IPC-TM-650 EN 2022 试验方法.pdf - 第489页

set. Three times the standard deviation around each side of the mode is the repeatability. The ability to resolve a measurement value is fundamental to the accuracy of any measurement process. The TDR instru- ment should…

odd,Ch2

(

inc,Ch2

− V

r,TL,Ch2

inc,Ch2

+ V

r,TL,Ch2

)

TS,Ch2

where:

TS,Ch1

the characteristic impedance of the transfer stan-

dard connected to Ch1 as determined in Step 3 of 5.3.3,

TS,Ch2

the characteristic impedance of the transfer stan-

dard connected to Ch2 as determined in Step 3 of 5.3.3,

r,TL,Ch1

the reflected voltage value from Ch1 as calculated

in Step 1,

r,TL,Ch2

the reflected voltage value from Ch2 as calculated

in Step 1,

inc,Ch1

the voltage amplitude computed for Ch1 in Step 1

of 5.3.3, and

inc,Ch2

the voltage amplitude computed for Ch2 in Step 1

of 5.3.3.

Step

3 –

Calculate

the differential impedance, Z

TL,diff

, of

the

transmission line under test using:

TL,diff

= Z

odd,Ch1

+ Z

odd,Ch2

Special Considerations and Notes

6.1 General

6.1.1 Quality Control

Measurements

for manufacturing

control are performed to identify and correct process or mate-

rials problems occurring during a manufacturing run, as well

as to assure that a product will perform electrically as

designed. To facilitate the large number of measurements

required in a production environment and to maximize mea-

surement repeatability and reproducibility between different

operators and test systems, it is particularly useful to auto-

mate the TDR calibration and measurement by using com-

puter control. This can be easily achieved using a personal

computer and suitable equipment as described in Section 4.

Examples of parameter variations detectable by TDR, and that

are evidence of process or materials problems, include the

following:

a. Over/under-retching (line width problems).

b. Over/under-plating (line width and thickness problems).

c. Permittivity of the dielectric.

d. Thickness of the dielectric.

e. Degradation from excessive heating and humidity.

f. Damage from excessive pressure during the multilayer pro-

cess.

g. Variations in the laminate glass-to-resin content.

h. Variations in additional coatings applied to the board sur-

face, e.g., solder mask.

Measurement repeatability is described in IPC-TM-650 Test

Method 1.9, Measurement Precision Estimation for Variables

Data. This test method also describes a process to evaluate

the reproducibility of a measurement system for multiple

operators, on different days, and when using different instru-

ments. This evaluation process should be followed and a

precision-to-tolerance ratio acceptable to the customer

obtained.

6.1.2

Single-Ended and Differential Lines

Increased

performance

requirements for computer and other electronic

products often demand even greater signal fidelity, time pre-

cision, and noise immunity, than can be obtained with a

single-ended transmission line. A single-ended transmission

line is a transmission line design consisting of a single signal

conductor placed over one ground plane, as in microstrip, or

between two ground planes, as in stripline. Single-ended lines

may be called unbalanced transmission lines. Differential lines

are used to increase signal fidelity with improved time preci-

sion and increased noise immunity. Differential lines may also

be called balanced or coupled transmission lines. The

required TDR method is different for differential lines.

6.1.3

Environmental Factors

Temperature

and humidity

should be monitored during the test. Long exposures to tem-

peratures and humidities other than standard laboratory con-

ditions (temperature range of 20 °C to 23 °C [68 °F to 73.4 °F]

and relative humidity range of 35 % to 65 %) can impact the

dielectric properties of the materials in the test objects. Fur-

thermore, the electrical characteristics of the TDR, such as

sampler gain, are temperature dependent. Therefore, for the

most repeatable measurements, the TDR instrumentation

should be maintained within the manufacturer recommended

temperature and humidity ranges. Low relative humidity may

result in electrostatic discharge damage to the TDR unit.

6.1.4

Measurement Accuracy and Repeatability

Accu-

racy

and repeatability depend on the impedance of the line

being measured, the type and condition of probes, cables,

sampling head, and the experience of the test technician.

Accuracy is the difference between the most likely measure-

ment and the defined standard. The most likely measurement

is also called the mode of all measurements within a sample

IPC-TM-650

Number

2.5.5.7

Subject

Characteristic

Impedance of Lines on Printed Boards by TDR

Date

03/04

Revision

Page

20 of 23

电子技术应用　　　　　　　ｗｗｗ．ＣｈｉｎａＡＥＴ．ｃｏｍ

set.

