on the question of sac solder alloy - cu pad solder joint fragility

On The Question OfOn The Question Of

SAC Solder Alloy SAC Solder Alloy -- Cu Pad Cu Pad

Solder Joint FragilitySolder Joint Fragility

Donald W. HendersonDonald W. HendersonIBM IBM -- East FishkillEast Fishkill

September 2004September 2004

OverviewOverview

SnSn--AgAg--Cu ( SAC ) Alloy Cu ( SAC ) Alloy -- Cu Pad BGA Solder Joints Demonstrate FragilityCu Pad BGA Solder Joints Demonstrate Fragility

NEW Reports In Literature of SAC NEW Reports In Literature of SAC -- Cu Pad Solder Joint EmbrittlementCu Pad Solder Joint EmbrittlementReports Focused On Kirkendall Voiding MechanismReports Focused On Kirkendall Voiding MechanismPresentations At TMS Meeting Presentations At TMS Meeting ---- March 2004March 2004Papers From ECTC Papers From ECTC ---- June 2004 June 2004

Texas Instruments PaperTexas Instruments PaperTzTz--Cheng, et. al.Cheng, et. al.

"Effect of Thermal Aging On Board Level Drop Reliability For Pb"Effect of Thermal Aging On Board Level Drop Reliability For Pb--Free Packages"Free Packages"ECTC (2004) p. 1256ECTC (2004) p. 1256

Hitachi Hitachi -- UCLA PaperUCLA PaperM. Date, et. al. M. Date, et. al. "Impact Reliability Of Solder Joints""Impact Reliability Of Solder Joints"ECTC (2004) ECTC (2004)

Overview Overview (continued)(continued)

Reported Findings Newly Confirmed By IBM Reported Findings Newly Confirmed By IBM ---- In PartIn Part

PreliminaryPreliminary IBM Experimentation Indicates At Least Two Embrittlement MechanIBM Experimentation Indicates At Least Two Embrittlement Mechanismsisms

Kirkendall Voiding Related MechanismKirkendall Voiding Related Mechanism––Focus Of Industry ReportsFocus Of Industry Reports––Gross Interfacial Voiding ... At CuGross Interfacial Voiding ... At Cu33Sn Sn ---- Cu Interfacial Boundary Cu Interfacial Boundary ––Requires Thermal Aging To Induce Weak InterfacesRequires Thermal Aging To Induce Weak Interfaces––Mechanism Not Always Present / Active In Solder Joints (IBM FindMechanism Not Always Present / Active In Solder Joints (IBM Finding)ing)

Embrittlement Mechanism(s) Without Kirkendall Voiding Embrittlement Mechanism(s) Without Kirkendall Voiding ––No Detailed Descriptions In LiteratureNo Detailed Descriptions In Literature––Ball Pull Test Used To Define Behavior ( IBM )Ball Pull Test Used To Define Behavior ( IBM )––Some Embrittlement ( Interfacial Failure ) Experimentally Found Some Embrittlement ( Interfacial Failure ) Experimentally Found At "Time Zero"At "Time Zero"––Brittle Fracture Through Interfacial Intermetallic Compound LayeBrittle Fracture Through Interfacial Intermetallic Compound Layersrs––Effects Made Worse By Thermal Aging Effects Made Worse By Thermal Aging ---- At Least In Early Stages Of AgingAt Least In Early Stages Of Aging

Based On Present IBM Data Based On Present IBM Data -- Neither Mechanism Always OperativeNeither Mechanism Always Operative

Presently => At the Earliest Stages Of Understanding Of Both MecPresently => At the Earliest Stages Of Understanding Of Both Mechanismshanisms

Results Of IBM Testing On Kirkendall Voiding MechanismResults Of IBM Testing On Kirkendall Voiding MechanismBGA Attachment EvaluationBGA Attachment Evaluation

No Significant Kirkendall Voiding Noted In Early IBM InvestigatNo Significant Kirkendall Voiding Noted In Early IBM Investigationsions

