IPC-TM-650 EN 2022 试验方法.pdf - 第491页

6.3.6.2 Probes for Coupled-Signal-Line (Differential) Transmission Line Measurements The probe consider- ations described in 4.3.3 apply for probes used in differential transmission line measurements. However, the necess…

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

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6.3.6.2

Probes for Coupled-Signal-Line (Differential)

Transmission Line Measurements

The

probe consider-

ations described in 4.3.3 apply for probes used in differential

transmission line measurements. However, the necessity to

simultaneously probe two signal lines and one or two refer-

ence plane contacts makes differential probing more difficult

than probing single signal line structures. In a PCB manufac-

turing environment, the use of two probes that were previ-

ously used for single-ended measurements may not be pos-

sible. This is because the operator is required to use both

hands for probing, which leaves them unable to operate the

instrument. Contact your instrument manufacturer for their

probing solutions and advice. Probes from one manufacturer

can also be used with another manufacturer’s TDR if the

impedance values and connectors are compatible.

6.4

Adjustable Measurement Parameters

6.4.1 Sampling Interval (Point Spacing)

The

temporal

resolution of the TDR unit is an issue only if it impacts the

duration of the constant-valued regions in the TDR waveform

(see 4.1.2) that are used for computing Z

The temporal reso-

lution of the TDR is affected by the transition duration of the

TDR step response, the transition duration of the step

response of all intervening electrical components (connectors,

cables, adapters), measurement jitter, the interval between

sampling instances, and timebase errors. For typical TDR

measurements, timebase errors and sampling intervals should

not be an issue (both are or can be made to be less than 10

ps). The effect of measurement jitter can be modeled by con-

volving the jitter distribution with the TDR step response to

yield an effective TDR step response. The effect of jitter on the

bandwidth of the TDR measurement can be assessed from

the jitter spectrum, which can be described by:

J(ƒ)=e

−2(πσƒ)

where:

J is

the jitter spectrum,

f is frequency, and

σ is the rms jitter value

If the effective step response impacts the duration of the mea-

surement zones, then jitter must be reduced. If the jitter has

an observable effect, then the user must reduce the duration

of the measurement zone (by increasing the lower limit and

decreasing the upper limit, (see 5.1.3) from which Z

com-

puted or reduce the system jitter. Reduction in the duration of

the measurement zone may introduce a bias in the voltage or

reflection coefficient values and this affect the computed value

of Z

If the rms jitter value is less than 20 % of the transition

duration of the TDR step response, then the jitter is small and

can be ignored. For typical TDR systems, however, rms jitter

is less than 10 ps and will not affect the Z

measurements.

Similarly,

the effect of cables, connectors, and adapters on

the measurement can be modeled by convolving their step

responses with that of the TDR unit. If the transition duration

of this new step response meets the requirements of 4.1.2,

then the performance of the cables, connectors, and adapters

is adequate.

6.4.2

Waveform Averaging and Number of Samples in

the Measurement Zone

Waveform

averaging reduces the

effective noise level of the measurement by M

-1/2

where M is

the number of acquired waveforms (typically, 8 ≤ M ≤ 256).

Consequently, averaging can reduce measurement noise.

This reduction is limited by the number of bits of the analog-

to-digital converter of the TDR system. However, if the TDR-

system exhibits drift in the timebase, averaging too many

waveforms may result in a reduction of t

sys

and

a commensu-

rate reduction in the temporal/spatial resolution of the TDR.

The number of samples (data points) in the measurement

zone will affect the standard deviation of the computed value

of Z

because

this value is the result of averaging all the

samples in the measurement zone. Therefore, the more

samples in the measurement zone, the smaller will be the

standard deviation of the computed Z

value.

6.4.3

Selection of Constant-Valued Region (Measure-

ment Zone)

Inconsistency

in defining where the constant-

valued region is located in the TDR waveform may cause a

significant but unknown error than can exceed 5.0 Ω. Speci-

fying the measurement zone improves measurement repeat-

ability of the same or similar samples, and this can improve

assessment of design and fabrication quality and vendor

capability. This measurement zone should be far enough

away from the launch and the open end of the transmission

line under test to minimize the effects of these discontinuities.

The measurement zone is to be given as the separation

between two positions on the transmission line, and these

positions are to be given as a percentage of the transmission

line length referenced from the TDR/transmission line inter-

face. The measurement zone is defined in 5.1.3.

