1 quartz crystal : iqc procedures and reliability issues arthur lee technical director copyright,...

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Quartz Crystal : IQC procedures

and Reliability Issues

Arthur Lee Technical DirectorCopyright, 1999 ©

Hong Kong X’tals Limited.

2Hong Kong X’tals Limited

What is a Quartz Crystal ???

• Production Engineers : – It is the first component you replace when you have a failure

board !

• Purchasing Officers :– It is a component that you could never get a reliable vendor !

• R&D Engineers :– It is a mysterious little black box !


What is a Quartz Crystal ?

A crystal is a Piezo-Electric Device, i.e. :

Energy is converted between electrical and mechanical, millions of times per second.

Hence, its characteristic is very different form other components, as oppose to :

• Resistor -- Pure Electrical• Screw -- Pure mechanical


The Structure of a Quartz Crystal

Cover

Base

Silver Electrodeon both sides

Silver EpoxyQuartz Wafer( Blank )

Sealed with Clean, Dry, inert gas

Electrical Energy Voltage & Current

Mechanical Energy Stress and Motion


What is special with a Quartz Crystal?

Due to the Electro-mechanical nature, a quartz crystal has some unique characteristic that require attention, such as :– Sleeping Crystals ( DLD characteristic )– Spurious modes at room temperature– Drop Test / Shock and vibration Tests– Aging ( Frequency and ESR drift )– Temperature Cycle– Over Temperature characteristic


Full Analysis at IQC ?

• A full analysis on all crystal parameters, especially the reliability performance, requires a lot of effort and equipment.

• The function of IQC is to check some critical parameters that may affect your products’ quality / performance, but NOT to act as your suppliers’ outgoing QA.


What parameters can be included in IQC ?

– All room temperature parameters, including• Sleeping Crystals ( DLD characteristic )• Spurious modes at room temperature

– Drop Test / Shock and vibration Tests– Aging ( Frequency and ESR drift )

• Optionally depending on package type and vendor, solderability test may be needed.

• Over temperature test and thermal cycle test are usually skipped due to cost and time.


IQC -- Relaxed Level

• AQL depends on crystal user or crystal supplier, typical 0.4%, or 0.15%; or optional C=0 Sampling method

• Depending on Crystal user's company policy, data of 10 pcs pass crystals, and any failure should be printed out and kept together with supplier's batch code for tracibility.

• Skip Drop Shock and Accelerated Aging tests as compared to Normal Level, so as to reduce IQC work load

Mil-Std-105D Level I, Sampling for each incoming batches

Visual Check for marking and outlook

Electrical Parameter Test using KH1200includes :

DLD, Spurious, FL, ESR, Co, C1, Ts, ( and optional DL=FL1-FL2 etc.. )

Pass ?


What is …. ???

• Sleeping Crystals, and DLD• Spurious modes ??


Sleeping Crystals

• Sleeping crystal is a well known phenomenon in Crystal Industry :– The Crystal will NOT start, but after the crystal is started by some mechanical

or electrical excitation, it works fine for some time, then, after some unknown time, it sleeps again – not functioning.

– The time that it begins to sleep again is unpredictable, could be minutes or months.

– Not all crystal makers know how to avoid producing sleeping crystals, that’s why not many crystal makers like to talk about this.

• Root Cause : Improper Cleanliness• Cure : None, a sleeping crystal will sleep again.• Test Method : DLD, or high drive

( note high drive is a test, not a cure ).


DLD or Starting ESR

• Same root cause as Sleeping Crystal, less serious.• If your circuit has starting problem, you should specify

extra requirement of ESR at a lower drive level.• This specification depends on your circuit’s drive level,

and starting gain.• Do NOT over specify, it costs.

0

1

2

3

4

5

6

7

Drive Level

ESR


Crystal DLDTypical Example

Changes due to Design limit, not because of poor process

Kolinker KH1200 Test data

At 10nW Rs=37At 100uW Rs=31, non-ideal process, depending on applications, could be good or no good


Typical DLD Specification

• Ask your crystal vendor the operating Drive level (DL) of your circuit, and also whether your circuit has a good starting gain.

• If your circuit has a good starting gain, specify : – DLD test range : from 2.5nW/Ohm to

1 µW/Ohm or operating DL ( whichever the lower);and, ESR change in above power range < ( 20% to 120%,

depending on ESR )• If your circuit does not has a good starting gain :

– discuss with your vendor.• This specification safeguard against Sleeping Crystal with

very high safety factor, hence may affect cost


Details in DLD or Sleeping Crystals

• See Handout for details, or

• Contact E-mail : [email protected]

for a separated paper.


