IPC-TM-650 EN 2022 试验方法--.pdf - 第420页

5.1.2 Pre-M easurement Checks 5.1.2.1 Instr ument W arm-Up and Stability Before per- forming delay measurements, the user ensure adequate in st ru me nt wa rm- up ti me a s s pec ifi ed by t he in st ru men t manufacture…

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

5.2.1.1 Establishing the Electrical Length of the Mea-

surement System

To assist in tracking the repeatability and

suitability of the measurement system, the method includes

the following procedure to determine the electrical length of

the TDR measurement system from the TDR sampler to the

end of the probe tip. The user may record this time value for

a given set-up over subsequent measurement sessions in

order to verify consistency in system performance over long

time periods.

Turn on the TDR source and enable triggering.

Hold the probe in the air away from other objects

and surfaces and set the waveform epoch to include both the

incident signal from the TDR source and the superimposed

reflection signal from the probe tip (see Figure 5-1).

For this epoch, adjust the number of sample points

to achieve a sample density of no less than 2 S/ps. For

example, this is achieved with a time record of 4,000 sample

points and a waveform epoch that is 2,000 ps long (10 divi-

sion x 200 ps/div).

Identify the arrival time t

inc

of the incident pulse edge

as 50% of the incident amplitude. For step signals, use the

difference of average pre- and post-step voltage levels to

establish the incident amplitude.

Identify the arrival time t

refl

of the reflection pulse

edge as 50% of the reflection amplitude. For step signals, use

the difference of average pre- and post-step voltage levels to

establish the reflection amplitude (this is often near the region

of maximum dV/dt.)

Record the system’s electrical length as one half of

the round-trip time: t

syslen

= (t

ref

- t

inc

)/2.

IPC-25511-5-1

(Note: The optional static isolation unit (SIU) is a protection device designed to eliminate static discharge damage to the TDR sampling

head.)

Number

2.5.5.11

Subject

Propagation Delay of Lines on Printed Boards by TDR

Date

04/2009

Revision

IPC-TM-650

Step

Page