New IBM Evaluations Undertaken New IBM Evaluations Undertaken Instigated By New Industry Reports Instigated By New Industry Reports BGA Solder Joint Interfacial Failures In BGA Module Socket TestiBGA Solder Joint Interfacial Failures In BGA Module Socket Testingng

Results From Initial Experiment Results From Initial Experiment -- SAC BGA Testing / EvaluationSAC BGA Testing / EvaluationCommon Plating Bath For All Lots Common Plating Bath For All Lots –– Cu Plating Bath 1 Cu Plating Bath 1 ---- "Stringent" Bath Control"Stringent" Bath ControlHTS Testing => 150 C For 1000 Hrs.HTS Testing => 150 C For 1000 Hrs.---- Used To Define Propensity For VoidingUsed To Define Propensity For VoidingInitial Experiment Initial Experiment –– Cu Plating Bath 1Cu Plating Bath 1Highly Variable Results After HTS TestingHighly Variable Results After HTS Testing

––One (1) Cu Plating Lot Showed Massive Voiding At CuOne (1) Cu Plating Lot Showed Massive Voiding At Cu33Sn Sn ---- Cu InterfacesCu Interfaces8 BGA Rows From 5 Substrates Sectioned For Evaluation8 BGA Rows From 5 Substrates Sectioned For EvaluationEvery BGA Site In All Rows Shows Massive Interfacial VoidingEvery BGA Site In All Rows Shows Massive Interfacial Voiding

––Five Cu Plating Lots Show Very Minimal / Nonexistent VoidingFive Cu Plating Lots Show Very Minimal / Nonexistent VoidingMultiple BGA Rows Sectioned From Each Substrate LotMultiple BGA Rows Sectioned From Each Substrate LotNo Significant Voiding Found On Any BGA Site In Any SubstrateNo Significant Voiding Found On Any BGA Site In Any Substrate

Kirkendall Voiding Embrittlement MechanismKirkendall Voiding Embrittlement MechanismInitial Experiment Initial Experiment -- Plating Bath 1Plating Bath 1

SAC BGA SAC BGA -- Cu Pad Solder JointCu Pad Solder Joint

The Cu PadThe Cu Pad--SAC Interfacial StructureSAC Interfacial StructureCuCuCuCu33Sn Layered InterfaceSn Layered InterfaceCuCu66SnSn55

SAC Solder Alloy SAC Solder Alloy

Test Conditions Test Conditions -- 6 Cu Plating Lots6 Cu Plating LotsAll Lots All Lots -- Plated In Bath 1 Plated In Bath 1 -- Different DatesDifferent DatesSame Plating Current DensitySame Plating Current DensitySame BGA Solder Ball Lot / Flux LotSame BGA Solder Ball Lot / Flux LotSame Oven / Reflow Profile / Assembly DaySame Oven / Reflow Profile / Assembly DaySame HTS Test Chamber Same HTS Test Chamber -- 1000 Hrs / 150 C1000 Hrs / 150 C

Kirkendall VoidingKirkendall VoidingOnly 1 Lot In 6 Showed VoidingOnly 1 Lot In 6 Showed VoidingVoiding Found In CuVoiding Found In Cu33Sn PhaseSn Phase

At CuAt Cu33Sn Sn -- Cu Interface Cu Interface

Results Of IBM Testing On Kirkendall Voiding MechanismResults Of IBM Testing On Kirkendall Voiding Mechanism

Worst Case Section Of BGA Solder Joint Interface After 1000 HouWorst Case Section Of BGA Solder Joint Interface After 1000 Hours At 150 Crs At 150 CCentral Pad RegionCentral Pad RegionVoid Formation At Cu Void Formation At Cu -- CuCu33Sn InterfaceSn Interface

