. If the materials have different magnetic behavior thedouble shield is with 6dB better (for example Cu+ Ni). In general, equal thickness is best.

● Triple shield works best with materials of equal thickness and alternating magnetic behavior, the most conductive toward the noise source.

● There is a difference between laminated shields and double shields (air separation).

Page 20: Engineering Aspects of Electromagnetic Shielding Engineering Aspects of Electromagnetic Shielding Dr. Sergiu Radu Principal Engineer, EMC Design Sun Microsystems, Inc

20

Limits of the Analytical Methods

● Analytical methods:

• impedance method (Schelkunoff) - infinite sheet of metal

• separation of variables (Kaden) – theoretically possible for 11 geometries)

● Physical insight and precision - perfect as a benchmark for the evaluation of the numerical methods.

● Numerical simulation (FDTD, FEM, MoM, etc.) - can’t be used for real life situations, no physical insight, difficult to characterize the noise source, internal reflections, large size of the file, etc.

● Very good for simple cases to analyze a trend or principle.

● Experimental methods - the only real validation of the analytical or numerical models.

● In the end, all these are about material and thickness of the shield, and to a little extent about the geometry (circuit method).

Page 21: Engineering Aspects of Electromagnetic Shielding Engineering Aspects of Electromagnetic Shielding Dr. Sergiu Radu Principal Engineer, EMC Design Sun Microsystems, Inc

21

Shielding and Reciprocity● Reciprocity principle is applicable only to passive and linear circuits (a CPU, ASIC, IC is active and non-linear).

● Reciprocity requires us to consider only those parts of the system that meet the criteria of “passiveness” and “linearity”. In this sense, within an electronic system, the properties of radiators/sensors can often be assumed to be reciprocal, just as is the case with antennas.

● In shielding practice, we can find non-linear effects, like the "rusty bolt" effect, which results in nonlinear and non-reversible conductivity (diode). Another typical example from shielding practice, is a multilayer shield, with one of the layers having nonlinear magnetic permeability.

● If the shield is linear, the reciprocity principle requires that Z21

=Z12

. The same requirements for passive, linear, circuits hold. As long as the shield is a passive network and its material exhibits linear electric and magnetic properties, the reciprocity theorem holds, and the same shielding parameters are true for emissions and susceptibility. However, this is true only at the levels of the shield transfer function, but not at the system level shielding effectiveness.

● For example, a dominant radiator in the system (emissions case) was not necessarily the dominant victim in the same system (immunity case). For instance, one of the reasons that the radiator is dominant for emissions may be a high level of signal within it which, on the other hand, makes this particular part of the system more immune to EMI. Typically, the Tx (transmitting) output signal is 20dB stronger than Rx (receiving) input signal.

Page 22: Engineering Aspects of Electromagnetic Shielding Engineering Aspects of Electromagnetic Shielding Dr. Sergiu Radu Principal Engineer, EMC Design Sun Microsystems, Inc

22

Evaluation of Shielding Materials (MIL285, IEEE 299, ASTM D4935)

ASTM D4935 30MHz - 1.5GHz

I Input

Outer conductor

\_p r '

Sample

_/ p , -V'--

d~

Output

~

Page 23: Engineering Aspects of Electromagnetic Shielding Engineering Aspects of Electromagnetic Shielding Dr. Sergiu Radu Principal Engineer, EMC Design Sun Microsystems, Inc

23

Evaluation of Shielding Materials

Dual TEM Cell*

ASTM D4935 Coaxial TEM

* Two coupled stirred mode reverberation chambers as a HF alternative (Qinetiq)

Page 24: Engineering Aspects of Electromagnetic Shielding Engineering Aspects of Electromagnetic Shielding Dr. Sergiu Radu Principal Engineer, EMC Design Sun Microsystems, Inc

24

IEEE 299 – Measuring SE of Enclosures

Schematic diagram of the test configuration for magnetic testsshowing dimensions of transmit (TX) and receive (RX) antennas

Measurement setup for frequencies >1 GHz

Page 25: Engineering Aspects of Electromagnetic Shielding Engineering Aspects of Electromagnetic Shielding Dr. Sergiu Radu Principal Engineer, EMC Design Sun Microsystems, Inc

25

Experimental Setup

------

Page 26: Engineering Aspects of Electromagnetic Shielding Engineering Aspects of Electromagnetic Shielding Dr. Sergiu Radu Principal Engineer, EMC Design Sun Microsystems, Inc

26

Chassis Resonances

The resonance frequencies in a rectangular box are given by:

L = lengthH = heightW = width

where the case i=j=k=0 is forbidden

f ijk =c

2 r r iL 2

jH 2

kW 2

f ijk = 150 iL 2 jH 2 kW 2 ( for vacuum )

Page 27: Engineering Aspects of Electromagnetic Shielding Engineering Aspects of Electromagnetic Shielding Dr. Sergiu Radu Principal Engineer, EMC Design Sun Microsystems, Inc

27

Rectangular Box - Lowest Resonance Frequencies

f0

0

3.368 108.

6.737 108.

1.011 109.

8.605 108.

9.241 108.

1.093 109.

1.327 109.

1.721 109.

1.754 109.

1.848 109.

1.996 109.

2.581 109.

2.603 109.

2.668 109.

2.772 109.

=

f1

2.231 108.

4.041 108.

7.097 108.

1.035 109.

8.889 108.

9.506 108.

1.115 109.

1.346 109.

1.735 109.

1.768 109.

1.862 109.

2.008 109.

2.591 109.

2.613 109.

2.677 109.

2.781 109.

=

f2

4.463 108.

5.591 108.

8.081 108.

1.105 109.

9.693 108.

1.026 109.

1.18 109.

1.4 109.

1.778 109.

1.81 109.

1.901 109.

2.045 109.

2.62 109.

2.641 109.

2.705 109.

2.808 109.

