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What is Just Enough Test?

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TopicsBoard Test TrendsDifferent types of Test

Inspection/StructuralAdvantages/Disadvantages

Big Iron vs Low-CostDistributed TestFalse FailuresSampling

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Fault Spectrum and Issues

Solder Paste Height

Solder Missing

Solder BridgeIC Pin Lifted

IC Pin ShortedTrace Short

Trace Open

Not Enough Solder

Board MarkingsReversed Part

Bad Part

Missing Parallel Cap

Part Placement

Part Tolerance

On Power Testing

Wrong Part

Flash Programming

ECO Version

Transistor testing

Boundary Scan

Memory Programming

I2C Programming

GUID Programming

Programming Verification

Test Time

Test Costs

Schedule AOI

X-Ray

MDA

ICT

Flying Probe Functional Test

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Board Trends

Higher Speed PartsMore pins per deviceShort life cyclesHigher Reliability Parts

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Test TrendsTest is a commodity

Issues are mature – little difference between vendorsHigher density boards -lack of test probe access Verify assembly process not parts

Screened parts don’t fail (normally)Programmable devices (where to program)Designers using standard Test interfaces

Boundary ScanTree-TestingSPI, I2C, ISP (serial interface protocols)

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Parts Fail Less

Better Controlled Manufacturing ProcessHigher YieldsLess need for testHigher Reliability

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Why Test?

Verification of productBusiness ReputationLow Volumes – less control of processPay Me Now or Pay Me Later

Basic Rule of 10

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Test Considerations

TimeVolumeCostQualityThe earlier a fault is found in manufacturing process the cheaper it is to fix (Rule of 10)

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Modern SMT line – Quality & Efficiency

AOI MDA/ICT•Defective device•Wrong device•Power-Up•Programming

•Shorts•Opens•Lifted lead•Polarity

•Tombstone•Billboard•Skid/Skew•Marking•Insufficient/excessive solder

Mfg Issues

• Repair after solder is difficult and costly

• Not enough board real estate for ICT test points

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Inspection Testing

Solder Paste InspectionAutomated Optical InspectionX-Ray Inspection

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SPI Advantages

No fixture costIn-LineNo componentsBeginning of SMT process

Fast program development (less than 1 hr)

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SPI Defect Coverage

Solder Defects

• Solder bridge

• Insufficient solder

• Excessive solder

•Volume

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SPI Disadvantages

Requires Known Good Board (KGB)Unpopulated board100% success does not guarantee 0% faults

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AOI Advantages

No fixture costIn-Line100% Visual coverage vs humanEarly in SMT process

Fast program development (3 -7 hours)

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AOI Defect Coverage

Component Defects Solder Joint Defects Surface Defects

• Missing component

• Skid/Skew component

• Extra component

• Tomb-stoned component

• Bill-boarded component

• Alternative/Marking

• Lifted/bent lead

• Broken lead

• Solder bridge

• Insufficient solder

• Excessive solder

• Lifted/bent lead

• Broken lead

• Solder ball

• Through-hole pin

• Gold-finger contamination

• Scratch

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AOI Disadvantages

Higher False Failure RatesRequires Known Good Board (KGB)Requires 10-30 board to adjust programOnly Visual (shorts under part not found)BGA pin shorts/opens

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X-Ray Advantages

AOI advantages minus markings and polarity

Non-visible assembly faults BGA pin shorts and opensInner layer shorts and opens

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X-Ray Coverage

AOI coverageBGA pin coverageInterlayer assembly faults

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X-Ray Disadvantages

False Failures More Expensive than “Big Iron” ICT testerRequires KGBProgram development 3-10 days

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Structural Testing

Flying ProbeManufacturing Defect AnalyzerBoundary Scan (virtual connections)X-Ray Inspection

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Flying Probe Advantages

No fixture Physical verification• Basic shorts and opens• Basic R/L/C testing

1 day program development

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Flying Probe Coverage

Shorts & OpensR/L/C testing

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Flying Probe Disadvantages

Long test times up to 30 min Limited DiagnosticsLower quality measurementsPossible to miss some shorts

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MDA AdvantagesLow CostPhysical testing of board• Shorts• Analog• Open Pins (Vectorless & Clamping Diode)

Short test time1-2 Days to programBetter diagnosticsBetter measurements - guarding

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MDA Coverage

Shorts & Opens (Pins and traces)Passive Component Testing• R/L/C , diodes, transistors, fets

Component ValuesVector-less testing (digital presence)

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Open IC Pin Detection

Vector-less Testing

Clamping Diode Testing

Both offer low cost analog

measurement to detect an open IC pin

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MDA Disadvantages

Requires a fixtureUsually requires mid-high volume to justify fixture costs

Requires nodal access1-2 Days to programRequires 20-50 boards for stable program

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ICT Advantages

Most Bang for the Buck Highest Quality TestingMDA test advantagesPowered up testingShort test time (10-60 sec)Digital component testingBest component level diagnostics

