IPC-TM-650 EN 2022 试验方法--.pdf - 第471页
Figure 4-1 Resolution and Electrical Length of T ransmission Line t V adequate resolution t V inadequate resolution 2 T p transmission line IPC-TM-650 Page 3 of 23 Number 2.5.5.7 Subject Characteristic Impedance of Lines…
IPC-2141
IPC-TM-650
IPC-TM-650
Page 2 of 23
Number
2.5.5.7
Subject
Characteristic
Impedance
of
Lines
on
Printed
Boards
by
TDR
Date
03/04
Revision
A
a.
The
transmission
line
under
test
varies
along
its
length
whereas
the
value
ofZo
obtained
assumes
a
uniform
trans¬
mission
line.
Therefore,
the
measured
Zo
only
approxi¬
mates
the
characteristic
impedance
of
an
ideal
line
that
is
representative
of
the
line
under
test.
b.
Lines
on
a
printed
circuit
board
may
deviate
significantly
from
design.
For
example,
microstrip
lines
longer
than
15
cm
[5.91
in]
on
boards
with
plated-through
holes
often
have
variations
in
line
width;
this
variation
is
due
to
plating
and/or
etching
variations.
c.
If
the
transmission
line
is
too
short,
the
accuracy
of
the
cal¬
culated
impedance
value
may
be
degraded
(see
4.1.2).
If
the
transmission
line
is
too
long,
skin
effect
and
dielectric
loss
may
cause
a
bias
in
the
impedance
measurement.
d.
Depending
on
where
the
measurements
are
made,
the
value
of
Zo
obtained
may
be
affected
by
dielectric
and
conductor
loss
and
other
effects.
The
farther
away
from
the
interface
between
the
probe
and
the
transmission
line
under
test,
the
worse
these
effects
will
be.
e.
Duration
of
the
measurement
window
(waveform
epoch)
may
need
to
be
adjusted
for
sample
length
and
location
of
midpoint
vias
along
the
transmission
line.
2
Reference/Applicable
Documents
Controlled
Impedance
Circuit
Boards
and
High
Speed
Logic
Design
IPC
Test
Methods
Manual
1
.9
Measurement
Precision
Estimation
for
Variables
Data
3
Test
Specimens
The
test
specimen
can
take
one
of
sev¬
eral
forms,
depending
on
the
application,
but
contains
at
least
one
transmission
(or
interconnect)
test
structure.
As
examples,
four
types
are
mentioned
in
3.1
.1
through
3.1
.4.
The
transmission
lines
to
be
measured
may
be
of
either
strip¬
line
or
microstrip
construction
and
configured
as
either
single-
ended
or
differential.
See
IPC-2141
for
a
recommended
test
coupon
design.
3.1
Test
Specimen
Examples
3.1.1
Example
1
Representative
samples
of
the
actual
PCB
being
manufactured
are
selected.
In
some
cases,
this
sample
set
may
contain
all
of
the
boards.
Agreed
upon
func¬
tional
or
nonfunctional
transmission
lines
within
the
sample
are
used
for
the
measurement.
Criteria
for
selection
of
such
lines
includes:
a.
Inclusion
of
the
PCB's
critical
features.
b.
Accessibility
of
terminations
for
the
line.
c.
Absence
of
branching.
d.
Absence
of
impedance
changes
within
the
transmission
line
under
test.
e.
Representation
of
controlled
Zo
signal
layers
in
a
multi-layer
board.
3.1.2
Example
2
Representative
samples
should
be
as
in
3.1
.1
,
except
that
the
test
lines
are
nonfunctional
lines
designed
into
the
board
for
easy
termination
for
TDR
mea¬
surements.
Such
test
lines
should
be
planned
to
include
criti¬
cal
features
typical
of
functional
lines
and
should
lie
in
con¬
trolled
Zo
signal
layers.
3.1.3
Example
3
Representative
samples
should
be
as
in
3.1
.1
,
except
test
coupons
are
cut
from
the
master
board
at
the
time
the
individual
PCBs
are
separated.
Such
test
cou¬
pons
will
have
one
or
more
sample
transmission
lines
with
termination
suited
for
testing.