=

f3

6.694 108.

7.494 108.

9.497 108.

1.212 109.

1.09 109.

1.141 109.

1.282 109.

1.487 109.

1.847 109.

1.877 109.

1.966 109.

2.105 109.

2.667 109.

2.688 109.

2.751 109.

2.852 109.

=

● At higher frequency (for example >1GHz), the enclosure supports a very large number of modes. Lowest modes: 223, 336, 404, 446, 559, 669, 673, ... MHz.

● An empty box has very high Q, very sharp resonances. The energy stored is proportional to volume, and losses are proportional with the surface area.

Page 28: Engineering Aspects of Electromagnetic Shielding Engineering Aspects of Electromagnetic Shielding Dr. Sergiu Radu Principal Engineer, EMC Design Sun Microsystems, Inc

28

Field Distributions at Resonant Frequencies

HzXZ

TMy201 - Ey(x,z) (rear)

EyXZ

TMy201 - Hx(x,z) (rear)

HxXZ

HxXZ

HzXZ

● Empty box: Q=5000, Fully loaded, densely populated box: Q=5-25 (practical implications)

● Use extensively internal compartmentalization (limit excitations, lower Q, upper f)

Page 29: Engineering Aspects of Electromagnetic Shielding Engineering Aspects of Electromagnetic Shielding Dr. Sergiu Radu Principal Engineer, EMC Design Sun Microsystems, Inc

29

Significance of modes

0 200 400 600 800 1000 1200 1400 1600 1800 2000

0.3

0

1j 1+( ) k 1+( )⋅ i 1+( )⋅

1j 1+( ) k 1+( )⋅

1k 1+( ) i 1+( )⋅

1j 1+( ) i 1+( )⋅

20000 F i j, k,( ) F 0 j, k,( ), F i 0, k,( ), F i j, 0,( ),

Y-amplitudedenotessignificanceof mode

MHz

● Most energy stored in the lower modes – Can we limit the upper frequency of interest? Yes, use 3GHz as a practical upper limit for the internal resonances (Exceptions)

Page 30: Engineering Aspects of Electromagnetic Shielding Engineering Aspects of Electromagnetic Shielding Dr. Sergiu Radu Principal Engineer, EMC Design Sun Microsystems, Inc

30

Far Field Estimations using Analytical Methods

● Based on measuring Q of the enclosure

● Based on the measurement of the field in the aperture and the Huygens principle

● Based on the evaluation of the field in the aperture and Bethe’s theory of diffraction through small holes

● Problems: multiple sources without accurate characterization, multiple coupling paths, and multiple apertures - the real field in the aperture can’t be correctly estimated in early phases of the design.

● The main results: trends and possible estimation of the relative effect of a modification.

Page 31: Engineering Aspects of Electromagnetic Shielding Engineering Aspects of Electromagnetic Shielding Dr. Sergiu Radu Principal Engineer, EMC Design Sun Microsystems, Inc

31

Why Apertures Radiate?

I 1/ II I I \ \ \ \ \ I I 1 \ \ \ \ \,

I / / 1 , \ \ / I I I \ \ \ \ \

I I I j / / \ \ \ ' ' '/- ' / ' ' ' ' '

I {/' /-<(_ _ _// ""'--~---~ \ \ { \

t ( (~-----------o-----~-.. ' \ \ ) I ' " ........__ -~' / I I \ \ '· )/f .

\ \~ '----~~~~~~"'"-, /,---;~~------- / / \ ~ \ ' / / / ;' \ \ \ ' \ // I / I

\\ \ \ \ \ I I I I \ \ \ I I I II ;'

\

\ \ \ \ 1

I I \ \ \ \ / I I I I \ \ \ \ I I I I I

~ l l ~ i . J J 1 1 J

: I

S LOT C ONTAINS RESISTANCE & INDU C TANCE

z = R + j X L

V= IZ

E - fie ld - V

Page 32: Engineering Aspects of Electromagnetic Shielding Engineering Aspects of Electromagnetic Shielding Dr. Sergiu Radu Principal Engineer, EMC Design Sun Microsystems, Inc

32

Why joints radiate?

● Similar behavior for screws & rivets

~~--'\ \ ,, __

Page 33: Engineering Aspects of Electromagnetic Shielding Engineering Aspects of Electromagnetic Shielding Dr. Sergiu Radu Principal Engineer, EMC Design Sun Microsystems, Inc

33

Slot and Aperture Radiation

S.E. = 20 log 2 L S.E. = − 10 log n

● A slot behaves like an equivalent dipole with orthogonal polarization (Babinet).For example, at 1GHz, a slot of 0.6” length or less will still provide a S.E.=20dB

Page 34: Engineering Aspects of Electromagnetic Shielding Engineering Aspects of Electromagnetic Shielding Dr. Sergiu Radu Principal Engineer, EMC Design Sun Microsystems, Inc

34

Slot Shape and Shielding Effectiveness (TLM)

Slot resonant at 3.6GHz, SE at 5 cm away from the slot

Shape C

Shape B

Shape A

Page 35: Engineering Aspects of Electromagnetic Shielding Engineering Aspects of Electromagnetic Shielding Dr. Sergiu Radu Principal Engineer, EMC Design Sun Microsystems, Inc

35

Slot Shape and Shielding Effectiveness (TLM)

Radiated Emissions - Simulation for a real box with an aperture of different shapes (TLM)

Page 36: Engineering Aspects of Electromagnetic Shielding Engineering Aspects of Electromagnetic Shielding Dr. Sergiu Radu Principal Engineer, EMC Design Sun Microsystems, Inc

36

Example of joint treatment for a lid

● Can we avoid gaskets?

Page 37: Engineering Aspects of Electromagnetic Shielding Engineering Aspects of Electromagnetic Shielding Dr. Sergiu Radu Principal Engineer, EMC Design Sun Microsystems, Inc

37

Type of Joints

● Gasket compression is of critical importance (typical 30-70%).