Three times the standard deviation around each side of

the mode is the repeatability.

The ability to resolve a measurement value is fundamental to

the accuracy of any measurement process. The TDR instru-

ment should have sufficient measurement resolution to facili-

tate the accuracy requirements of the measurement method

described herein. The total risetime of the TDR system (includ-

ing cables, probes, etc.) and step aberrations define the

impedance resolution (see 4.1.2).

6.1.5

General Cautionary Statement

TDR

test systems

and associated accessories are precision high frequency

devices. Most TDRs include hardware to protect the static-

sensitive sampling heads. However, operators and mainte-

nance staff should take proper ESD precautions (see manu-

facturer’s recommendations). High frequency cables, because

they typically use solid center conductors, are not as flexible

as typical coaxial cable. Consequently, care should be taken

not to excessively bend and flex the high frequency cables.

The probes used in TDR systems typically use spring-loaded

contacting mechanisms and these should be checked peri-

odically to ensure proper operation. Statistical process control

methods and control charts can provide useful information

regarding the condition of the TDR system and its associated

accessories.

6.1.6

TDR Measured Values

The

units of the values out-

put by the TDR system may be in voltage, reflection coefficient

(commonly called ‘‘rho’’ for the Greek character, ρ, represent-

ing it), and impedance.

6.1.6.1

Impedance

the TDR system provides impedance

values directly, no further computation is required to obtain

the characteristic impedance of the transmission line under

test.

6.1.6.2

Reflection coefficient, ρ

the TDR unit provides

its output in terms of ρ, then the characteristic impedance of

the transmission line under test must be computed from ρ.

6.1.6.3

Voltage

the TDR unit provides its output in term

of voltages, these voltages must first be used to compute the

amplitude of the incident and reflected pulses. Note, all volt-

ages values measured in the test procedures are that of static

voltage levels. These voltage levels are used to compute pulse

amplitudes. The pulse amplitudes, in terms of voltage, are

then used to compute the reflection coefficient of the trans-

mission line under test relative to the TDR, as shown in the

test methods, and these reflection coefficients are then used

to determine the characteristic impedance of the transmission

line under test.

6.2

Calibration

6.2.1 System Verification

The

use of test reference speci-

mens corresponding to different impedance values, for

example 28 Ω,50Ω, and 100 Ω for single-ended transmis-

sion lines and 100 Ω for differential transmission lines, should

be measured according to the user-defined sampling plan

and compared to impedance control limits to ensure the sys-

tem is functioning correctly.

6.2.2

Calibration Artifacts

Air

line standards should be

checked for mechanical tolerances or replaced at regular

intervals. They should be handled with care. Worn out stan-

dards can cause a significant but unknown error than can

exceed 2 Ω. The air line should be compared to another air

line periodically to verify the air line in use has not been dam-

aged. The airline should also be calibrated and documented

periodically (not less than once every two years) by a qualified

certification laboratory and kept in an environment safe from

mechanical shocks, dust and dirt. Dust and dirt degrade the

fine threads of the connection and damage the electrical mat-

ing surfaces. Also, some TDR equipment manufacturers have

requirements for the minimum length of the airlines they rec-

ommend for calibration and standardization. Check with the

manufacturer regarding their calibration requirements. For dif-

ferential impedance of 100 Ω, each channel can be checked

with a 50 Ω airline.

Ideally, the effects of material properties of the air line should

be included in the calculation of the air line impedance

because some of the corresponding transmission line proper-

ties, such as conductor resistance, will be frequency depen-

dent. Also, beadless air lines should be used because their

geometries can be readily measured whereas the geometries

of beaded air lines are more difficult to measure.

6.3

Measurement System

6.3.1 Bandwidth/Risetime Resolution

The

frequency

components of the TDR step are approximately related to the

bandwidth by:

−3dB

≈

0.35

IPC-TM-650

Number

2.5.5.7

Subject

Characteristic

Impedance of Lines on Printed Boards by TDR

Date

03/04

Revision

Page

21 of 23

电子技术应用　　　　　　　ｗｗｗ．ＣｈｉｎａＡＥＴ．ｃｏｍ

where:

-3dB

the 3 dB attenuation bandwidth and t

the 10 %

to 90 % transition duration of the TDR step response.

