IPC-TM-650 EN 2022 试验方法.pdf - 第511页

the connector. Table 4-3 provides the typical connector torque specifications. 4.3.2 Cabling All test cables shall be high-quality, low- phase delay coax and with a nominal characteristic imped- ance of 50 Ω . Cables use…

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

the connector. Table 4-3 provides the typical connector

torque specifications.

4.3.2 Cabling All test cables shall be high-quality, low-

phase delay coax and with a nominal characteristic imped-

ance of 50 Ω. Cables used in the measurement circuit of the

transmission line under test shall have connectors that are

compatible with the instrument and probes. The bandwidth of

the cable must be great enough so that the cable does not

limit the accuracy of the propagation delay measurement. The

length of the cables should be kept to a minimum. The total

insertion loss (including connector loss) of the cabling con-

necting the transmission line under test to the TDR should be

kept to less than 3.3 dB/m (1db/foot) at 26.5 GHz. Table 4-4

contains suggested maximum cable lengths for the TDR test

set up as depicted in Figure 5-1 and described in 5.2.

4.3.3 Probes The probe assembly characteristic imped-

ance shall either be 50 Ω or the same value as the charac-

teristic impedance of the transmission line under test, with an

uncertainty of ± 1.0 Ω or less. The probe tips should be of

sufficient diameter and pitch (spacing between signal and

ground tips) to provide accurate and repeatable connections

to the desired probe contact pad geometry (see IPC-2141 for

additional recommendations on probe landing layouts for TDR

coupons). Single-ended probes should contain two electrode

tips, one each for the signal and ground lines. The probe tips

should have moderately sharp edges to cut through any

oxides. The probe bandwidth should be sufficient for the

desired temporal/spatial resolution (see 4.1.2). The probe

response time should be sufficiently short so as not to

increase the duration of the measurement period. The overall

performance of the probe can be incorporated into the TDR

system response for computing TDR system temporal/spatial

resolution (see 4.1.2). Inconsistent probe force and placement

is common and can cause a significant yet unknown error in

. Probe connections to the measurement system cables

should be tightened with a torque wrench following the con-

nector specifications. For hand held probe assemblies, the

probe handle should be ergonomically shaped.

4.3.4 Terminations TDR sources are not perfect voltage

source generators; they may perform differently under differ-

ent electrical load conditions. Therefore, the termination con-

ditions of any verification experiments should match those of

the interconnection test structures, and all test structures in a

given specimen should be of the same design. For example, if

the propagation delay test is to be performed on lines that are

electrically open at their far end, all lines should be terminated

in electrically open circuits, and any TDR field verification tests

(see 5.2.1.2) should be made using open circuit terminations.

4.3.5 ESD Protection Static build up on specimens and

test cables prior to test can damage the signal samplers in the

TDR equipment; ESD protection and transmission line dis-

charging procedures must be used. ESD protection can be

supplied internally to the TDR system or externally using a

Static Isolation Unit (SIU). If supplied externally using a coaxial

switch (as shown in Figure 5-1), the switch should be placed

between the transmission line under test and the TDR instru-

mentation. The SIU should have a return loss and insertion

loss less than 16 dB and 0.3 dB, respectively, at 18 GHz. A

maximum of 30.0 cm [11.8 in] of high quality, high frequency

cable may be used to connect the TDR instrument to the SIU

protection switch. Test interconnections should be first

grounded with the SIU and/or passed through some type of

deionization device prior to testing to remove any residual

static electrical charge. Use of proper ESD control methods,

control components and humidity control will help reduce

electrostatic discharge damage to the measurement system.

Automation software can be used to enhance the effective-

ness of the static isolation unit by switching the static isolation

unit on/off as required to minimize the amount of time that the

TDR sampling unit is exposed to potential ESD.

4.3.6 Transfer Standard The TDR measurement system

(see Figure 5-1) specified for measuring propagation delay

requires a precision coaxial transmission line to set the refer-

ence impedance of the reflectometer measurements. This

standard shall be a rigid, or semi-rigid, cable not more than

10 cm long with a uniform impedance profile along its length.

