IPC-TM-650 EN 2022 试验方法.pdf - 第590页
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 …
5.1.2
The
cross-sectional dimensions of the specimen may
be determined by micrometer measurements, and a sufficient
number of measurements shall be made to obtain the mean
cross section to within ± 0.10 percent.
5.1.3
In
case any dimension of the specimen is less than
0.100 in. and cannot be measured to the required accuracy,
the cross section shall be determined from the weight, den-
sity, and length of the specimen.
5.1.4
When
the density is unknown, it shall be determined
by weighing a specimen first in air and then in a liquid of
known density at the test temperature, which shall be at room
temperature to avoid errors due to convection currents.
5.1.5
Calculate
the density from the following formula:
δ=
W
a
Xd
W
a
− W
I
where:
δ =
density of the specimen, grams per cu cm,
W
a
=
weight of the specimen in air, grams,
W
I
=
weight of the specimen in the liquid, grams, and
d = density of the liquid at the test temperature, grams per
cu cm.
5.2
Test
5.2.1
When
potential leads are used, the distance between
each potential contact and the corresponding current contact
shall be at least equal to 1-1/2 times the cross-sectional
perimeter of the specimen.
5.2.2
The
yoke resistance (between reference standard and
test specimen) shall be appreciably smaller than that of either
the reference standard or the test specimen unless a suitable
lead compensation method is used, or it is known that the coil
and lead ratios are sufficiently balanced so that variation in
yoke resistance will not decrease the bridge accuracy below
stated requirements.
5.2.3
Make
resistance measurements to an accuracy of ±
0.15 percent.
5.2.4
In
all resistance measurements, the measuring current
raises the temperature of the specimen above that of the sur-
rounding medium. Therefore, care shall be taken to keep the
magnitude of the current low, and the time of its use short
enough so that the change in resistance cannot be detected
with the galvanometers.
5.2.5
To
eliminate errors due to contact potential, two read-
ings, one direct and one with current reversed, must be taken
in direct succession.
5.2.6
Check
tests are recommended whereby the specimen
is turned end for end, and the test repeated.
5.2.7
Surface
cleaning of the specimen at current and
potential contact points may be necessary to obtain good
electrical contact.
5.3
Evaluation
5.3.1 Reference Tests
For
reference tests, the report
should include the following:
1.Identification of test specimen,
2.Kind of material,
3.Test temperature,
4.Test length of specimen,
5.Method of obtaining cross-sectional area: the average val-
ues of micrometer readings, or, if by weighing a record of
length, weight, and density determinations that may be
made, and calculated cross-sectional area.
6.Weight, if used,
7.Method of measuring resistance,
8.Value of resistance,
9.Reference temperature,
10.Calculated value of resistivity at the reference temperature,
and
11.Previous mechanical and thermal treatments. (Since the
resistivity of a material usually depends upon them, these
shall be stated whenever the information is available. )
5.3.2
Routing Tests
For
routine tests, only such of the
items in paragraph 5.3.1 as apply to the particular case, or are
significant, shall be reported.
IPC-TM-650
Number
2.5.14
Subject
Resistivity
of Copper Foil
Date
8/76
Revision
A
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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
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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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