Voiding Levels Low at Pad PeripheryVoiding Levels Low at Pad Periphery


BGA Site Showing Kirkendall Voiding BGA Site Showing Kirkendall Voiding –– Cu Plating Bath 1 Cu Plating Bath 1 -- Initial ExperimentInitial ExperimentCenter BGA Position With Significant / Dense Void FormationCenter BGA Position With Significant / Dense Void FormationPad Periphery Shows Little, If Any, Void FormationPad Periphery Shows Little, If Any, Void Formation


BGA Ball Pull Results On Cu Plating Bath 1 Lot BGA Ball Pull Results On Cu Plating Bath 1 Lot -- Initial Experiment Initial Experiment Demonstrating Severe Kirkendall VoidingDemonstrating Severe Kirkendall Voiding

Results Results ---- Cu Plating Bath 1 Cu Plating Bath 1 ---- Severely Voided BGA InterfacesSeverely Voided BGA Interfaces

3 Failure Modes Found In Ball Pull Test3 Failure Modes Found In Ball Pull Test––Failure Through SolderFailure Through Solder––BGA Solder Pad LiftingBGA Solder Pad Lifting––Interfacial Fracture At Intermetallic Compound Interfacial Fracture At Intermetallic Compound -- Cu Pad InterfaceCu Pad Interface

Approximately 9 % Interfacial Failure Rate In Ball Pull TestApproximately 9 % Interfacial Failure Rate In Ball Pull Test––Results Similar For 700 u/s and 5000 u/s Pull RatesResults Similar For 700 u/s and 5000 u/s Pull Rates––Pull Force Values Dropped To Approximately 1/2 Of Other Failure Pull Force Values Dropped To Approximately 1/2 Of Other Failure ModesModes––"Donut" Shaped Texture On Fracture Surface "Donut" Shaped Texture On Fracture Surface

Probably Reflects Void DistributionProbably Reflects Void DistributionVoid Distribution May Have Prevented Void Distribution May Have Prevented Dramatically Lower Pull Force ValuesDramatically Lower Pull Force Values


In Continued Testing Of Cu Plating Bath 1 ( Described Above) OveIn Continued Testing Of Cu Plating Bath 1 ( Described Above) Over Monthsr MonthsMore Than 30 Plating Lots have Been Evaluated More Than 30 Plating Lots have Been Evaluated –– By HTS Testing By HTS Testing ---- To DateTo DateOnly Two Plating Lots Have Shown Any Propensity For VoidingOnly Two Plating Lots Have Shown Any Propensity For VoidingSecond Plating Lot Second Plating Lot ---- Bath 1 Bath 1 ---- Showing Voided Interface After 500 Hours At 150 CShowing Voided Interface After 500 Hours At 150 C

Note Void Spatial Distribution Differences With First LotNote Void Spatial Distribution Differences With First LotNot All Substrates In Lot Demonstrate Voiding After 500 Hours AtNot All Substrates In Lot Demonstrate Voiding After 500 Hours At 150 C150 C

Voided Region Voided Region Voided RegionVoided Region

Results Of IBM Testing On Kirkendall Voiding MechanismResults Of IBM Testing On Kirkendall Voiding Mechanism

Degree Of Oxidation Degree Of Oxidation -- Before Ball Attachment Before Ball Attachment –– Cu Plating Bath 1Cu Plating Bath 1Oxidized In Air At 150 C For ... 0 / 6 / 12 / 24 / 48 HoursOxidized In Air At 150 C For ... 0 / 6 / 12 / 24 / 48 HoursAt Longer Times Cu Pad Were Very Badly OxidizedAt Longer Times Cu Pad Were Very Badly OxidizedOxidation Had No Influence On Void FormationOxidation Had No Influence On Void Formation

Second Experimental Plating Bath Evaluation Second Experimental Plating Bath Evaluation –– Cu Plating Bath 2Cu Plating Bath 25 Plating Lots Examined, Experimentally5 Plating Lots Examined, ExperimentallyHTS Testing HTS Testing ---- 150 C For 1000 Hours150 C For 1000 HoursAll 5 Experimental Plating Lots Demonstrated Very Significant KiAll 5 Experimental Plating Lots Demonstrated Very Significant Kirkendall Voidingrkendall Voiding

