IPC-TM-650 EN 2022 试验方法.pdf - 第509页

3.14 Additional Guidelines for Testing Panel Coupons Test interconnects may be contained within one or more panel coupons. It is recommended that there be at least one coupon per PB on the panel as long as it does not ad…

interconnects, but a practical issue of operator ability to use

hand-held probes may be considered. Test reports must

report any deviation from the nominal contact land and PTH

geometry.

3.9 Contact Land Pitch Whenever possible, the center-

to-center distance between the signal and reference lands

of the test interconnect should be consistent to simplify

probing requirements and ensure measurement repeatability

and reproducibility. Nominal center-to-center pitch shall be

1.91 mm [0.075 in]. The use of different contact and probe

pitches must be specified and documented.

3.10 Single-Signal Conductor Transmission Line The

single-signal conductor transmission line is also known as the

single-ended, unbalanced, and asymmetrical structure. The

probing area for these lines should consist of a contact land

(see Figure 3-2) for each signal line. The contact land should

provide connection to the reference, or ground, connection for

the test structure. This method requires the use of one con-

tact pitch to ensure measurement consistency between the

test structures of the specimen.

3.11 Orientation The contact land orientation (placement

and angle of the contact land of the signal line relative to the

contact land of the reference plane) must be the same for all

test interconnects of the specimen in order to ensure mea-

surement consistency between test interconnects.

3.12 Test Interconnect Routing

a. The test interconnects shall only be routed over and under

contiguous ground and voltage planes following controlled

line impedance guidelines (see IPC-2141). The test inter-

connects must not extend into PTH clearance areas.

b. The test interconnects shall be kept at least six times the

width of the signal conductor or 2.5 mm [0.0984 in],

whichever is greater, from any PTHs and any other inter-

connect on the same plane. All conductive material (such

as copper nomenclature, copper thieving, etc.) shall be

kept at least 2.5 mm [0.0984 in] from each test intercon-

nect.

c. Test interconnects shall be straight or contain gradual and

rounded bends.

3.13 Nomenclature Labeling of all test interconnect con-

tact lands on at least one surface layer is required for opera-

tor identification during manual probing operations. The label

shall minimally contain information about which signal layer

the test interconnect is modeling (for example, L1-3in, L1-6in,

etc.). Nomenclature should be etched in copper and be

spaced a minimum of at least six times the width of the signal

conductor (of the test interconnect) or 2.5 mm [0.0984 in],

whichever is greater, from the test interconnect area. When-

ever practical, the terminations at both ends of test intercon-

nect shall be marked.

IPC-25511-3-2

Figure 3-2 Transmission Line Structures

2.54 mm

[0.100 in] MIN

2.54 mm [0.100in] MIN

2.54 mm [0.100 in] MIN

1.91 mm [0.075 in]

152.4 mm [6.00 in] REF

76.2 mm [3.00 in] REF

1.91 mm [0.075 in]

L1/3.0 Layer One – 3 Inch Line

L1/6.0 Layer One – 6 Inch Line

L3/3.0 Layer Three – 3 Inch Line

L3/6.0 Layer Three – 6 Inch Line

L1/3.0

L1/6.0

L3/3.0

L3/6.0

L7/3.0

L7/6.0

THIEVING

NOTES:

PN#. LOT#.

SN#.

IPC-TM-650

Number

2.5.5.11

Subject

Propagation Delay of Lines on Printed Boards by TDR

Date

04/2009

Revision

Page4of16

3.14 Additional Guidelines for Testing Panel Coupons

Test interconnects may be contained within one or more

panel coupons. It is recommended that there be at least one

coupon per PB on the panel as long as it does not adversely

affect panel utilization. With this configuration, the following

additional design guidelines apply. More than one coupon

may be necessary on a PB to ensure uniformity. Also, more

test interconnects may be required than can fit inside one test

coupon. In that case, more than one test coupon is neces-

sary.

3.14.1 Reference and Ground Planes All reference

planes existing in the coupon are to be connected together

within the coupon area and be electrically independent of con-

ductor planes in the functional circuit area.

3.14.2 Surface Condition The panel test coupons shall

have the same surface plating and use the same solder mask

requirements as the functional PB.

3.14.3 Thieving Differences in circuit density between the

inside of a panel coupon and the functional area may produce

surface plating and etching differences. In order to compen-

sate for these differences, thieving (the use of nonterminated

copper structures, such as planes, pads, and/or traces adja-

cent to test lines) may be used. All thieving structures shall be

kept at least six times the width of the signal conductor (of the

test interconnect) or 2.5 mm [0.0984 in], whichever is greater,

from each test interconnect.

4 Apparatus 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 test structures under test. IPC-

2141 provides a discussion of the TDR system architecture,

system considerations, and the TDR measurement processes

used herein.

4.1 Measurement System Requirements

4.1.1 Voltage Measurement Accuracy

The voltage mea-

surement accuracy and linearity of the TDR sampling oscillo-

scope shall be sufficient to provide the required accuracy in

the value of propagation delay. Nominally, the voltage mea-

surement accuracy should be better than ± 1%.