Table 4-3 Connector Torque Speciﬁcations

[The conversion factor is 0.1128 N-m/(lb-in)]

Connector Type Required Torque

SMA 0.56 N-m (5 lb-in)

3.5, 2.92, and 2.4 mm 0.90 N-m (8 lb-in)

Table 4-4 Maximum Suggested Cable Lengths

for TDR System (As Depicted in Figure 5-1)

TDR Cable Assembly TDR Cable Length

Sampling Unit to Static

Isolation Unit

30.0 cm [11.8 in]

Static Isolation Unit to In-Line

Secondary Standard

91.0 cm [35.83 in]

Transfer Standard (such as

semi-rigid coaxial cable)

10.0 cm [3.94 in]

IPC-TM-650

Number

2.5.5.11

Subject

Propagation Delay of Lines on Printed Boards by TDR

Date

04/2009

Revision

Page7of16

The transfer standard shall have precision coax connectors

that match the test cables and probes. The uncertainty in the

nominal characteristic impedance of the transfer standards

shall be less than or equal to ± 0.015 Z

ref

, where Z

ref

is the

characteristic impedance of the transfer standard (nominally

50 Ω.)

4.3.7 Check Standards The method makes use of two

precision coaxial air lines of two different lengths to verify the

operation of a test set-up (see 5.2.1.2). The air lines are pre-

cision coaxial lines where the center conductors are held in

place with an isolation bead or the center pins of the end

connectors, and are not filled with any other dielectric mate-

rial. The coaxial air lines serve as a precise delay standard that

can be measured during field checks (see 5.2.1.2) to verify the

measurement set-up. The coaxial air line standards are avail-

able commercially with any of the precision coaxial connec-

tors. Probe contact to coaxial transitions must be fabricated

to use with a given probe tip configuration.

5 Procedures In TDR, the observed voltage waveform is

the sum of incident and reflected signals. The reflections are

related to the difference between the characteristic imped-

ance Z

of a transmission line and any impedance discontinui-

ties along the transmission line or at its end.

The method procedures establish the means of determining a

time delay per unit length t

from TDR measurements of two

transmission lines that differ in length. The transmission lines

are the interconnect test structures fabricated in PB materials

as specified. The far end of the transmission line is either

electrically open- or short-circuited in order to create a clearly

observable reflection feature in the measured TDR waveform.

The procedures in this section establish the propagation delay

per unit length as the differential propagation time obtained

using the TDR measurements of two interconnect test lines

divided by the length of the same interconnects:

= t

/2L

Here, t

is the measured propagation time difference given by

− t

where t

is the round-trip propagation time for the first trans-

mission line and t

is the round-trip propagation time of the

second transmission line.

is the propagation length difference of the transmission line

pair given by

− L

where L

is the length of the first transmission line and L

the length of the second transmission line.

5.1 Measurement Preliminaries This section provides

common considerations for the calibration and initial configu-

ration of the TDR measurement system, and the method to

establish the waveform epoch (time window) used in the delay

measurements (see 5.2 and 5.3).

5.1.1 System Calibrations

5.1.1.1 Manufacturer Calibrations

The TDR oscilloscope

or other TDR equipment used shall be calibrated and ser-

viced following the recommended schedule of the instrument

manufacturers.

5.1.1.2 Field Calibrations Manufacturer ecommended

field calibrations shall be performed in addition to scheduled

factory calibrations. TDR system field calibrations shall be

performed at the frequency recommended by the instrument

manufacturers and after a change of any system component,

such as a sampler of TDR source unit. The user must ensure

adequate system warm-up time before performing field cali-

brations, as specified by the instrument manufacturers.

Users-accessible field calibrations for TDR oscilloscopes may

include the application of an internal voltage calibration for

each sampler and TDR source. Though not required for this

method, TDR field calibrations may also include a reflection

coefficient or impedance normalization/calibration procedure

where standards are connected to the instrument’s test port

following a menu-driven procedure. Field calibrations are

required for the following reasons:

a. TDR instrument specifications vary with temperature

b. TDR instrument specifications vary with time (drift)

c. TDR instrument specifications vary due to minor ESD dam-

age

d. TDR instrument factory calibration usually does not include

user supplied auxiliary components (i.e., cables, probes,

etc.)

IPC-TM-650

Number

2.5.5.11

Subject

Propagation Delay of Lines on Printed Boards by TDR

Date

04/2009

Revision

Page8of16