6.5

Acknowledgments

The

majority of the figures used

herein were provided by Mr. Bryan C. Parker of the Introbot-

ics Corporation, Albuquerque, NM.

IPC-TM-650

Number

2.5.5.7

Subject

Characteristic

Impedance of Lines on Printed Boards by TDR

Date

03/04

Revision

Page

23 of 23

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Scope

This

procedure outlines a test method to deter-

mine the permittivity (dielectric constant or E’r) and loss tan-

gent (dissipation factor or Tanδ) of printed wiring materials at

various frequencies (from 1 MHz to 1.5 GHz) using a single

test fixture for the measurement.

The permittivity and loss tangent are measured using a narrow

sweep of frequency around the target or desired frequency.

The test method is built around the capability of currently

available materials analyzers, which use a capacitance

method to determine permittivity.

This test method is not intended for low loss materials, such

materials may be tested at fixed frequencies using other IPC

test methods.

Applicable Documents

HP 4291A-5 Product Note

‘‘Dielectric

Constant Evaluation

of Rough Surface Materials,’’ which describes how to make

accurate measurements using the HP 4291A and HP

16453A.

Application Note 380-1

‘‘Dielectric

Constant Measure-

ments of Solid Materials,’’ which contains a technical back-

ground, suitable for this subject.

Test Specimen

3.1

Each

specimen shall be 50 mm x 50 mm by the thick-

ness of the substrate material. Within the limits of the test fix-

ture, the thicker the sample the less error in the measure-

ments. Multilayer samples can be used to increase the

thickness of the sample, but these cannot be simple stacked

layers; they must be physically bonded with no air gaps

between the layers. A target thickness would be 1.0 mm, but

both thinner and thicker samples will work.

3.2

Three

specimens are required for this test.

3.3

All

materials are affected by moisture, including all rein-

forced laminates and most films. Therefore, all samples shall

be conditioned at 23°C ± 2°C and 50% RH ± 5% RH for a

minimum of 24 hours prior to testing. However, if a sample

has recently been etched or exposed to excessive moisture, it

should be dried in an air-circulating oven for two hours at

105°C +5°C, -2°C prior to testing and conditioned at room

temperature as mentioned above.

3.4

Sample Surface Preparation

3.4.1

is preferred that the sample be patterned with a

conductive material in the shape and size of the test elec-

trode. This conductive material is preferably 100 angstroms of

vapor deposited copper. Other metals may be used. In all

cases, the conductor on the sample must make good electri-

cal contact with the fixture electrode. Such a conductive pat-

tern eliminates air gaps and other potential sample mounting

errors.

3.4.2

Bare

dielectric materials may be tested with this test

method. The fixture electrodes must be applied with some

level of force to ensure a gap-free contact area. Determining

the correct force setting may require some trial and error test-

ing for each type of sample (see 6.4).

Equipment/Apparatus

4.1

The

Hewlett-Packard Impedance Material Analyzer,

model 4291A, or equivalent is recommended.

4.2 Hewlett-Packard

model number 16453A test fixture, or

equivalent

4.3

appropriate calibration-verification kit and a fixture-

correction kit as recommended in the instrument’s manual

(i.e., HP4291A Calibration kit). Such a kit usually includes the

following devices:

• OPEN and SHORT for fixture correction

• 50 Ohms impedance

• Dielectric (PTFE) of known characteristic for the purpose of

the calibration verification

4.4

Micrometer,

capable of 0.001 mm resolution

4.5

Circulating

oven capable of 105°C +5°C, -2°C

Procedure

5.1

Calibrate

the instrument using the calibration kit accord-

ing to the recommendations of the instrument manufacturer.

The

Institute for Interconnecting and Packaging Electronic Circuits

2215 Sanders Road • Northbrook, IL 60062

IPC-TM-650

TEST

METHODS MANUAL

Number

2.5.5.9

Subject

Permittivity

and Loss Tangent, Parallel Plate,

1 MHz to 1.5 GHz

Date

11/98

Revision

Originating Task Group

HDI Test Methods Task Group (D-42a)

Material

in this Test Methods Manual was voluntarily established by Technical Committees of the IPC. This material is advisory only

and its use or adaptation is entirely voluntary. IPC disclaims all liability of any kind as to the use, application, or adaptation of this

material. Users are also wholly responsible for protecting themselves against all claims or liabilities for patent infringement.

Equipment referenced is for the convenience of the user and does not imply endorsement by the IPC.

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