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Introduction

Relative complex permittivity(permittivity and loss tangent) of

printed circuit (PC) board andsubstrate material is a critical

parameter that affects circuit

performance. Characterizing this parameter at RF is becoming moreimportant because of increasedclock frequencies used in todayshigh speed computers.Bandwidths of 10 to 20 times theclock frequency are required forgood signal integrity. Thus indigital designs, analog bandwidths

to 1 GHz and higher are becomingcommon with clock frequencies of 50 to 100 MHz. In addition,

performance of dielectricmaterials at RF is equally

important for wirelesscommunication circuits andcomponents.

Measuring sheet dielectricmaterial at RF has been difficult.No RF parallel plate measurementsolution has existed until therecent introduction of the

HP 4291A RF Impedance/Material Analyzer with the HP 16453ADielectric Material Test Fixture.Combined with the optionalmaterial measurement firmware,

the analyzer offers a simple andhighly accurate solution tosheet-material permittivitymeasurements from 1 MHz to1.8 GHz. This note summarizesthe current measurementtechniques and discusses thesolutions now available using theHP 4291A analyzer system.

Permittivity Measurements of PC Boardand Substrate Materials using theHP 4291A and HP 16453A

Application Note 1255-3New solutions for materialmeasurements to 1.8 GHz

The HP 4291A RF impedance and material test system.

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Background

Relative complex permittivity(permittivity and loss factor/loss

tangent) of the dielectric materialaffect the bandwidth (necessaryfor high speed signaltransmission) and circuit density.

For high operating frequencies :High speed signal transmissionand low signal attenuation aredesired in many applications. Therelationship of relative complex

permittivity to signal transmissionspeed and attenuation is shown

below:

Velocity: V = C

r

Attenuation: A =f x r x tan

C

These equations show that low permittivity and low loss arerequired to achieve high operatingfrequencies. For this reason, PC

boards and substrate materialhave lower permittivity (typically2 to 10) and loss tangent (typically0.01 to 0.0001) compared withother dielectric materials.

For high circuit density: Smallinterconnect size is required. Theequation and graph in Figure 1show that for a constantcharacteristic impedance,increasing the permittivity willreduce the interconnect size.Thus for high-densityinterconnects (IC packages,MCM), the substrate material willhave higher permittivity values,typically 6 to 10.

A range of measurements arerequired to characterize dielectricmaterial used for PC boards andsubstrates. Table 1 shows typicalmaterial parameters and operatingconditions. Note that due to thewide temperature range found inelectronic applications, evaluationof permittivity over temperature(and humidity) extremes is oftendesired.

Table 1. Summary of typical dielectricmaterial parameters for electronic

interconnect applications.

Application PC board Substrate

Material Type Glass Epoxy,PI, EP

Alumina,Ceramic

Frequency 1 MHz to>1 GHz

1 MHz to>1 GHz

Permittivity 2 to 6 6 to 10

LossTangent

0.01 to 0.0001 0.01 to0.001

Thickness 0.02 to 2 mm 0.5 to 2 mm

OperatingTemperature

55C to150C

55C to200C

Limitations of traditional

sheet-material permittivitymethods

In the past, two general methodswere used to measure permitivityof sheet-material; a parallel-platefixture (HP 16451B) used with anLCR meter, and resonant methodsusing a vector network analyzer.

Figure 2. Resonant method using anetwork analyzer.

Figure 2 shows the resonant (stripline) method using a networkanalyzer (N.A.). The resonantmethod is best at higherfrequencies due to the practicalityof the sample/trace physical size.The resonant method has otherlimitations besides the problem of not covering lower frequencies.The resonant method, by

2

w

h

1000

0.1 1.0 10

10

50

100

C h a r a c

t e r i s

t i c

Z 0

(

)

1246

108

Trace Density

r=

w/h

Microstrip

Z0 =r

60 5.98h

0.8w+t

Substrate

t

ln

r

+ L J K / R Z

Figure 1. Characteristic impedance of a trace on a dielectric material.

N.A. + Strip Line (resonator) Method

L

Transmission

fr

PC Board

Microstrip

r = ( )2n C

fr(L+ L)

2

NA

:

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definition, is a single frequencytechnique. It requires sample

preparation with tight tolerances.Relative permitivity results must

be computed from other parameters, often requiring anexternal computer.

The LCR meter parallel-platemethod has been limited by thefact that accurate impedancemeasurements were not possibleabove 30 MHz, the limit of theHP 4285A Precision LCR meterand HP 16451B fixture. Thus nosimple, accurate solution wasavailable for the frequencies from

30 MHz to over 1 GHz. SeeFigure 3.

Parallel Plate Method

Z Meter + 16451B

Resonator Method

N.A. + Strip Line

1M 10M 100M 1G FREQ[Hz]

Figure 3. Frequency coverage of twotraditional PC board and substratepermittivity measurement methods.

In summary, permittivity

measurements of thinsheet-material in the RF rangehave been difficult at best, andaccurate swept-frequency resultsnearly impossible.

