IPC-TM-650 EN 2022 试验方法.pdf - 第591页
1 Scope It is the intent of these guidelines to describe the material properties and test procedures required to ensure effective RFI and EMI shielding of flat cable. 1.2 Definitions 1.2.1 Relative Shielding Effectivenes…
6.0 Notes
6.1
Volume Resistivity
Volume
resistivity is used in place
of ‘‘weight resistivity’’ and ‘‘percent conductivity.’’ The value
of 10.381 ohm–circular mil/ft at 20°C (68°F) is the volume
resistivity equivalent to the International Annealed Copper
Standard (IACS) for 100 percent conductivity. This term
means that a wire 1 ft in length and 1 cir mil in cross-sectional
area would have a resistance of a wire1minlength, and 1
sq mm in cross-sectional area.
6.2
Weight Resistivity
Weight
resistivity is expressed in
English units in ohm-pound/mile
2
and
in metric units in ohm–
gram/meter
2
.
It may be calculated as follows:
ρw =
W
L
1
L
2
R
where:
ρw
= weight resistivity in ohm-pound/mile
2
,
or ohm-gram/
meter
2
,
W
= weight of the test specimen in lb, or gm,
L
2
=
length of the test specimen in miles, or m, and
L
1
=
gauge length, used to determine R, in. miles or m, and
R = measured resistance in ohms.
T
able 1 Equivalent Resistivity Values For Copper
Conductivity
at 20°C (68°F), percent IACS...................100.00
VOLUME RESISTIVITY
Ohm-Circular mil/f ..................................................10.371000
0hm-mm
2
/meter
.......................................................0.017241
Microhm-inch ...........................................................0.678790
Microhm-cm.............................................................1.724100
WEIGHT RESISTIVITY
0hm–pound/mile
2
.........................................................875.20
0hm–gram/meter
2
......................................................0.15328
6.3
Conversion
Resistivity
and Conductivity Conversion –
Conversion of the various units of volume resistivity, weight
resistivity, and conductivity, may be facilitated by employing
formulas and factors. Table 2 lists values of density, for the
common electrical conductor materials.
6.4
Density
For
the purpose of resistivity and conductivity
conversion, the density of copper materials may be taken as
shown in Table 2, based on a temperature of 20°C (68°F).
T
able 2 Density and Temperature Coefficient of
Resistance for Electrical Materials
Density
at
20°C, gm
per cu cm
Temperature
Coefficient of
Resistance
at 20°C
Copper, % IACS:
100................................. 8.89 0.00393
98.40.............................. 8.89 0.00387
98.16.............................. 8.89 0.00386
97.80.............................. 8.89 0.00384
97.66.............................. 8.89 0.00384
97.40.............................. 8.89 0.00383
97.16.............................. 8.89 0.00382
96.66.............................. 8.89 0.00380
96.61.............................. 8.89 0.00380
96.16.............................. 8.89 0.00378
94.16.............................. 8.89 0.00370
93.15.............................. 8.89 0.00366
IPC-TM-650
Number
2.5.14
Subject
Resistivity
of Copper Foil
Date
8/76
Revision
A
P
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1
Scope
It
is the intent of these guidelines to describe the
material properties and test procedures required to ensure
effective RFI and EMI shielding of flat cable.
1.2
Definitions
1.2.1 Relative Shielding Effectiveness
The
attenuation
difference in the electromagnetic field strength between an
unprotected cable and a shielded cable system, which is
expressed,S=R
x
+A+B
,where:
R
x
=
the losses caused by reflection in db
A = the losses caused by absorption in db
B = the secondary reflection losses of the shields in db.
The reflection losses are a function of the material, frequency,
and type of field. Generally, the field within one wave length
from a generating source will either be predominantly electric
or magnetic, and at greater distance will propagate as a plane
wave made up equally of electric and magnetic components.