Spurious Modes ( Couple Modes )

• Couple modes always exist !– Couple modes are caused by energy coupling from the main mode of

vibration ( AT cut thickness shear ) to other unwanted vibration mode. Just like when you hit a guitar wire, other then the fundamental frequency, it will always produces other harmonic and inharmonic frequencies.

• Must specify Minimum ratio of :ESR(worst unwanted mode) / ESR(operation) > 2.5

• ( to 10 times for very demanding applications )• If the unwanted modes are too active, the oscillator may :

– jump from one frequency to another, or too much jitter / phase noise

– produce impure spectrum that may jam your circuits. • Do Not over specify, it costs !


Couple modes at Room temperature

0

0.2

0.4

0.6

0.8

1

1.2Main mode : AT Cut, Thickness

Shear

All other couple modes are NOT AT Cut, Thickness Shear mode

Couple Mode is also known as :•Spurious mode•inharmonic mode•unwanted mode


Couple modes at Room temperatureGood Example

Main mode : AT Cut,

Thickness Shear

Kolinker KH1200 Test data


Deficiency of Relaxed Level

• Only Room temperature test• No Drop Test, Aging Test, and over temperature test

• Do Not forget to set up Spurious Test and DLD test, this helps.


IQC -- Normal Level

• Add :– Drop Shock– Vibration ( option for

automobile industry )– Accelerated Aging

Test

• Refer to hand out for detail chart

All environmental condition, e.g. Drop Shock, Aging, Vibration, etc

Test after environmental condition,For all electrical parameters

Compare results with specified limits


Drop Test

• Very Important Reliability Specification• Do Not over specify, it costs !

– Different for different grade of crystals and different application and different vendors.

• Typical Drop shock performance :Clip Mount :

– Below 12 MHz 50 cm– >= 12 MHz 35 cm

Spring Mount– Below 16 MHz 75 cm– >= 16 MHz 50 cm


Accelerated Aging Test Standards

• Industrial Standards, usually – 105 degree C, 7 days, or

– 85 degree C, 30 days, or 1000 hours

• These require too much time

• Aging test check for excess stress and contamination in the crystal, if too serious, the crystal may become dead after some time.

-15

-10

-5

0

5

10

15

1stday

2ndday

3rdday

4thday

5thday

6thday

7thday

ppm


Short Cut : Accelerated Aging Test

• Usually :– 85 degree C to 120 degree C,– for 1 to 2 hours– Then check for difference before and after test, usually :

• Difference in Fs : < +/- 5 to 10 ppm• Difference in ESR : < +/- 10 to 20% + 2 Ohm

• Important note :– Since this is not a standard test, some crystal vendor agree, some do not.

You will have to discuss with them.– You need a good instrument to test, the R&R ( Repeatability and

Reproducibility ) of the instrument should be well within the requirement.


IQC -- Tighten Level

• Add : Room temperature parameter

record between each environmental condition, and compare difference of Fs and ESR.

• Usually, difference in Fs < 5 to 10 ppm, and difference in ESR < 5 to 20% + 2 Ohm

• Note : This is too tight for commercial grade applications

• Refer to hand out for detail chart

Test before environmental condition,For Fs and ESR

One environmental condition, e.g. Aging

Test after environmental condition,For Fs and ESR

Compare difference between : Before and after

of each individual condition


Reliability Issues Not covered by above IQC

• Temperature Cycle– Usually NOT specified and tested by crystal vendor for

commercial grade crystal

• Over Temperature Characteristic– For commercial grade, usually only specified as ?? ppm

over temperature– But the crystal may change dramatically over some

narrow temperature ranges.


Temperature CycleGood Example

Kolinker KH1200TS Test data


Temperature CycleBad Example



Couple modes over temperatureBad Example



Couple modes over temperature

0

0.2

0.4

0.6

0.8

1

1.2

When Temperature Change :Frequency changes, ESR changes

Main mode

0

0.2

0.4

0.6

0.8

1

1.2

All other couple modes change according to the

TC curves of each of their own modes


The Frequency and ESR of the Main mode is affected by how the other couple modes are changing over temperature. If a crystal is not designed properly, the crystal may exhibit frequency jump or severe ESR change over temperature.