Note that this relationship may not accurately represent the

intended operational frequencies of the transmission line

being tested. The bandwidth and risetime characteristics must

be adequate to ensure that the TDR can provide a measure-

ment zone long enough to accurately determine Z

for

transmission line of a given length. This constant-valued

region corresponds to the round-trip propagation time of the

TDR step on the transmission line being tested. If the TDR

transition duration is too long relative to this propagation time,

the constant-valued region (also called the measurement

zone) will be very short thereby increasing measurement error

and uncertainty. Risetime considerations, however, are not

the best method for determining TDR resolution. It is better to

consider the temporal/spatial resolution of the TDR (see 4.1.2)

than bandwidth/risetime resolution when determining the per-

formance of the TDR measurement system.

6.3.2

Temporal/Spatial Resolution

The

TDR unit may

not be the only limiting factor for temporal resolution. The

probe connecting the TDR unit to the test specimen may also

limit resolution and this needs to be considered. Because of

the nature of TDR, it is easy to include the effects of the TDR

unit and all of the probe devices collectively, by defining t

sys

the

fall time of the TDR step that has reflected from a short

circuit placed at the end of the probe and returned to the TDR

head. The mean value of Z

less susceptible to TDR reso-

lution than are the minimum and maximum values.

6.3.3

Amplitude Scale

a coarse vertical scale is used,

quantization error can be significant. Many instruments

change accuracy when their scales are changed, and this can

result in significant but unknown errors that can exceed 3 Ω.

6.3.4

Baseline and Amplitude Drift

The

ability of the TDR

instrument to maintain a constant baseline voltage and con-

stant amplitude step pulse are critical to the repeatability of

the TDR measurement process. TDR step generators and

sampling units are sensitive to time and temperature drifts.

Drift should be minimized and have a value that corresponds

to less than one-tenth the desired impedance uncertainty.

6.3.5

Electrostatic Discharge Damage

ESD

damage to

TDR instrumentation is often not easily detected and may

unknowingly affect measurement accuracy. Therefore, system

calibration should be performed regularly to check for this (see

5.1.1). All cables should have a termination attached to one

end when not in use and while they are being connected to

the TDR instrumentation. The use of a static protection switch

helps eliminate ESD damage to the TDR. Operators should

have anti-static awareness training and should perform all

measurements in anti-static work areas while wearing anti-

static wrist straps.

6.3.6

Probes

Hand-held

(manual) probing solutions are

more sensitive to operator technique than are automated

methods and, consequently, the measurements made using

manual probing methods will exhibit higher variability than

automated methods. Operators should be trained and tested

in their ability to repeatably probe and obtain a consistent

measurement.

6.3.6.1

Probes for Single-Ended Transmission Line

Measurements

The

probe assembly impedance is often

chosen to be 50 Ω to match the impedance of the TDR sys-

tem. Impedance matching minimizes reflections at the inter-

face between the probe and the transmission line under test.

These reflections, which appear at and around the transition

region in the TDR pulse and can extend for some time after

this transition, are perturbations in the TDR waveform and are

undesirable because they affect the length of the measure-

ment zone and may increase measurement uncertainty. When

the characteristic impedance of the transmission line under

test is nominally 50 Ω, these perturbations will normally decay

rapidly. If the impedance of the transmission line under test is

significantly different from 50 Ω, the magnitude of the pertur-

bations can be large and their duration long enough to affect

the measurement zone. The effect of these perturbations

must be taken into account when determining the measure-

ment zone (see 4.1.2). The design and quality of manufacture

of the probe has a large effect on the magnitude and duration

of reflections generated between the TDR system and the

transmission line under test.

When probing non-50 Ω lines, it is possible to separate, in the

TDR waveform, the large signal perturbations caused by the

TDR/probe interface from those caused by the probe/

transmission line interface. To do this, a specially designed

probe is required that is impedance matched to the transmis-

sion line under test and that also has a long propagation delay

between the TDR/probe connection and the probe tip. The

long propagation delay can effectively move the large pertur-

bations at the TDR/probe interface out of the measurement

zone.

IPC-TM-650

Number

2.5.5.7

Subject

Characteristic

Impedance of Lines on Printed Boards by TDR

Date

03/04

Revision

Page

22 of 23

电子技术应用　　　　　　　ｗｗｗ．ＣｈｉｎａＡＥＴ．ｃｏｍ