Such
test
lines
should
include
critical
features
typical
of
functional
lines
and
will
be
fabricated
in
the
same
configuration
and
structure
as
the
master
board
on
the
same
controlled
Zo
layers.
3.1.4
Example
4
A
sample
of
the
substrate
laminate
to
be
characterized
before
use
in
manufacturing
PCBs
is
fabricated
with
test
transmission
lines.
The
fabrication
may
involve
lami¬
nating
several
board
layers
together
in
the
same
manner
anticipated
for
PCB
manufacture.
3.2
Identification
of
Test
Specimen
For
specimens
of
types
called
out
in
3.1.1
,
3.1
.2,
or
3.1
.3,
a
board
serial
num¬
ber,
part
number,
and
date
code
should
be
adequate.
Speci¬
mens
from
3.1
.4
should
include
whatever
lot
or
panel
identifi¬
cation
is
available
for
the
substrate
laminate
being
evaluated.
3.3
Conditioning
If
conditioning
is
required,
test
speci¬
mens
shall
be
stored
before
testing
at
23
(+1/-5)
[73.4
°F
(+
1
.8/-0
°F)]
and
50
%
RH
±
5
%
RH
for
no
less
than
16
hours.
If
a
different
conditioning
procedure
is
used,
it
must
be
specified
by
the
user.
4
Equipment
and
Instrumentation
The
TDR
measure¬
ment
system
contains
a
step
generator,
a
high-speed
sam¬
pling
oscilloscope,
and
all
the
necessary
accessories
for
con¬
necting
the
TDR
unit
to
the
device
under
test.
IPC-2141
provides
a
short
discussion
of
the
TDR
system
architecture,
system
considerations,
and
the
TDR
measurement
process.
Figure 4-1 Resolution and Electrical Length of Transmission Line
t
V
adequate resolution
t
V
inadequate resolution
2 T
p
transmission line
IPC-TM-650
Page 3 of 23
Number
2.5.5.7
Subject
Characteristic
Impedance
of
Lines
on
Printed
Boards
by
TDR
Date
03/04
Revision
A
1.1
Measurement
System
Requirements
4.1.1
Measurement
Accuracy
The
measurement
accu¬
racy
of
the
TDR
should
be
sufficient
to
provide
the
required
accuracy
in
the
value
of
characteristic
impedance.
The
required
measurement
accuracy
of
the
TDR
unit
will
depend
on
the
TDR
measurement
method.
In
general,
the
measure¬
ment
accuracy
of
the
TDR
unit
should
be
better
than
1
%
of
amplitude
(either
voltage
or
reflection
coefficient).
Noise
in
the
measured
values
will
affect
the
uncertainty
in
the
calculated
Zo
values.
The
value
of
Zo
may
be
affected
by
the
length
of
the
transmission
line
under
test
and
the
section
of
the
transmis¬
sion
line
from
which
Zo
is
calculated
(see
3.1.1.d).
4.1.2
Temporal/Spatial
Resolution
The
resolution
limit
of
a
given
TDR
unit
is
defined
as
that
particular
time
or
distance
wherein
two
discontinuities
or
changes
on
the
transmission
line
being
measured,
that
would
normally
be
individually
dis¬
cernable,
begin
to
merge
together
because
of
limited
TDR
system
bandwidth.
The
resolution
limit
is
specified
in
either
time
or
distance,
and
is
always
related
to
the
one-way
propa¬
gation
time
between
the
two
discontinuities,
TP
(see
Figure
4-1),
and
not
the
round
trip
propagation
time.
Per
this
definition,
the
resolution
limit
is:
a.
half
the
system
risetime,
tsys,
where
%ys
is
the
10
%
to
90
%
risetime
or
90
%
to
10
%
falltime
(depending
on
whether
the
TDR
response
is
calibrated
with
a
short
or
open
circuit),
or
b.
0.5
kys
x
%,
where
vp
is
the
signal
propagation
velocity
in
the
transmission
line
being
measured.
These
definitions
are
complementary.
For
a
given
length
of
transmission
line
to
be
measured,
the
resolution
should
not
exceed
one
fourth
(0.25)
of
the
available
length,
Ltl
of
the
transmission
line.
