IPC-TM-650 EN 2022 试验方法.pdf - 第492页

1 Scope This procedure outlines a test method to deter- mine the permittivity (dielectric constant or E’r) and loss tan- gent (dissipation factor or Tan δ ) of printed wiring materials at various frequencies (from 1 MHz …

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

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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.

age1of5

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Calibrations

only last 24 hours, so calibration shall be per-

formed within 24 hours of the measurement. See 6.1 for more

calibration notes.

5.2 Set up the unit to sweep the target frequency ± 0.5% of

the target.

5.3

Measure

the sample thickness with the micrometer and

insert into the test fixture. The sample must make good con-

tact with the fixture electrodes (see 6.1 concerning the proper

force to be applied). The sample must not touch the back wall

of the fixture. The sample electrode placement and thickness

measurement shall be obtained from the same area of the

sample.

5.4

Run

the test and record the average permittivity and loss

over the narrow frequency range sweep. The scanned data

may also be saved on disk. See 6.2 for comments on

expected behavior for permittivity as a function of frequency.

5.5

Repeat

5.1 through 5.4 for all desired frequencies.

5.6

Report

the average permittivity and loss at the frequen-

cies requested.

6 Notes

6.1

Correct

calibration and operation of the test equipment

is required to obtain accurate measurement of permittivity and

loss. Proper sample preparation is also very important for

obtaining useful data from this test. Calibrate the materials

analyzer in accordance with manufacturer’s instructions. An

automatic program has been developed for the HP 4291A,

which will ease calibration and setup (see 6.6).

6.2

The

permittivity should decrease slightly with increasing

frequency. If it increases greatly or decreases more than 0.2

units from approximately 20 MHz to 1.2 GHZ, reposition

(reset) the sample in the fixture and measure again (check for

debris between the electrodes; blow out with air).

6.3

Testing

at temperatures and humidities other than room

temperature may be performed with this instrument, as a spe-

cialized fixture can be placed in a temperature chamber. A

temperature chamber must be used when testing with this

fixture under conditions where condensation might contami-

nate the electrodes, as such contamination gives spurious

results.

6.4

The

pressure of the test fixture on the specimen affects

the measured permittivity and loss values, in particular for

un-metallized test specimens. Too light of pressure reduces

the area of electrode/sample contact, thus leaving air gaps,

which result in erroneous measurements. If the pressure is too

high, the sample can be reduced in thickness and the mea-

sured values would be incorrect because the thickness is

unknown. Making measurements while adjusting the force

should lead the operator to a force setting where the mea-

sured value is independent of the force applied.

6.5

The

HP 4291 Materials Analyzer and related fixtures and

calibration kits are available from Hewlett Packard, (800) 452-

4844.

6.6

Reference Program for Automatic Calibration and

Operation

This

automatic calibration and operation pro-

gram was developed for the HP 4291A and is published in

this method as a reference. Although the program listed in this

section has been tested and used, it is given here for ‘‘refer-

ence only.’’

6.6.1

Procedure Using Automatic Program

Turn

on the

analyzer with the program/calibration disk in the drive and fol-

low the user friendly calibration instructions, which appear on

the monitor. The calibration on the HP4291 lasts about 24

hours; after that, it begins to drift and provides slightly higher

values as a function of time. The calibration procedure in

6.6.1.1 through 6.6.1.10 should therefore be performed, at a

minimum, on a daily basis. During the calibration procedure,

the line traces on the monitor should be observed during each

step. Noisy or erratic traces are an indication of external inter-

ference. If noise is observed, the calibration procedure should

be aborted and rerun. Clean the electrode on the test head,

standards, and fixture connections and electrodes on a regu-

lar (weekly) basis. Blow dry.

6.6.1.1

Allow

at least 30 minutes for the unit to warm up

and stabilize.

6.6.1.2

the unit is a frequency sweeping unit, enter the

start and stop frequency (in megahertz) of the test (single fre-

quency tests use close start/stop frequencies, which should

be ± 0.5% of the target frequency).

6.6.1.3 Place

the OS (open) calibration standard on the test

head as prompted on the unit’s monitor and press Return on

the unit’s keyboard or ‘‘x1’’ on the unit.

IPC-TM-650

Number

2.5.5.9

Subject

Permittivity

and Loss Tangent, Parallel Plate,

1 MHz to 1.5 GHz

Date

11/98

Revision

age2of5

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