● Need tolerance analysis. Over-compression less a concern than under-compression, however compression stops are recommended.

●The type of gasket also critical: fabric over foam, conductive elastomer, conductive foam beryllium-copper spring-fingers (remove oxide), knitted wire, woven fabric, etc.

Page 38: Engineering Aspects of Electromagnetic Shielding Engineering Aspects of Electromagnetic Shielding Dr. Sergiu Radu Principal Engineer, EMC Design Sun Microsystems, Inc

38

Gaskets Resistance Between Fasteners

F=force (N), X=distance between fasteners (mm)E=elasticity module (Steel – 200kN/mm2, Plastic – 3kN/mm2 )I=moment of inertia (geometry specific, kg m2 )δ=displacement of a loaded beam (mm)

max =F X 3

E I 384

Page 39: Engineering Aspects of Electromagnetic Shielding Engineering Aspects of Electromagnetic Shielding Dr. Sergiu Radu Principal Engineer, EMC Design Sun Microsystems, Inc

39

Example of gasketing for a lid and rivets

Page 40: Engineering Aspects of Electromagnetic Shielding Engineering Aspects of Electromagnetic Shielding Dr. Sergiu Radu Principal Engineer, EMC Design Sun Microsystems, Inc

40

Perf Panel Behavior for EMC and Thermal

● Thermal has become the main concern (vibration and noise implications)● Cable management also critical for the EMI performance

E ..,c:. Cl) ..... :I t ~ tv ... 0 Cl 1: ·-1:: Cl) c. 0 X «< ~

100

Thermal dissipation threshold\

Thermal requires larger aperture

Aperture size alone no longer provides solution

fMI containment requ1res smaller apenure

1000 Fre<1uency (MHz)

10,000

Ho

Cable

E

~~ 0

Cable

I

Page 41: Engineering Aspects of Electromagnetic Shielding Engineering Aspects of Electromagnetic Shielding Dr. Sergiu Radu Principal Engineer, EMC Design Sun Microsystems, Inc

41

Effect of Perf – Wire Coupling on Shielding

Wire at 20 mm from the hole for single holes

iii ~

B c .. E ... 0 't ~

0.. a> c '5 "i :.: U)

60

50

40

30

20

10

"" \ '

"' , \ "' ' \. 0 \ vT/ ......... -

\'Yr -10

r--....

Shield ing from Various SINGLE Hole Sizes 1 mm thick metal

- 10x10 mm

- 20x10mm

- 40x1 0mm

- 60x10mm

- 80x10mm

- ~/""'--.......

P'" -

-20 O.OE+OO 5.0E+09 1.0E+10 1.5E+10 2.0E+10

Frequency (Hz)

2.5E+10 3.0E+10

__.../S

3.5E+10 4.0E+10

Page 42: Engineering Aspects of Electromagnetic Shielding Engineering Aspects of Electromagnetic Shielding Dr. Sergiu Radu Principal Engineer, EMC Design Sun Microsystems, Inc

42

Effect of Perf – Wire Coupling on Shielding

Wire at 20 mm from the hole pattern

a! ~ C)

£ 'tl Gi :E UJ

90

80

70

60

50

40

30

20

/'\.

"" "'--"" -............

\ "" ~ ~ ~ "'~ ~

""' ~ -............ 10

0

Shielding as Metal Thickness Increases 44 ho les @ 4x4 mm with 5 mm Spacing

- 0.5 mm thick

- 1 mmthick

---- - 2mmthick - - 4mmthick ..... ~ ......... - 6mmthick ---- ......

~ ~ - ammthick

' ---- -"'"' --- ""' ---- -~ "'~ ---- ~ ~ ~ --~ ~

-10 O.OE+OO 5.0E+09 1.0E+10 1.5E+10 2.0E+10

Frequency (Hz)

2.5E+10 3.0E+10 3.5E+10 4.0E+10

Page 43: Engineering Aspects of Electromagnetic Shielding Engineering Aspects of Electromagnetic Shielding Dr. Sergiu Radu Principal Engineer, EMC Design Sun Microsystems, Inc

43

Effect of Perf – Wire Coupling on Shielding

Wire at 20 mm from the hole pattern

iii' :!:!. C) c: ;:; Qj ~ en

Shielding as Metal Thickness Increases 21 holes @ 6x6 mm with 7 mm Spacing

70 .-------,-------.-------.r-------.-------.--------.-------.-------.

60

- 0.5 mm thick

so - 1 mmthick

- 2 mmthick

40 - 4 mmthick

- 6mmthick

30 - smmthick

20

10

0

-10 +--------+--------~------~------~--------+--------+--------~------~

O.OE+OO 5.0E+09 1.0E+10 1.5E+1 0 2.0E+10

Frequency (Hz)

2.5E+10 3.0E+10 3.5E+10 4.0E+10

Page 44: Engineering Aspects of Electromagnetic Shielding Engineering Aspects of Electromagnetic Shielding Dr. Sergiu Radu Principal Engineer, EMC Design Sun Microsystems, Inc

44

Waveguides Below Cut-off

f cc =175.768D [ m ] [ MHz ] f cr =

150.114W [ m ] [ MHz ]

S.E.c = 32LcD [ dB ] S.E.r = 27.2

LrW [ dB ]

S.E.≃ 27 La − 20 log10 N [ dB ]● For Honeycomb:where: a - the largest waveguide transverse dimension, L – the length of the waveguide,and N – the number of waveguides employed in the honeycomb

● Never pass a wire through a waveguide below cutoff (will support TEM mode)

Page 45: Engineering Aspects of Electromagnetic Shielding Engineering Aspects of Electromagnetic Shielding Dr. Sergiu Radu Principal Engineer, EMC Design Sun Microsystems, Inc