Thus, the reflection losses for each of these fields may be
designated by:
R
E
=
electric or ‘‘E’’ field
R
H
=
magnetic or ‘‘H’’ field
R
p
=
plane wave field
The absorption losses are a function of the material and fre-
quency but are independent of field type. If these losses (A)
are greater than 10 db, the secondary reflection losses are
negligible, and the expression for shielding effectiveness
reduces toS=R+A.
The following are standard equations that may be used to
obtain a rough approximation of a shield’s effectiveness.
Absorption Losses:
A=3.38X10
-3
t
(uGf)
1/2
Reflection
losses:
1. Plane wave
R
p
= 108.2 + 10
log
Gx10
6
uf
2.
Magnetic fields
RH = 20 log
0.462
r
(
U
Gf
)
1
⁄
2
+0.136
r (
Gf
u
)
1
⁄
2
+ 0.354 (r<λ)
3.
Electric fields
R
E
= 353.6 + 10
log
G
ur
2
f
3
(r
<λ)
where:
G = conductivity relative to copper
u = magnetic permeability relative to free space
f = frequency in Hertz
r = distance from source to shield in 2.5 cm
t = thickness of metal shield in 0.0025 mm
λ = wavelength
A field surrounds every source of electric energy. The simple
situation of an electric current flowing through a wire causes a
field to exist around the wire, whose magnitude and direction
follow well-known principles. Part of the energy in any field is
propagated through space and eventually dampens to zero.
The remaining part of the energy of a field either returns to its
origin or is absorbed by some receiving source. A dipole
antenna behaves in this manner; part of its energy becomes a
radiation field, while another portion (that periodically returns
to the antenna) becomes the induction field. The general
mathematical expression that describes an electromagnetic
field is rather complex and is usually discussed in texts on field
theory. It is easier to discuss this expression in terms of its
electric vector E and its magnetic vector B, where E has the
dimension of V/1 and units of volt/meter and B has the dimen-
sions of VT/1
2
and
units of volt-second/meter
2
.
E and B can
then be written as the sum of two components:
E=E
i
+E
R
B=B
i
+B
R
The
components of the induction field are E
i
and
B
i
,
while the
components of the radiation field are given as E
R
and
B
R
,E
R
,
and
B
R
are
proportional to B
o
/R
(B
o
=
w/v
o
R,
where w is the
angular frequency of the field in radians and v
o
is
the velocity
of propagation in meters per second.) E
i
and
B
i
are
propor-
tional to 1/R
2
,
where R is the distance from the source in
meters. The ratio of the two is B
o
R
or wR/v
o
.
It can be con-
cluded from this that for very small values of R and any given
values for w and v
o
,
the induction field will be so much greater
than the radiation field, that the latter may be neglected. How-
ever, if R is very large, the radiation field is important and the
induction field can be discarded.
The
Institute for Interconnecting and Packaging Electronic Circuits
2215 Sanders Road • Northbrook, IL 60062
IPC-TM-650
TEST
METHODS MANUAL
Number
2.5.15
Subject
Guidelines
and Test Methods for RFI-EMI Shielding
of Flat Cable
Date
10/86
Revision
A
Originating Task Group
N/A
Material
in this Test Methods Manual was voluntarily established by Technical Committees of the IPC. This material is advisory only
and its use or adaptation is entirely voluntary. IPC disclaims all liability of any kind as to the use, application, or adaptation of this
material. Users are also wholly responsible for protecting themselves against all claims or liabilities for patent infringement.
Equipment referenced is for the convenience of the user and does not imply endorsement by the IPC.
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Induction
fields are either high- or low-impedance fields. A
high-impedance field is defined as a field whose impedance is
higher than the impedance of the dielectric in which it exists.
A low-impedance field has an impedance lower than the
impedance of the dielectric. High-impedance fields are asso-
ciated with a voltage source and most of their energy is con-
tained in their electric component, while low-impedance fields
are associated with a current source and most of their energy
is contained in the magnetic component.