Table
4-I
provides
examples
of
required
resolution
for
typical
surface
microstrips
in
air,
and
on
FR4
circuit
board
(%
=
2x1
08
m/s),
for
a
given
TDR
system
risetime.
IPC-2257a-4-1
Table 4-I Resolution of TDR Systems
TDR
System
Risetime Resolution 4X Resolution
Figure 4-2 Potential TDR Step Aberrations
overshoot
undershoot
ringing
low frequency drift
IPC-TM-650
Page 4 of 23
Number
2.5.5.7
Subject
Characteristic
Impedance
of
Lines
on
Printed
Boards
by
TDR
Date
03/04
Revision
A
10
ps
5
ps
/
1
mm
[0.04
in]
4
mm
[0.16
in]
20
ps
1
0
ps
/
2
mm
[0.08
in]
8
mm
[0.31
in]
30
ps
1
5
ps
/
3
mm
[0.12
in]
12
mm
[0.47
in]
100
ps
50
ps/
1
0
mm
[0.39
in]
40
mm
[1
.57
in]
200
ps
1
00
ps
/
20
mm
[0.79
in]
80
mm
[3.15
in]
500
ps
250
ps
/
50
mm
[1
.97in]
200
mm
[7.87
in]
Intermediate
values
can
be
linearly
interpolated
from
Table
4-1
or
using:
法
-三0
For
example,
if
a
32
mm
[1
.26
in]
long
transmission
line
was
being
measured,
a
TDR
system
with
tsys
80
ps
should
be
used.
Note
that,
if
the
probe
launch
caused
excessive
ringing
in
the
TDR
waveform,
or
if
the
launch
does
not
repeatably
replicate
the
connection
to
the
standard,
then
the
0.25
factor
may
need
to
be
smaller.
4.2
TDR
Requirements
4.2.1
Impedance
The
impedance
of
the
TDR
unit
should
be
50
Q
with
an
impedance
uncertainty
less
than
or
equal
to
土
0.5
Q.
This
TDR
impedance
value
is
selected
because
it
is
the
impedance
used
by
most
high-speed/high-frequency
test
instrumentation
and
compatibility
with
this
instrumentation
is
necessary
for
characterizing
the
dynamic
TDR
properties,
such
as
its
impulse
response
(or
transfer
function).
The
imped¬
ance
of
the
TDR
unit
should
be
calibrated
using
an
artifact
standard,
such
as
an
air
line
(see
4.3.6).
However,
the
TDR
impedance
is
a
function
of
frequency
and
calibration
using
a
fixed
region
of
the
TDR
waveform
(the
measurement
zone)
will
only
yield
an
average
impedance
value
for
the
TDR
unit
for
the
corresponding
frequency
range.
4.2.2
Timebase
Accuracy
The
horizontal
timebase
accu¬
racy
defines
how
well
the
TDR
instrument's
horizontal
time
scale
can
display
the
correct
length
of
the
trace.
This
affects
both
the
accuracy
of
the
measurement
zone
calculations
and
any
propagation
delay
values.
The
timebase
accuracy
should
be
less
than
0.25
tsys
(see
also
4.1.2).
4.2.3
Step
Aberrations
The
ability
of
the
TDR
instrument
to
measure
the
impedance
of
a
transmission
line
is
related
to
how
well
the
instrument
can
generate
a
step-pulse
with
a
minimum
of
aberrations
(ringing,
overshoot,
undershoot,
set¬
tling,
etc.).
Any
ringing,
overshoots,
or
undershoots
will
cause
corresponding
aberrations
in
the
TDR
waveform
(see
Figure
4-2).
These
aberrations
can
cause
significant
errors
in
the
impedance
value
computed
from
the
TDR
waveform.
Addi¬
tionally,
low
frequency
step
aberrations
may
produce
a
ramp
in
measurement
zone.
This
ramp
can
cause
a
significant
bias
in
the
computed
impedance
value.
The
TDR
instruments
step
aberrations
should
be
less
than
1
%
of
the
total
step
ampli¬
tude.
For
example,
the
impedance
error
shown
in
Table
4-2
is
for
a
1
mV
error
of
a
250
mV
step.
Poor
settling
and
large