IPC-TM-650 EN 2022 试验方法--.pdf - 第555页
Probe p erformance may degr ade over time. It is necessary to periodically check the p robe quality to assure the electrical requirement in Figure 4-3 is met. 5 Procedure The proc edure section is to be used to detail al…
As the solid metal planes may block the moisture penetration,
for the conductors routed on inner layers it typically takes a
long time (with rare exceptions) for the sample to absorb the
moisture. Therefore, making measurement of insertion loss of
inner routing layers under the conditions described in 3.8.1 is
recommended over making such measurements under the
conditions described in 3.8.2.
3.8.1 Insertion Loss Measurement of Vacuumized Test
Specimens
Test specimens can be vacuumized right after
baking them at 105 °C RH 0% over 2 hours, or 140 °C RH
0% over 1 hour. However, if the coupon has been stored over
a long period of time without proper vacuum packaging, the
baking condition needs to be adjusted to be 140 °C RH 0%
for 12 hours. Consistent results can be obtained by testing
specimens at 23 °C (± 2 °C) [73.4 °F (± 3.6 °F)] and 20~80%
RH for less than 12 hours since opening the vacuum package
or finishing a baking treatment. It is recommended to allow
test coupons to cool to room temperature for at least 30 min-
utes before test if measurement is done after a baking treat-
ment.
3.8.2 Insertion Loss Measurement of Test Specimens
Stored in Environmental Chamber
For conductors routed
on outer layers, consistent results of insertion loss at typical
humidity condition can also be obtained by storing test speci-
mens at 23 °C (± 2 °C) [73.4 °F (± 3.6 °F)] and 40% RH (± 5%
RH) for no less than 48 hours. Note that the test under this
condition takes longer time compared to that described in
3.8.1.
4 Apparatus
4.1 VNA Measurement Apparatus
The measurement
equipment needed includes a VNA, calibration kit, cabling,
and a probing solution, as shown in Figure 4-1. High perfor-
mance connectors and cables that are rated above the maxi-
mum frequency of interest are required in performing VNA
measurements.
Using TDR/TDT system in place of a VNA to acquire fre-
quency domain attenuation and loss data is beyond the scope
of this test method. A future IPC-TM-650 Test Method
2.5.5.15 for best design practices for Time Domain method is
envisioned under the IPC D-24D Task Group.
4.2 Probe Quality
The quality of probe (whether using
probing station or handheld probe) is critical for accurate and
repeatable measurement. It is recommended to have the
insertion loss of the probe and launching pad to be less than
3.5 dB at highest frequency of interest, to make sure the
probe and launching pad design have good electrical perfor-
mance.
A direct measurement of electrical performance of probe and
launching pad can be cumbersome. Alternatively, Figure 4-2
shows an example of test setup to check the electrical perfor-
mance. A 50.8 mm [2.0 in] microstrip line with known insertion
loss is used to provide a connection between two probes.
VNA is calibrated to the end of coaxial cable, and the inser-
tion loss of the 50.8 mm [2.0 in] microstrip line with probes at
both ends is measured.
Insertion loss requirement for the test setup in Figure 4-2
depends on the highest measurement frequency, as well as
the microstrip trace loss. A test coupon with known loss can
be used, or a separate measurement can be done to deter-
mine microstrip loss. Figure 4-3 shows an example of the
probe quality requirement, assuming the highest measure-
ment frequency is 20 GHz, and the insertion loss of the
50.8 mm [2.0 in] microstrip is 5 dB at 20 GHz. The measured
insertion loss must be above the red dash line in the figure.
Note at DC level, the required loss is less than 1 dB, and at
20 GHz, the required loss is less than12 dB (where 3.5 dB is
allocated for each probe, and 5 dB is coming from the
50.8 mm [2.0 in] microstrip).
IPC-25514-4-1
IPC-25514-4-2
Number
2.5.5.14
Subject
Measuring High Frequency Signal Loss and Propagation on
Printed Boards with Frequency Domain Methods
Date
02/2021
Revision
IPC-TM-650
Calibration
to
the
end
of
coaxial
cable
Figure
4-2
Test
Setup
for
Probe
Quality
Check
Page
6
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Probe performance may degrade over time. It is necessary to
periodically check the probe quality to assure the electrical
requirement in Figure 4-3 is met.
5 Procedure
The procedure section is to be used to detail
all of the specific steps necessary to perform the actual test.
It
include any specific conditioning requirements, or
other specimen preparation not previously detailed. It
then describe in detail the successive steps of the procedure,
grouping related operations into logical divisions in a concise
manner. It
include times, temperatures, voltages, pres-
sures, concentrations, linear measurements and quantitative
criteria when necessary in applicable units (both Metric and
English).
It
then state any detailed information required in report-
ing the test results. When two or more procedures are
described in the same test method, the report
indicate
which of the procedures was used. When a test method
allows variations in operating or other conditions, the report
state the particular conditions utilized for the test.
This specification currently outlines measuring Frequency
Domain characteristics using a VNA.
5.1 VNA Settings
Follow the VNA manual for proper
operation of equipment. Recommended settings for the VNA
include an IF bandwidth of 1 kHz (can be decreased based on
instrument and applications), and a step size of 10 MHz.
Smoothing is not allowed.
The cables and connectors used in the measurement should
be sufficiently rated for the maximum intended measurement
frequency.
5.2 Conditioning of Test Sample
Refer to 3.8 for proper
conditioning of test sample before test.