1.2.2
Shield Impedance
An
important parameter associ-
ated with these radiating fields is the characteristic imped-
ance, which is the ratio of the electric to magnetic field com-
ponents. For a plane wave in free space, the characteristic
impedance is 377 ohms, and correspondingly for intense
electric or high impedance fields, it is greater than 377 ohms,
and for strong magnetic or low impedance fields, it is less than
377 ohms. The difference in characteristic impedance
between an incident field and a shield is directly proportional
to the reflection losses. The characteristic impedance of a
shield varies with the material’s permeability, conductivity, and
frequency. Shield impedances are generally low at low fre-
quencies and increase directly with frequency. Since at all fre-
quencies, electric (E) fields are high impedance and magnetic
(H) fields are low impedance, the corresponding reflection
losses are high for electric fields at low test frequency and low
or poor for magnetic fields at the same test frequency. As test
frequencies increase, the impedance mismatches decrease
for electric fields (decrease in R
E
)
and increase for magnetic
fields (increase in R
H
).
The absorption losses for both electric
and magnetic fields increase with frequency. It can be con-
cluded from this that good shielding effectiveness against pre-
dominantly electric fields can be obtained with most high con-
ductivity shielding materials. At low frequencies, R
E
losses
are
so high that small absorption losses may be neglected and, at
high frequencies, even though most of the transmitted energy
is coupled to the shield, absorption losses are high enough for
adequate shielding if all nonconductive openings in the shield
are eliminated. Shielding against magnetic fields presents a
different situation at low frequencies, where absorption and
reflection (R
H
)
losses are small. Here, uniform 100% shielding
is essential and in most cases ferromagnetic, highly perme-
able materials are employed to increase absorption losses. At
high frequencies, both reflection and absorption losses are
high, and shielding effectiveness is good for magnetic fields.
Table 1 shows properties of various metals at 150 KHz and
400 MHz and the corresponding absorption loss in db. The
significance of this table is to show the necessity for highly
permeable materials to shield against low frequency magnetic
fields.
3
Test Specimen
None
4
Equipment/Apparatus
None
5
Procedure
None
6 Notes
6.1
Shielding
effectiveness is usually determined more pre-
cisely by measurement than by calculation, especially when
100% shielding is impractical. To obtain the attenuation capa-
bility of a shielding material about a flat cable, it is more prac-
tical to test a cable system for its susceptibility to radiated
energy.
Figure 1 shows a test setup designed to measure shielding
effectiveness in a flat cable for electric and magnetic radiating
fields. Two 1.5 m cable specimens, one shielded and one
unshielded, are terminated in their characteristic impedance at
the generator source end and attached through a coaxial
switch to a field intensity meter (or similar device) at the other
end. These two cable samples are mounted and suspended
2.5 cm above a conducting ground plane and 7.5 cm to either
side of a bare unshielded copper wire (see Figure 2). This
radiating copper wire is connected at one end to a RF signal
generator and is terminated at the opposite end in either a
short or non-radiating open circuit.
6.2
When
the bare wire is open circuited, the majority of the
radiated field is electric, and when it is short circuited, mag-
netic fields dominate. Since the cables are only 7.5 cm away
from the radiating source, electric and magnetic shielding
effectiveness can be measured separately at frequencies up
to approximately 4 GHz. It is assumed that if a shield is effec-
tive under these conditions, it will be equally effective against
plane wave radiation.
6.3
Four
readings must be taken at each test frequency.
First the voltage pick-up in the unshielded specimen is
observed and is used as the reference level for the voltage
measurement on the shielded line. The shielding effectiveness
in decibels is given by:
S=20logV
u
/V
s
where:
V
u
=
voltage induced into unshielded cable
V
s
=
voltage induced into shielded cable
IPC-TM-650
Number
2.5.15
Subject
Guidelines
and Test Methods for RFI-EMI Shielding of Flat Cable
Date
10/86
Revision
A
P
age2of5
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