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ICT Coverage

MDA coverage Powered up testingDevice programmingBoundary Scan and other industry standard interface testing (I2C, SPI)

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Easy to Use On Board Programming Software Modules

EEPROM Programming (Mac Address)

ISP Programming

SPI Programming

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ICT Disadvantages

Requires a fixtureRequires mid-high volume to justify fixture and program costs

Requires nodal access1-2 weeks to programMore device data requiredRequires 20-50 boards for stable program

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Fault Spectrum vs TesterFaultSolder Height X XSolder Quality X X XBd Markings XComp Placement XIC Pin Shorts X X X X XIC Pin Opens X X X X XTrace Shorts X X X XTrace Opens X X X XMissing Part X X X X XBad Part X X XPart Tolerance X X XTransistors X XPower UP X XProgramming X XBoundary Scan X XECO Version X XAt Speed Verfication X

SPI AOI X-Ray Flying Probe MDA ICT Functional100-250K 100-250K 300K-600K 100-200K 20-50K 100K-500K low to high CostLow-High Low-High Low-Mid Low Low-Mid High Low-High VolumeMinute Minute Minutes > 30 Minutes Minute< Minute Minutes-Hours Test Timehours hours days day day week days-weeks* DevelopmentNo No No No Yes Yes Box/Interface Fixture

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Traditional Test Choice

Big Iron ICT TesterFastest and largest pin count testerMost bang for the buckAlso costs the most bucksNot cost-effective with smaller volumes

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Low Cost Tester Options

MDAMDA with 3rd party (Boundary Scan)Combinational (MDA/ICT/Functional)Stand-alone Boundary ScanUsed Testers

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Pin Card Cost700 net board with 200 pin IC

$70,000$10,0008967328:2128Big Iron

$38,500$3,50070411641:164Combo

$12,000$2,00076861281:1128MDA

Pin Card Cost

Bd CostPins req

Bds req

Real Pins/bd

Mux Ratio

Pins/bdTester

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Pin Card Costs700 net board with 300 pin IC

$100,000$10,000128010328:2128Big Iron

$38,500$3,50070411641:164Combo

$12,000$2,00076861281:1128MDA

Pin Card Cost

Bd CostPins req

Bds req

Real Pins/bd

Mux Ratio

Pins/bdTester

* 1 drive/1 sense

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Software Costs

>$60KAwareMagicShared Wiring

Basic Scan/Scan Pathfinder/ Silicon Nails

Flash 70Opens Xpress/Test Jet

Big Iron

$10KincincFlash libTest JetCombo

$45KNAOption$15K ?

Option$10K ??

Test Jet$10K

MDA

TotalsOtherBScanFlashVector-less

Tester

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Tester Costs700 nets with 300 pin IC

Industry Standard

$260K>$60K$100K$100KBig Iron

Integrated Solutions

$99K>$10K$39K$50KCombo

Multi-vendor$77K$45K$12K$20KMDA

NotesTotalSoftwarePinsBaseTester

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Boundary Scan Issue

More acceptance than everBut most customers don’t implementPC motherboards don’t (Nand Tree)Cell phones have no test padsStill parts without compliant BSDL

Micro-controllersAutomotive industry likes CAN

Its has many versions

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Big Iron vs Low Cost

Big Iron is still the Tried and True standardDo you need 10MHz?Do you need timing sets?

Low Cost is worth investigating if new linesStill need standard driver/sensorsWhat about multiple vendor support

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Distributed TestUse strengths of different types of testAOI for presence/bypass & reverse capsPhysical test for shorts and valuesDigital test for program/verificationNo software talks to all vendors (assy/test)Testers need to be able to turn on/off any test

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Manufacturing Test

Once process is good little goes wrongWhy Test all partsMaybe test changes in processExample: Replacing a reel of partsWhy Test all parts on the reel

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Test SamplingTesters will ability to turn on/off testsRequired for distributed testDrawbacks

QualityCustomer ExpectationsWho takes the blame for a test escape

Could a report even be generated to explain what happened. If its dynamic the conditions changed by the time the fault is found

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Functional Failures

How many bad parts are found?Are the failures structural?Is any particular test better at catching these failures and why didn’t it?Did test introduce any failures?

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False FailuresWhat do False Failures cost?Rules usually state you must replaceAre the parts really bad?Reasons for False Failures

Marginally testFixtures/ProbesAllow retest and fixture recycling

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What will you do?

Same as usualKeep buying “Big Iron”Investigate Low Cost solutionsExplore Distributed Test

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Conclusion

Test is mature and is a commodityBoundary Scan only solutions are not viable if there aren’t any boundary scan partsTest can’t be more flexible with current mind sets –not necessarily a bad thingDistributed Test has a long way to go

More work to be done – mainly software and controlLow Cost solutions are viable and available