IPC-TM-650 EN 2022 试验方法--.pdf - 第512页

The t ransfer st andard have precision coax connec tors that match the test cables and probes. T he uncertainty in the nominal characteristic impedance of the transfer standards be less than or equal to ± 0.015 Z ref , w…

the connector. Table 4-3 provides the typical connector

torque specifications.

4.3.2 Cabling

All test cables 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

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

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

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

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

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

SMA 0.56 N-m (5 lb-in)

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

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]

Number

2.5.5.11

Subject

Propagation Delay of Lines on Printed Boards by TDR

Date

04/2009

Revision

IPC-TM-650

―

Table

4-3

Connector

Torque

Specifications

Connector

Type

Required

Torque

Table

4-4

Maximum

Suggested

Cable

Lengths

for

TDR

System

(As

Depicted

Figure

5-1)

TDR

Cable

Assembly

TDR

Cable

Length

shall

Page

The transfer standard have precision coax connectors

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

nominal characteristic impedance of the transfer standards

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

be calibrated and ser-

viced following the recommended schedule of the instrument

manufacturers.

5.1.1.2 Field Calibrations

Manufacturer ecommended

field calibrations

be performed in addition to scheduled

factory calibrations. TDR system field calibrations

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.)

Number

2.5.5.11

Subject

Propagation Delay of Lines on Printed Boards by TDR

Date

04/2009

Revision

IPC-TM-650

shall

Page

5.1.2 Pre-Measurement Checks

5.1.2.1 Instrument Warm-Up and Stability

Before per-

forming delay measurements, the user

ensure adequate

instrument warm-up time as specified by the instrument

manufacturers, and ensure that the TDR waveform is not drift-

ing in amplitude or time.

5.1.2.2 Environmental Conditions

The user ensure

that the temperature and humidity of the test environment is

within TDR instrument specifications and that the conditions

will be stable for the duration of the measurements. If the test

environment is substantially different than that used for speci-

men conditioning (see 3.3), the user document this in

the test reports.

5.1.2.3 Test Structure Isolation

The user ensure

that the signal line and reference planes of the test structures

are located an adequate distance from objects and surfaces

(such as the work surface of a test bench) that could electri-

cally couple or interact with the test structure and probes. If

surface layer microstrip lines are used, the recommendation is

to keep extraneous objects and surfaces at least 6 w from the

test coupon or PB, were w is the width of the signal line. If the

tests are being conducted with hand probes, care must be

taken to ensure that the hands and arms of the operator do

not come in close proximity to the coupon or PB being tested.

Any fixtures used to ensure electrical isolation of the test fix-

tures must also be sufficiently strong to accommodate the

probing force required for repeatable electrical connections.

5.1.3 Suitable Waveform Epochs

The waveform epoch is

the measurement interval over which the propagation time for

a given discontinuity will be computed. The time epoch may

be described in terms of the TDR instrument parameters delay

and time per division. The user

ensure that the instru-

ment settings can be adjusted so the waveform epochs can

contain the arrival of the far end reflection signals of both the

shorter line and the longer line in the test structure; the user

must ensure an epoch includes the reflection signal and suffi-

cient pre- and post-waveform data to establish the required

reference amplitude levels; and the user

ensure that the

delay and time/div settings can re-adjusted to repeat the

desired epochs. This requires probing both test structures

using the TDR measurement set-up (similar to that depicted in

Figure 5-1). As shown in Figure 5-1, the user may first find the

arrival point of the reflection signal for the open-circuit probe

to help locate the subsequent reflection signal of the intercon-

nection test structure.

5.1.4 Suitable Amplitude Resolution

In order to com-

pute propagation delay, this method requires the recording of

the instants when the TDR waveform crosses a specified volt-

age reference level. The reference level, V

REF

, is given gener-

ally by:

REF

= xV

refl

+ V

off,refl

where V

refi

is the amplitude of the reflected pulse (measured

when it is superimposed on an incident step pulse), x is the

fraction of V

refl

used to determine the transition instant (for

example, x = 0.5 corresponds to the 50% reflection amplitude

value), and V

off, refl

is the amplitude of an incident TDR step

pulse.

This method specifies two possible values for x:

= 0.05

50%

= 0.50

The method also allows the user to specify their own x as long

as the same value of x is used in all delay measurements and

verification field tests. The user must document which value of

x is used in the test reports.

The user

ensure that the TDR equipment amplitude set-

tings can be adjusted to capture the reference level V

REF

with

sufficient resolution to minimize errors in recording time of the

crossing instant.

5.2 Propagation Delay TDR Measurement Procedures

This section contains the methods for measuring the propa-

gation delay of single-ended transmission lines. The following

steps should be used when the interconnect test structures

under test are unbalanced (single-ended) transmission lines.

This process can be followed or automated (recommended).

Additionally, the use of quality fixtures based or robotic prob-

ing systems may reduce probe placement uncertainty com-

pared to hand probe techniques of certain users.

5.2.1 Multiple Line Method

To mitigate the effects of

imperfect measurement system cables, probes, and contact

pad discontinuities, the propagation delay measurements are

defined using the ratio of differences of two measurements

made on separate lines that are very similar except for their

physical length. Therefore, the procedure requires careful and

repeatable connections and measurements of TDR waveform

from two lines of the interconnection test structure.

Number

2.5.5.11

Subject

Propagation Delay of Lines on Printed Boards by TDR

Date

04/2009

Revision

IPC-TM-650

shall

Page