45

Waveguides Below Cut-off

Fc [GHz] = 5.9/g [inches]g = gap, largest transverse dimension

SE [dB] = 27.2 d/gd = depth, length of the waveguide

Page 46: Engineering Aspects of Electromagnetic Shielding Engineering Aspects of Electromagnetic Shielding Dr. Sergiu Radu Principal Engineer, EMC Design Sun Microsystems, Inc

46

Honeycomb – Air Pressure Drop and SE

T = length of the waveguide, depthW = the largest transverse dimension

$ :'2. c 0 ·~ ::: c $ ;;:

100

90

80

70

60

v 1\ ..,. /

50 - r\

40

30 Typical Part Dimension:12x12 i ld 1(305 x 305 mm) overal with 9/16 irch (14 mm) Frame

20

10

0 10 HX> 1.000 10.000

MHZ

1.0

Piress tJIII·e DI!"OiP A c ross i H AIIUtm B U tm •O A &VC •O AT'I Ib

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All r F f ·ow - ·ctim 11 8 "" d tl a ,mete.- du1c t

Page 47: Engineering Aspects of Electromagnetic Shielding Engineering Aspects of Electromagnetic Shielding Dr. Sergiu Radu Principal Engineer, EMC Design Sun Microsystems, Inc

47

Honeycomb – The Effect of Polarization

● For single layer, non-plated honeycomb, the SE can be up to 40dB better in one direction● Omni Cell (Chomerics) uses two layers of honeycomb at 90 degrees of each other

Page 48: Engineering Aspects of Electromagnetic Shielding Engineering Aspects of Electromagnetic Shielding Dr. Sergiu Radu Principal Engineer, EMC Design Sun Microsystems, Inc

48

An Example: Single Layer 1/4” Honeycomb

Page 49: Engineering Aspects of Electromagnetic Shielding Engineering Aspects of Electromagnetic Shielding Dr. Sergiu Radu Principal Engineer, EMC Design Sun Microsystems, Inc

49

Honeycomb Frames & Gaskets

T = length of the waveguide, depth

T/ ' w cirection

l • L~~

dire on

Page 50: Engineering Aspects of Electromagnetic Shielding Engineering Aspects of Electromagnetic Shielding Dr. Sergiu Radu Principal Engineer, EMC Design Sun Microsystems, Inc

50

Perf Panel Example

SE = 8dB @ 1.2GHz

I

It gets worse if the frequency,diameter or panel dimensions increase, or if center distances decrease.

For a 1/4” plated honeycombwith less than 1/4” cells: SE > 50dB @ 10GHz

SE = 8dB @ 1.2GHz

For staggered round holes with 4mm diameter, and center to center distance of 4.7mm, for a 92x206mm panel:

SE = 13dB @ 1.2GHz

SE = 8dB @ 1.2GHz

(Type of perf pattern used in some power supplies)

Page 51: Engineering Aspects of Electromagnetic Shielding Engineering Aspects of Electromagnetic Shielding Dr. Sergiu Radu Principal Engineer, EMC Design Sun Microsystems, Inc

51

Typical Perf Used for Cosmetic Areas

4.80mm

I 0 1

Oo 5/32" (0.156in, 3.969mm)

I

The 5132" hex perf pattern is a standard one and should be readily available wherever it needs to be manufactured.

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Page 52: Engineering Aspects of Electromagnetic Shielding Engineering Aspects of Electromagnetic Shielding Dr. Sergiu Radu Principal Engineer, EMC Design Sun Microsystems, Inc

52

Single and Double Perf Panels

I I measurement t ..... FDTD modeling •-fv ~in as Na"3

1.5 2.0 Aperture Size (em)

l5 3.0

,.....

30.0 - !im&le S~:n:en - • dui\hcrttil, I em apart

20.0 · ... · duahtmii, 2 em ap.ttt -- dw s.c:~.n.4cm

10.0

0.0 s ~ -10.0 ::l

l\ ( .

! 8 ~ ~30.0 ~

§; -40.0

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' :! ~ ~ '-. J ':.\ I ' ' I I i ' \ I

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,;: \ , .' \'· ~ ~ • •. • t: I ;r~ \ ,. I ,. I ·. .. ". ; \ ... ... :

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• ' • \ " II\ , .. t 1

~/ / ll \ .. , .... / I ' \ ~J ~,· -60.0 ·r ,/ I \ / ' . ~~ t nv,\, 1 ,~ ... I v..._,.,, ~ f -70.0 f :'

-20.0 -

-50.0

0.3 OA 0.5 0.6 0.7 0.8 0.9 1.0 1.1 Frequency (GHz)

Page 53: Engineering Aspects of Electromagnetic Shielding Engineering Aspects of Electromagnetic Shielding Dr. Sergiu Radu Principal Engineer, EMC Design Sun Microsystems, Inc

53

An Example: Double Perf Panels

Page 54: Engineering Aspects of Electromagnetic Shielding Engineering Aspects of Electromagnetic Shielding Dr. Sergiu Radu Principal Engineer, EMC Design Sun Microsystems, Inc

54

Know Your Noise Source: CPU Radiated Emissions

● Open Chassis Emissions for a [email protected]) (older system). Average: 18.6dB over Class A.

Page 55: Engineering Aspects of Electromagnetic Shielding Engineering Aspects of Electromagnetic Shielding Dr. Sergiu Radu Principal Engineer, EMC Design Sun Microsystems, Inc

55

EMI mechanisms for VLSI chips

● The Heatsink acting as a monopole antenna against the first solid plane of the PCB (tall Heatsink).

● The Heatsink and the first solid plane of the PCB creating a patch antenna (wide Heatsink).

● The low inductance power distribution of the VLSI chip allows noise injection into the PCB, which re-radiates (especially through closely placed DC-DC converters).