Couple modes over temperatureGood Example



IQC Consideration

• Reliability issues that you may include into your IQC procedures :

• DLD -- Sleeping Crystals• Spurious mode• Drop Shock / Vibration• Accelerated aging

• Reliability issues that will cost you if you include into your IQC procedures :

• Over temperature characteristic• Temperature Cycle• Solderability

• What kind of tester do I need ?


Standards for the Crystal Industry

• Measurement Standards– IEC 444 is the commonly recognized and adapted by the

crystal industry– Sometimes, EIA 512 is used in very high frequency ( above

200 MHz )

• Specification Standards– There are various standards ( e.g. Mil-Std 3098 ... ), but most

of them are outdated, and not suitable for nowadays’ application and crystal packages

– An example is : drive level = 1 mW is well too high for nowadays crystal packages like HC-49U, HC-49US, 57 SMD, UM-1, UM-5 ….


Measurement Instrument

• IEC 444 Measurement Standard– IEC 444 describes how crystal can be measured with transmission

technique. It also defines explicitly a matching network to match the crystal impedance to the 50 Ohm coaxial cables commonly used in most network analyzers

– For measuring, Co, FR and ESR, it is not too difficult to follow.– However, for FL and other parameters, it is extremely difficult for

electronic product engineer to follow through all the details and develop compensation techniques for specific test jig and fixtures.

• There are test instruments commercially available complying with this standard. – The discussion and measurement herein are based on the Kolinker

KH1200 Pi network crystal measurement system.


Can I build my own tester ?

• Yes, but you must do it properly.– Do NOT use any test circuit other than EXACTLY the same

one as the particular product.– Do NOT target to build some standard general test jig that

can measure crystal parameter accurately for all types of crystal. You will have big difficulties in determining phase shift and stray component compensation. Usually, most crystal vendors will have difficulty in accepting readings from such testers.

– It is acceptable if you have one tester for each model/series of product, using the PCBA of the product.


Your tester with the PCBA of the product

Oscillator Circuit buffer

Do Not Test here : avoid loading the

oscillator

Test at other functional check point to avoid loading the oscillator. Check for function including frequency, pullability and amplitude

Socket for crystal :•Low Inductance•Low stray capacitance•Prefer gold plated•Obtain from your crystal supplier

Safety Factor for ESR :Resistor = about 2maximum ESR at room temperature

DUT


Building your own tester

• Build ONE set of test jigs for each individual model/series. Use THE PCBA of the product, target for a GO / NG test, not a parameter test.

• The extra Resistor is to safe guard against changes due to :– Power supply variations– Temperature change thus affecting the components on the PCB– Component tolerance in mass volumn– Start up time– The crystal’s ESR may change due to drive level, temperature

• However, the extra Resistor may induce extra stray impedance ( both inductance and capacitance ) which affect frequency, so if you want accurate frequency measurement, you may have to consider building 2 test jig :– one without Resistor for frequency test– one with Resistor for maximum ESR test

• Do provide one extra set to your crystal vendor for crystal specification identification.


Difference : Self-build testers and Standard tester

IEC444 compliance instruments Self-build testersRequire One time investment No one time investmentNo need to build tester for each newmodel

Have to build tester for each new model

Easy communication with crystalvendor

More work required to communicationwith crystal vendor(s)

Easy second sourcing More work required during secondsourcing

Require a good written specification foreach type of crystal. Most of the timeyou will have to send your PCBA toyour vendor for specificationverification.

Send the tester to your crystal vendor, lethim specify the crystal for you ( do thiswith every new model, or every newvendor )

Parametric test for better control Simple GO/NG testBoth method acceptable by most crystal vendors


Crystal Passing IQC, but fail in Production !

• This is No good, some corrective action MUST be done !

• Possible cause :– Crystal quality problem not covered by IQC ( Improper IQC

procedures / coverage )– Improper crystal specification ( matching )– Improper testing instrument– Problems with other components

• Contact your Crystal Vendor, see whether they could give you proper support

39

Technical Support from HKC

Send HKC your production failuresSend us your latest PCBA for crystal specification verification.

Web Page : http://www.HongKongCrystal.comE-mail : [email protected] : ( 852 ) 24985908Copyright, 1998 © Hong Kong X’tals Limited

1 quartz crystal : iqc procedures and reliability issues arthur lee technical director copyright,...

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