5.3 VNA Calibration and De-embedding
Calibration
and/or de-embedding techniques outlined in 1.2.1 must be
performed to remove the effects of cable, connector, and test
fixtures.
5.4 Smoothing and Fitting of Insertion Loss Measure-
ment Curve
5.4.1 Insertion Loss Smoothing Basics
Printed board
testing facilities often report insertion loss per inch at a hand-
ful of frequencies (e.g., 4 GHz, 8 GHz, 12.89 GHz, etc.). An
ideal insertion loss curve for a printed board conductor is
expected to follow transmission line behavior and be smooth.
However, in some testing houses, the de-embedded insertion
loss curves may have oscillations and deviations due to vari-
ous sources of measurement and de-embedding error, as
shown in blue curve in Figure 5-1. Without proper post-
processing of the data, the measurement house can easily fail
to report the true loss performance of the test coupon at des-
ignated frequencies. One common methodology for obtaining
a smooth de-embedded insertion loss curve is to use an iter-
ated moving average. The result is a very smooth red curve
shown in Figure 5-1.
While smoothing with an iterative moving average addresses
most of the challenges posed by the measurement errors,
there remain some disadvantages. The resulting smooth curve
is non-physical and unlikely to be representative of the true
loss of printed board conductor. For example, the smoothed
curve usually deviates from the correct answer at low
IPC-25514-4-3
IPC-25514-5-1
Red denotes the smoothed curve
Number
2.5.5.14
Subject
Measuring High Frequency Signal Loss and Propagation on
Printed Boards with Frequency Domain Methods
Date
02/2021
Revision
0
0
5
10
15
20
Frequency
(GHz)
Frequency(GHz)
Figure
4-3
Insertion
Loss
Requirement
for
the
Probe
Quality
Test
Setup
in
Figure
4-2
shall
shall
shall
shall
shall
shall
Figure
5-1
An
Iterative
Moving
Average
Applied
to
a
Typical
Insertion
Loss
Curve
Note
1.
IPC-TM-650
—
3
5
2
5
1
5
N
1
.
6
m
p)
q
o
u-
sso.
(8P)
sso
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uo
Sil-
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frequencies where the conductor losses dominate. Addition-
ally, in the high frequency range, the smoothing may preserve
unrealistic features of the de-embedded insertion loss.
5.4.2 Cumulative Dielectric and Conductor Loss Fit-
ting
As it has been discussed in [14], the cumulative dielec-
tric and conductor losses can be generally approximated by
IL
dB
(,) = a
√
, + b, + c,
2
(Eq. 6)
where , is the frequency in GHz and a, b and c are constants.
For most of the cases coefficient c << 1 and can be
neglected. Therefore, as a first approximation the total loss
curve can be fitted to
IL
dB
(,) = a
√
, + b, (Eq. 7)
There are number of algorithms that can be used to perform
the printed board loss fit to Eq. 7. One of the most well-known
and widely available algorithms is the least squares fit,
example of which is shown in the Figure 5-2 below.
Even though least squares generally provide a good curve
approximation with the specified behavioral function, there are
many other fitting algorithms that can be applied.
5.4.3 An Alternative Cumulative Dielectric and Conduc-
tor Loss Fitting
Alternatively, when losses cannot be fitted
to the conventional physical based behavioral functions in (Eq.
6) and (Eq. 7), especially when measurement raw data has
high ringing resonances, other empirical approximations can
be used. Fox example, in [15], the following function is set as
the target function for the fitting algorithm:
IL
dB
(,) = a(, – ,
0
)
b
+ c(, – ,
0
)
2
+ d(, – ,
0
) + IL
0
(Eq. 8)
The first term represents the AC conductor loss (i.e., the skin-
effect losses), where ‘b’ is an additional fitting parameter
(instead of a constant 0.5 where ideal conductor loss is a
function of ,
0.5
) added to take into account the surface rough-
ness impact of the conductor. The second and the third terms
represent dielectric losses, and the constant represents the
conductor’s DC loss. Furthermore, a certain offset point (,
0
,
IL
0
) is introduced, where ,
0
is the first frequency point of the
measurement. The offset is added to accommodate the fact
that VNA measurements made at the printed board fabricator
usually do not provide results lower than 10 MHz.
The abovementioned methods fit the data to a smooth curve
over the entire bandwidth of the measurement where each
data point is allocated equal weight. As measurement errors
usually increase significantly at high frequencies, a weighting
scheme can be introduced to force the algorithm to prioritize
the curve fitting at the low frequencies and minimize (or ignore)
the impact of high frequency:
W(,) =
(
1–
(
,
,
max
))
3
(Eq.9)
where ,
max
is the maximum measurement frequency. Figure
5-3 shows the suggested weighted function where ,
max
= 20
GHz.
IPC-25514-5-2
Red represents the fitted curve.
IPC-25514-5-3
Number
2.5.5.14
Subject
Measuring High Frequency Signal Loss and Propagation on
Printed Boards with Frequency Domain Methods
Date
02/2021
Revision
IPC-TM-650
Figure
5-2
Least
Squares
Fit
Based
on
(eq.
7)
Applied
to
a
Representative
Insertion
Loss
Curve
Note
1.
Figure
5-3
The
Suggested
Weight
Function
for
Insertion
Loss
Curve
Fitting
.5
2
.5
1
.5
O
2
L
S
s
p)
uow
j
d
SSO1
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