Page 56: Engineering Aspects of Electromagnetic Shielding Engineering Aspects of Electromagnetic Shielding Dr. Sergiu Radu Principal Engineer, EMC Design Sun Microsystems, Inc

56

Typical problems with heatsink grounding

● Components on Top need to be placed at a distance (some decoupling may be impacted)

● No pin escapes on Top

● The routing might be impacted by the multiple vias from the GND ring to GND plane

● Not effective above approx. 1GHz (any grounding will be too inductive to really matter)

● No direct DC grounding if the chip can use Back Bias or Forward Bias

Page 57: Engineering Aspects of Electromagnetic Shielding Engineering Aspects of Electromagnetic Shielding Dr. Sergiu Radu Principal Engineer, EMC Design Sun Microsystems, Inc

57

Heatsink as a Monopole and Cavity Resonance

Heatsink radiation at 5GHz from PCI_e Switches

Page 58: Engineering Aspects of Electromagnetic Shielding Engineering Aspects of Electromagnetic Shielding Dr. Sergiu Radu Principal Engineer, EMC Design Sun Microsystems, Inc

58

Heatsink as a Monopole and Cavity Resonance

Removing the heatsinks from the PCI_e switches, 5GHz decreases

Page 59: Engineering Aspects of Electromagnetic Shielding Engineering Aspects of Electromagnetic Shielding Dr. Sergiu Radu Principal Engineer, EMC Design Sun Microsystems, Inc

59

Heatsink as a Monopole and Cavity Resonance

Adding carbon loaded foam over the heatsinks of the PCI_e Switches

Page 60: Engineering Aspects of Electromagnetic Shielding Engineering Aspects of Electromagnetic Shielding Dr. Sergiu Radu Principal Engineer, EMC Design Sun Microsystems, Inc

60

Electromagnetic Wave Absorbers

P = P P P =12 ∣E ∣2 1

2 0 1 ' ' ∣E ∣

2 12 0 1 ' ' ∣H ∣

2

● Electromagnetic wave absorption materials absorb the energy in electromagnetic waves as resistive, dielectric or magnetic losses, and convert that energy, in the end, to heat. The amount of that absorption, P, can be expressed by the following formula:

P [W/m3]: Electromagnetic energy absorbed per unit volume

E [V/m]: Electric field strength of the incident electromagnetic radiation

H [A/m]: Magnetic field strength of the incident electromagnetic radiation

σ [S/m]: Conductivity of the material

ω [sec-1]: Angular speed of the electromagnetic wave (= 2πf)

εr″: Complex component of the dielectric constant of the material

ε0 [F/m]: Dielectric constant of the vacuum: 8.854 × 10-12 [F/m]

μr″: Complex component of the dielectric constant of the material

μ0 [A/m]: Magnetic permeability of the vacuum: 1.2566 × 10-6 [H/m]

Page 61: Engineering Aspects of Electromagnetic Shielding Engineering Aspects of Electromagnetic Shielding Dr. Sergiu Radu Principal Engineer, EMC Design Sun Microsystems, Inc

61

Heatsink as a Patch Antenna

● Example of heatsink cavity resonances -Compact PCI

Page 62: Engineering Aspects of Electromagnetic Shielding Engineering Aspects of Electromagnetic Shielding Dr. Sergiu Radu Principal Engineer, EMC Design Sun Microsystems, Inc

62

VLSI injecting noise into the PCB

● μPGA (more inductive) and LGA (less inductive)

•• ••••• ••••• ••••• ••••• •••• • ••• •• ••• •• ••• •• ••• •• ••• •• • •• •• ••• •• ••• •• • •• •• • •• •• • •• •• ••• •• ••• •• ••• •• ••• •• ••• • • • •• •• ••• •• ••• •• • •• •• ••• •• • •• •• ••• •• ••• •• • • • • ••••• ••• •• • •• •• ••• •• • •• •• ••• •• ••• • • • •• •• ••• •• ••• •• • • • • ••••• • •• •• ••• • • • • • • • • •• •• ••• •• • •• • • • • • • • • •• •• • • • • • • •• •• ••• • • • • • •• • •• • • • • • •• •• •• • • •• •• ••• • • •• • •• •• • • • ••• •• • • • •• •• ••• • • • •• ••• • • • • • •• •• •• • • •• •• ••• • • • • • •• • • • • • • • • • • ••• •• • • • • • • •• •• ••• • • • • • •• • •• •• ••• •• •••

• • • • • ••• • • • • • •• •• • • • • •• •• ••• • • • • •• • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • ••• •• ••• • • • •• ••• • • • • • ••• •• • • • • •• •• ••• •• ••• •• ••• • • • •• •• ••• •• • •• •• • •• •• • •• •• • •• •• ••• •• ••• •• • •• •• • •• •• • •• •• • • • •• ••• •• • •• •• ••• •• ••• • • • •• •• ••• •• • •• •• ••• •• ••• •• • •• •• ••• •• ••• •• • •• •• • •• •• ••• •• • •• •• ••• •• • •• •• • •• •• ••• • • • •• •• ••• •• ••• •• ••• •• • •• • •

Page 63: Engineering Aspects of Electromagnetic Shielding Engineering Aspects of Electromagnetic Shielding Dr. Sergiu Radu Principal Engineer, EMC Design Sun Microsystems, Inc

63

Heatsink: Grounding vs. Shielding

● Grounding will work at lower frequencies, but not above 1GHz

● Heatsink shielding works at higher frequency (grounding is implicit)if the contact is continuous, 360 degrees, creating a Faraday cage.