New RF measurement solutionprovides ease-of-use and highaccuracy

The HP 4291A system is acomplete solution for relative

permittivity measurements. Iteliminates complex calculations,material preparation, and fixturingissues common with previoustechniques. The system consistsof the HP 4291A, Option 002material measurement firmware,and the HP 16453A dielectricmaterial test fixture.

Option 002 material measurementfirmware provides direct readoutof relative complex permittivity asa function of frequency,

eliminating the need for anexternal controller. In addition,the firmware includes aneasy-to-use HP 16453A fixturecompensation function. Thefirmware uses impedancemeasurement data provided by theHP 4291A Impedance/Material

Analyzer to calculate permittivityresults.

This new analyzer is based onHPs RF I-V method of impedance

measurement. This methodextends accurate, wide-rangeimpedance measurements to1.8 GHz, providing the highaccuracy required to make parallel

plate permittivity measurements.

In the parallel plate method, thematerial-under-test is sandwiched

between the fixtures twoelectrodes to form a capacitor.The HP 4291A measures theadmittance of the capacitor, and

Option 002 firmware calculatesrelative complex permittivity, asshown in Figure 4.

Figure 4. Calculating complexpermittivity from admittance.

New fixture for easysheet-material testing

The new HP 16453A Dielectric

Material Test Fixture is shown inFigure 5. It connects directly tothe APC-7 connector on theHP 4291A test head, or can beused with the optional hightemperature test head for use in atemperature chamber. Fixturefeatures include:

Figure 5. HP 16453A dielectric materialtest fixture mounted on the HP 4291Atest head.

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Easy sample preparation : Thematerial-under-test is simplysandwiched between the fixtureelectrodes. The test is

non-destructive and doesn'trequire extensive preparation of the material. The sample sizespecifications are shown inFigure 6.

Figure 6. Sheet-material dimensions forthe HP 16453A fixture.

Adjustment-free electrode: Adjustment to insure parallelelectrodes is often required usingthe parallel plate method. Thisadjustment is not required

because the fixture has a flexibleelectrode that adjustsautomatically to the materialsurface. See Figure 7.

& %

Upper Electrode

Lower Electrode

spring

Figure 7. Automatically adjustingupper electrode matches MUT surface.

LOAD standard provided : Forhigh accuracy results, the fixtureincludes a LOAD standard forfixture compensation.

High accuracy measurementsup to 1.8 GHz

The HP 4291A system achieveshigh accuracy relative complex

permittivity measurements up to1.8 GHz (typically 8% for

permittivity and 0.003 for losstangent at 100 MHz and r = 10).This high accuracy combined withswept-frequency measurement isideal for PC board and substrate

testing. Figure 8 shows anexample of a PC board (low

permittivity and low loss)measurement. The high accuracyis a result of the high basicaccuracy of the impedancemeasurement, the fixture design,and calibration and compensationfunctions built into the systemsfirmware.

Simplified temperaturecharacteristic measurements

The HP 4291A is designed to be an

instrument controller with theaddition of Option 1C2,HP I-BASIC. As such, it cancontrol other test equipment or anHP-IB controllable environmentalchamber. Figure 9 shows the

block diagram of a temperaturecharacteristic evaluation system.The HP 4291A provides thefollowing features and options tosimplify evaluating temperaturecharacteristics:

1.8 m cable to the measurementhead: Convenient for systemconfiguration. Does not affect theaccuracy of the measurement.

High temperature highimpedance test head option(Option 013): A heat-resistantcable (55C to 200C) to extendthe APC-7 calibration plane whileat the same time maintaining highaccuracy.

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Figure 8. PC board relative permittivity measurement over the RF range.

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Wide operating temperature range of the fixture : TheHP 16453A fixture can be usedfrom 55C to 200C without lossof accuracy.

HP-IB and Option 1C2 HP IBASIC controller function :Provides an interface andcontroller function for automaticmeasurement and chambercontrol. A third party temperaturechamber is required. Tabai EspecCorporation offers a temperaturechamber (SU-240-Y) compatiblewith the HP 4291A. It is picturedin Figure 10.

Figure 10. Using the Tabai EspecSU-240-Y temperature chamber with theHP 4291A.

Application program: ThisHP IBASIC program fortemperature characteristicsevaluation and chamber control iscompatible with the Tabai Espectemperature chamber. It isincluded with the optional hightemperature test head(Option 013).

Graphic display: Displaysmeasured parameters as afunction of chamber temperature.See Figure 11.

Figure 11. Direct display of temperature characteristics.

Basic measurement procedure

Figure 12 shows the basicmeasurement steps for dielectric

material measurements.

Figure 12. Dielectric materialmeasurement process.

1. Perform analyzer calibration at the APC-7 on the HP 4291A test head.

2. Connect the HP 16453A fixture tothe APC-7 on the test head.

3. Perform open/short/load fixturecompensation at the fixtureelectrodes. Note: The loadstandard is provided with thefixture.

4. Input the thickness of material-under-test.

5. Place the material-under-test in between the fixture electrodes.

6. Select measurement parametersand display format.

7. Start the measurement. Note: Therelative complex permittivity isdirectly displayed as shown inFigure 8.

8. Repeat the measurement for othersamples.

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