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, timing jitter, or a reduced signal-to-noise

ratio. The resolution limit is specified in either time or distance,

and is always related to the one-way propagation time

between the two discontinuities t

(see Figure 4-1), and not

the round trip propagation time 2 t

Per this definition, the temporal resolution limit is:

a. one half of the system risetime, that is 0.5 t

sys

, where t

sys

is the 10 to 90% risetime or 90% to 10% falltime depend-

ing on the propagating edge of the TDR signal;

and the spatial resolution limit is:

b. 0.5 t

sys

, where v

is the signal propagation velocity in

the transmission line being measured.

For a given length of transmission line to be measured, the

resulting spatial resolution of the TDR measurement set-up

should not exceed one fourth (0.25) of the available length of

the transmission line L

. In other words, L

should be at

least four times the spatial resolution of the measurement sys-

tem.

Table 4-I relates TDR system risetime values to minimum

for typical surface microstrip lines in air on FR4 PB mate-

rial (v

≈ 2x10

m/s).

IPC-25511-4-1

Figure 4-1 Resolution and Electrical Length of

Transmission Line

transmission line

adequate resolution

inadequate resolution

2 t

IPC-TM-650

Number

2.5.5.11

Subject

Propagation Delay of Lines on Printed Boards by TDR

Date

04/2009

Revision

Page5of16

Intermediate values can be linearly interpolated from Table 4-1

or using:

sys

≤

For example, if the test structure was a 32.0 mm [1.26 in] long

transmission line, then a TDR system with t

sys

≤ 80 ps must

be used. Note that, if the probe launch and test set-up cables

cause excessive ringing in the TDR waveform, or if the vari-

ance in connection delay is significant, then t

sys

must be made

sufficiently small to clearly observe the desired discontinuities

in the TDR waveforms.

4.2 TDR Requirements

4.2.1 Impedance

The TDR source and measurement ports

shall be electrically terminated with precision 50 Ω loads. This

is normally the case with high-quality TDR instrumentation

maintained on the manufacturer’s maintenance and calibra-

tion schedules.

4.2.2 Voltage Step Repeatability For all passive electrical

terminations, the TDR source shall repeat its voltage wave-

form to within 0.5% of the TDR pulse amplitude V

step

4.2.3 Timebase Accuracy When oscilloscopes are used

in the TDR measurement system, errors in the reported time

of the samples may arise due to imperfections in the counters

and clock sources used to establish the timebase. These are

systematic errors and may depend on the exact time/div and

delay settings of the scope. When applying this method, the

TDR system’s timebase accuracy must be better than 8 ps +

0.01% of the measured interval.

4.2.4 Timebase Repeatability (Jitter) The RMS value of

random timing uncertainty in measured voltage samples shall

be less than 10% of t

sys

4.2.5 Waveform Averaging The TDR equipment shall

perform waveform or sample averaging to reduce jitter and

electrical noise effects in the recorded waveform measure-

ments.

4.2.6 Step Aberrations The TDR source waveform aber-

rations shall be less than 1% of the total step amplitude V

step

The ability of the TDR instrument to measure transmission line

discontinuities is related to how well the instrument can mini-

mize aberrations (ringing, overshoot, undershoot, settling,

etc.). These aberrations (see Figure 4-2) can cause significant

errors in determining the instant that the waveform crosses a

user-defined voltage value. Additionally, low frequency step

aberrations may produce a ramp in measurement zone and

this can cause a significant bias in the computed propagation

delay value.

4.3 Other Equipment Requirements

4.3.1 Connectors

Propagation delay test set-ups shall

use precision coaxial connectors whenever possible. TDR

systems typically come with SMA, 3.5 mm [0.138 in],

2.92 mm [0.115 in], or 2.4 mm [0.094 in] connectors at their

measurement ports. These connectors are all 50 Ω connec-

tors. They are precision connectors (they have a low imped-

ance uncertainty due to their mechanical precision) whose

bandwidth must be great enough so that the connectors do

not limit the accuracy of the TDR measurement. The useable

bandwidth of these connectors are approximately 33 GHz,

40 GHz, and 50 GHz, respectively. The reflection and insertion

losses of all connectors used in the test set up shall be less

than 27 dB and 0.3 dB, respectively. Other connectors with

comparable or better performance may be used, but must be

specified and documented. All coaxial connections shall be

tightened with a calibrated torque wrench to specification of

Table 4-1 Resolution of TDR Systems

TDR

System

Risetime Resolution

Minimum L

4x Resolution

10 ps 5 ps / 1.0 mm [0.04 in] 4.0 mm [0.16 in]

20 ps 10 ps / 2.0 mm [0.08 in] 8.0 mm [0.31 in]

30 ps 15 ps / 3.0 mm [0.12 in] 12.0 mm [0.47 in]

100 ps 50 ps / 10.0 mm [0.39 in] 40.0 mm [1.57 in]

200 ps 100 ps / 20.0 mm [0.79 in] 80.0 mm [3.15 in]

500 ps 250 ps / 50.0 mm [1.97 in] 200.0 mm [7.87 in]

IPC-25511-4-2

Figure 4-2 Potential TDR Step Aberrations

overshoot

undershoot

ringing

low frequency drift

IPC-TM-650

Number

2.5.5.11

Subject

Propagation Delay of Lines on Printed Boards by TDR

Date

04/2009

Revision

Page6of16