Page 64: Engineering Aspects of Electromagnetic Shielding Engineering Aspects of Electromagnetic Shielding Dr. Sergiu Radu Principal Engineer, EMC Design Sun Microsystems, Inc

64

Typical Heatsink Grounding and Shielding

● Grounding ring and EMI gasket on top layer PCB

Page 65: Engineering Aspects of Electromagnetic Shielding Engineering Aspects of Electromagnetic Shielding Dr. Sergiu Radu Principal Engineer, EMC Design Sun Microsystems, Inc

65

Example: UltraSPARC IIIe in Sun Ultra 60

Heatsink Actually Grounded

Page 66: Engineering Aspects of Electromagnetic Shielding Engineering Aspects of Electromagnetic Shielding Dr. Sergiu Radu Principal Engineer, EMC Design Sun Microsystems, Inc

66

Cavity Effect for Heatsink GroundingSl. N Micr o Float ng

Frcqucrcy 1 2c ' OJJ

SLN Micr o Float ng rrequercy 1 2e-+{)J3

Eloclric Field (1/.rn)

Eloclric Field (1/,om)

SUN Micro 24 Grounds Frequency 3.6e+009

Electric Field (VIm) • 254.75

191 .11

127.48

63.845

< 0.2105

(Li )

2

+(. H1·-. )2 +(! .).3

fc~.j.i:) = l11.50 -1 "

Page 67: Engineering Aspects of Electromagnetic Shielding Engineering Aspects of Electromagnetic Shielding Dr. Sergiu Radu Principal Engineer, EMC Design Sun Microsystems, Inc

67

● The main two aspects we were looking for: The effect of chassis resonancesThe effect of direct coupling from a potential noise source to a potential antenna

● For each chassis resonance, the maximum energy is at the fundamental frequency of the resonance. For the higher modes the energy decreases with the order of the mode. From a practical point of view, this means that the most dangerous internal resonances will be below 1GHz, and to be on the safe side we may extend this to 3GHz. However, we expect that the energy in the very high modes, above 10GHz to be so small that the chassis resonance in itself will be not a problem above 10GHz (for normal sized 19” rack chassis).

● We may use 3GHz as upper limit for calculating the riveting (tux) pitch for the areas which are not in the path of a direct coupling to an internal source.

● For near-field direct coupling between an internal source and an aperture, the most important parameter is the source-aperture distance.

● For any significant internal source we need to have a distance to the closest aperture of at least 3-5 times the longest dimension of the aperture

Source – Aperture Coupling

Page 68: Engineering Aspects of Electromagnetic Shielding Engineering Aspects of Electromagnetic Shielding Dr. Sergiu Radu Principal Engineer, EMC Design Sun Microsystems, Inc

68

Unified Multipoint Grounding

Old recommendations – impossible to use at the current frequencies

,--- Inductance in tbe pOwer planes.

Internal power plane capacitance.

~~~~~tiW~-~~~~ ..--+-.i7 Vcm produced by eddy cu~nts across impedance (l) from mounting post.

~--- Screw securement to metal chassis (ground connection)

APPLICA n ON MO.DEL OF MULTIPOINT GROUNDING ()~~._m __ om-o --•~0~••--m-o--~~o

~

V cml is reduced by the mounting post.~ (ground stitch locations).

Resonance is thus conrrolled, along with enllanced Rf suppression.

WO ~-.r---~ .... ~l 0 WO

;/ 0 ... wo .. () .

Distance between screws (chassis ground) in any axis (x- or y-axis) should not exceed

WO of the highest edge rate generated with in the printed circuit board.

wo

Page 69: Engineering Aspects of Electromagnetic Shielding Engineering Aspects of Electromagnetic Shielding Dr. Sergiu Radu Principal Engineer, EMC Design Sun Microsystems, Inc

69

Shielding Effectiveness Evaluation

~ Battery powered source

Battery powered source inside:

+No change to the enclosure

RX-ant

+ easy setup in existing emissions measurement chambers

- No synchronization between receive and transmit frequencies (fill-in done)

- SIN may be limited

Page 70: Engineering Aspects of Electromagnetic Shielding Engineering Aspects of Electromagnetic Shielding Dr. Sergiu Radu Principal Engineer, EMC Design Sun Microsystems, Inc

70

Shielding Effectiveness Evaluation – Small Monopole

Page 71: Engineering Aspects of Electromagnetic Shielding Engineering Aspects of Electromagnetic Shielding Dr. Sergiu Radu Principal Engineer, EMC Design Sun Microsystems, Inc

71

Shielding Effectiveness Evaluation – Small Monopole

Calibration – small monopole only

I>

Sll FD~WA~J REFLECTICN

LOG MAGrJI TUDE

i~\ ~ -t /

I I

~ ,4

4

·-,, I I

" ' 1 ,, '\"' _L

\ "· \ '-,

\~ PHASE

15 -.., ....

\ \

GHz

If '-" ~

··~

"· .... ,

SET MARKERS 6.00J d61DIV

MARKER 1 or~ ----.- --..., 6 ',...._

r/ 1. 191625 GHz

MARKER 2 0~~ 1.888825 GH2:

MARKER 3 or~ 1. 988425 GHz

MARKER 4 ON 2.0071~0 GHz

~IARKER S or~ 10.e00e~0 4.1485~0 GH2:

~- MARKER 6 ON "b y-~,

"·-"-,, ,,

6.146725 GH2:

DISPLAY ON MARKERS

" ' ' l.REF MODE OFF ' '""' ,..__ SELECT

,,REF MARKER

~IARKER REAOOJT FUNCTIONS

Page 72: Engineering Aspects of Electromagnetic Shielding Engineering Aspects of Electromagnetic Shielding Dr. Sergiu Radu Principal Engineer, EMC Design Sun Microsystems, Inc

72

Shielding Effectiveness Evaluation – Small Monopole

Page 73: Engineering Aspects of Electromagnetic Shielding Engineering Aspects of Electromagnetic Shielding Dr. Sergiu Radu Principal Engineer, EMC Design Sun Microsystems, Inc

73

Shielding Effectiveness Evaluation – Comb Generator

E > : t

al 1J .s:::. ... tn c Q) I.. ... (/)

1J Q)

u.

110

100

90

80

70

60

50 ...... 40

30

20

10

1000

~ ...

2000 3000 4000 5000

~

r

6000 7000 8000 9000

Frequency MHz

~

BP01 Sv2AH

jDlkEMc Services

10000 11000 12000

Page 74: Engineering Aspects of Electromagnetic Shielding Engineering Aspects of Electromagnetic Shielding Dr. Sergiu Radu Principal Engineer, EMC Design Sun Microsystems, Inc

74

Shielding Effectiveness Evaluation – Comb Generator

II II II 1'.1', ~I II II 1~.1'~

~I II II 1111 J i' ill llS cll , I

Page 75: Engineering Aspects of Electromagnetic Shielding Engineering Aspects of Electromagnetic Shielding Dr. Sergiu Radu Principal Engineer, EMC Design Sun Microsystems, Inc

75

Shielding Effectiveness Evaluation – Comb Generator

The effect of extra sealing the lid, CGE02 behind PCI_E, with and w/o . Typical A/B comparison to evaluate the effect of a gasket or aperture.

80.0-

~ I ~ ~ 70.0-

Level - dBf V/m I

60.0- I

50.0-

40.0- I tt l l 30.0

20.0 1- ll.~ "'·l~ .:./1 ~

........ 10.0 I .»J('

I

0.0-~ L I I I

I

: I -10.0- I I -; Frequency - ~Hz

- 20.0-,, I I I I I I I I I I I I I I I I I I I I I I I ' I I I I I I I I I I I I I I I I I I I I I I I I

30 100 200 300 400 500 600 zoo 800 900 1000

80.0-Level - dBuVIm

70.0-

60.0-

~ f

t 50.0-

40.0- t-t I 30.0-

20.0-

10.0- ,._. L

0.0-

Page 76: Engineering Aspects of Electromagnetic Shielding Engineering Aspects of Electromagnetic Shielding Dr. Sergiu Radu Principal Engineer, EMC Design Sun Microsystems, Inc

76

Shielding Effectiveness Evaluation – Comb Generator

Air filter evaluation – use of conductive foam between air inlet and CPU area

Page 77: Engineering Aspects of Electromagnetic Shielding Engineering Aspects of Electromagnetic Shielding Dr. Sergiu Radu Principal Engineer, EMC Design Sun Microsystems, Inc

77

Small Enclosure

Pin

Pout

Monopole antenna

Small Enclosure

Pin

Pout

Monopole antenna

Shielding Effectiveness Evaluation – Stirred Mode Reverberation Chamber

Page 78: Engineering Aspects of Electromagnetic Shielding Engineering Aspects of Electromagnetic Shielding Dr. Sergiu Radu Principal Engineer, EMC Design Sun Microsystems, Inc

78

Local Shielding Using Thermoplastic Material

Best results with multilayer shield: Steel (2μm)-Copper (4μm)-Steel (2μm)PVC support: 25 mils

Memory harmonics <1GHz (x266MHz)Grounding is critical

Page 79: Engineering Aspects of Electromagnetic Shielding Engineering Aspects of Electromagnetic Shielding Dr. Sergiu Radu Principal Engineer, EMC Design Sun Microsystems, Inc

79

0.0

5.0

10.0

15.0

20.0

25.0

300

600

900

1200

1500

1800

2100

2400

2700

3000

Frequency, Mhz.

De

lta

SE,

dB

0.005" 0.010" 0.015" 0.020"

Local Shielding Using Thermoplastic Material

Effect of layer separation through the thickness of the thermoplastic base

Page 80: Engineering Aspects of Electromagnetic Shielding Engineering Aspects of Electromagnetic Shielding Dr. Sergiu Radu Principal Engineer, EMC Design Sun Microsystems, Inc

80

Shield Coating and RoHS

● RoHS requires less than 0.1% Hexavalent Chromium present as an 'homogeneous material', not intentionally added.

● Forms of chromium that are RoHS compliant: pure chrome plating (shiny, metallic), trivalent chromium conversion coatings (“Clear” or “Clear blue bright”).

● “Safe” substitutions for hexavalent chromium conversion coatings for steel:● Use aluminized steel (T2-65 or T2-LC), not galvanized (electro-galvanized steel is

linked to zinc whiskers, but also NEBS GR78 CORE compliance prohibits the use of ANY zinc in our systems, even as an alloying element over 15%)

● If EMI is not an issue, phosphatizing and painting is also an option● If NEBS compliance not an issue, then use “Clear blue bright” trivalent chromium

with acid bath electrolytic zinc underplate: “Zinc plate per ASTM B-633 SCI Type III with clear trivalent chromate conversion coating. Must withstand min. 96 hr ASTM B-166 Salt Spray. No additional dips or coatings.”

Page 81: Engineering Aspects of Electromagnetic Shielding Engineering Aspects of Electromagnetic Shielding Dr. Sergiu Radu Principal Engineer, EMC Design Sun Microsystems, Inc

81

● Things to watch for with the trivalent chromium coating:

● Some less-than-reputable coaters will try to get away with a hexavalent (or trivalent) chromium conversion coating to which they add a blue dye to make it look like the clear blue bright trivalent. In reality, the blue color comes from added cobalt to the trivalent coating bath, which also enhances the corrosion resistance. We may need to send samples for an external hex chrome "spot check" at a lab to be sure.

● There should be no additional top coats over the clear blue bright trivalent chromium finish. Any additional top coat will most likely be organic and will interfere with the EMI attenuating properties. A surface resistivity check can likely determine the presence of a top coat.

● For aluminum:● Determine if a coating is really needed● If a coating is needed, preferred use is trivalent chromium coating

● NOTE: If EMI is not an issue, anodizing is also an option

Coating

Page 82: Engineering Aspects of Electromagnetic Shielding Engineering Aspects of Electromagnetic Shielding Dr. Sergiu Radu Principal Engineer, EMC Design Sun Microsystems, Inc

82

Coating

● Chromate type conversion coatings have been observed to require around 20 psi to obtain low readings. Even then, in comparison to bright tin, the resistivity is high.

● Chromate on aluminum has been measured to be around 170 milliohms at 20 psi, while bright tin on aluminum has been observed to be less than 10 milliohms at only 8 psi. Very important above few GHz.

● There is data that shows the surface contact resistance to be several hundreds of milliohms and does not reduce down to below 30 milliohms until a pressure of around 200-250 psi for a nickel coating on aluminum (because of oxide).

● Use mechanical stops either stamped into or bends added to prevent exceeding the maximum compression value, typically 90-95% of the gasket height.

Page 83: Engineering Aspects of Electromagnetic Shielding Engineering Aspects of Electromagnetic Shielding Dr. Sergiu Radu Principal Engineer, EMC Design Sun Microsystems, Inc

83

Coating

● The coating must pass multiple type of tests: Conductivity, Anti-Finger printing test, Adhesive tape testing without peel off, Good corrosion resistance, Stamp / Paintability Test(scratch and peel off), Humidity Test (60oC / 95%H / 192 hours) without Blister or Rust, Anti-Chemical Resistance.

Page 84: Engineering Aspects of Electromagnetic Shielding Engineering Aspects of Electromagnetic Shielding Dr. Sergiu Radu Principal Engineer, EMC Design Sun Microsystems, Inc

84

Effect of Coating on Surface Resistivity (ASTM D257)

R = v1tLW = s

LW

s = R if :L=W square s = surface resistivity [ / square ]

● For thin, homogeneous and volume conductive materials, the apparent surface resistivity is equal to the true volume resistivity divided by the thickness.

R = v ˙tA ˙2

● If the coating is more resistive than the base metal, and L is much greater than t, the circuit can be approximately shown as in Fig. b. It is actually volume resistivity which is being measured and surface resistivity, in fact a nonexistent material property, acquires its apparent value depending on volume resistivity of the coating and specimen geometry (two serial resistors, connected by the conductive substrate). For a more conductive coating σ

s makes sense.

● Look for 100 mohms/square or less

Page 85: Engineering Aspects of Electromagnetic Shielding Engineering Aspects of Electromagnetic Shielding Dr. Sergiu Radu Principal Engineer, EMC Design Sun Microsystems, Inc

85

Plastic Coating – An Example

● A performance increase of 10 dB can be observed on critical frequencies.

● Vapor deposition is the most environmentally sound process for high quality conductive coatings. Aluminum is not considered a hazardous material and is thus completely compliant with the EU RoHS Directive. The coating is approximately 0.5 μm to 3 μm thick. ● European Union (EU) Waste Electrical and Electronic Equipment Directive (WEEE)

Page 86: Engineering Aspects of Electromagnetic Shielding Engineering Aspects of Electromagnetic Shielding Dr. Sergiu Radu Principal Engineer, EMC Design Sun Microsystems, Inc

86

A N O D IC (M o s t s u s c e p tib le to c o rro s io n )

M a g n e s iu m +2 .3 7 v

B e ry lliu m +1 .8 5 v

A lu m in u m +1 .6 6 v

Z in c +0 .7 6 v

C h ro m iu m +0 .7 4 v

Iro n /S te e l +0 .4 4 v

C a d m iu m +0 .4 0 v

N ic ke l +0 .2 5 v

T in /T in le a d +0 .1 4 v

L e a d +0 .1 3 v

C o p p e r -0 .3 4 v

S ilv e r -0 .8 0 v

C A T H O D IC (L e a s t s u s c e p tib le to c o rro s io n )

Shielding and Corrosion● Corrosion is also a concern because it leads to reduced shielding effectiveness due to causing the gasket material to become an insulator or creating new problem frequencies through nonlinear mixing.

● There are two types of corrosion: galvanic (most common) and electrolytic.

Galvanic corrosion is due to contact between two dissimilar metals in the presence of moisture. The potential developed depends upon their relative position in the electrochemical or galvanic series.

● Electrolytic corrosion is due to current flow between two metals in the presence of an electrolyte (which could be just slightly acidic ambient moisture). The major contributors to this problem are surface contact area, material dissimilarity and the presence of an electromotive, usually moisture.

● Galvanically compatible materials are those that are within 0.25 volts of each other. For commercial applications where the environment is controlled, the range can be increased up to 0.5 to 0.6 volts. If large contact voltages occur, the more anodic material will eventually be destroyed. To prevent this problem, either the gasket material or mating surface, or both, will need to be plated with a material or finish that is compatible with the base material. The closer the materials that are in contact with each other are in the galvanic series, then the lower the probability of corrosion. A typical galvanic activity table:

Cathodic: Gold (+1.5V), Silver (0.8V), Copper (0.34V), Hydrogen (0V), Tin (-0.137), Nickel (-0.257), Iron (-0.44), Zinc (-0.76V), Aluminum (-1.66V), Magnesium (-2.37) Anodic

Page 87: Engineering Aspects of Electromagnetic Shielding Engineering Aspects of Electromagnetic Shielding Dr. Sergiu Radu Principal Engineer, EMC Design Sun Microsystems, Inc

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Shielding and Safety Aspects

● Steel ball impact (important for plastic honeycomb)

● Finger test (important for hole size)

● Hazardous voltage (chassis interlock)

● Flammability (type of plastic materials, fire enclosure, mesh)

● UL recognized coating (acceptable if the conductive coating is “adequately” bonded to the substrate material).

Page 88: Engineering Aspects of Electromagnetic Shielding Engineering Aspects of Electromagnetic Shielding Dr. Sergiu Radu Principal Engineer, EMC Design Sun Microsystems, Inc

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Thanks !

Sergiu Radu

EMC Design Engineering

Sun Microsystems, Inc.

(650) 786 3465

[email protected]