Preliminary Experiments With Sputtered Cu Pad StructuresPreliminary Experiments With Sputtered Cu Pad StructuresHTS Testing For 1000 Hrs. At 150 CHTS Testing For 1000 Hrs. At 150 CNo No KirkendallKirkendall Voiding Found After HTS TestingVoiding Found After HTS Testing

Empirical Findings, To Date, On Kirkendall Voiding Phenonenon Empirical Findings, To Date, On Kirkendall Voiding Phenonenon ------ Strongly Indicate:Strongly Indicate:Not All Plated Cu Is Subject To Kirkendall Voiding PhenomenonNot All Plated Cu Is Subject To Kirkendall Voiding PhenomenonCu Plating Microstructure / Composition Probably Controls PhenonCu Plating Microstructure / Composition Probably Controls PhenonenonenonDay To Day "Condition" Of Plating Bath Day To Day "Condition" Of Plating Bath May Control Cu Microstructure / CompositionMay Control Cu Microstructure / Composition

IBM Identification Of IBM Identification Of NonNon--Kirkendall Voiding ProblemKirkendall Voiding ProblemBGA Attachment EvaluationBGA Attachment Evaluation

Failed BGA SiteFailed BGA SiteInterfacial FractureInterfacial Fracture

Occurred During Socket TestOccurred During Socket TestNext 3 Foils Show Additional F/ANext 3 Foils Show Additional F/A

On This Test ModuleOn This Test Module

IBM Identification Of NonIBM Identification Of Non--Kirkendall Voiding Problem Kirkendall Voiding Problem BGA Attachment Evaluation BGA Attachment Evaluation

SEM Image Of Failed PadSEM Image Of Failed Pad

Fracture Thru IMC LayersFracture Thru IMC Layers

EDX Analysis Of EDX Analysis Of Fracture SurfaceFracture Surface

Shown BelowShown Below

IBM Identification Of NonIBM Identification Of Non--Kirkendall Voiding ProblemKirkendall Voiding ProblemBGA Attachment Evaluation BGA Attachment Evaluation

FIB Section Thru Pad SurfaceFIB Section Thru Pad SurfaceAt Pad CenterAt Pad CenterMicrograph BelowMicrograph Below

BSE SEM Image Shows BSE SEM Image Shows Contrasting Layers => Cu/CuContrasting Layers => Cu/Cu33Sn/CuSn/Cu66SnSn55

Only 2 Small Voids Found Only 2 Small Voids Found At Cu At Cu -- CuCu33Sn Interface In FIB SectionSn Interface In FIB Section

IBM Identification Of NonIBM Identification Of Non--Kirkendall Voiding ProblemKirkendall Voiding ProblemBGA Attachment EvaluationBGA Attachment Evaluation

Failed Solder Joint Shown BelowFailed Solder Joint Shown BelowSocket Test FailureSocket Test FailureFracture Near CuFracture Near Cu--CuCu33Sn InterfaceSn Interface

Neighboring Solder JointNeighboring Solder JointShown At RightShown At Right

Note Consumption Of CuNote Consumption Of CuBy Intermetallic Compound StructuresBy Intermetallic Compound Structures

Mechanical Testing Of Solder Ball ArraysMechanical Testing Of Solder Ball Arrays

BGA Solder Ball Shear Test BGA Solder Ball Shear Test

Industry StandardIndustry StandardSolder Ball Pad Interface Submitted To Both Shear and Tensile Solder Ball Pad Interface Submitted To Both Shear and Tensile StressesStressesNo Problems Identified With This Test Method For SAC BGA'sNo Problems Identified With This Test Method For SAC BGA's––High Shear ValuesHigh Shear Values––Shear Values Comparable to SnShear Values Comparable to Sn--PbPb––Ball Failure => Ductile Fracture Through SolderBall Failure => Ductile Fracture Through Solder––Finding Consistent With Industry ReportsFinding Consistent With Industry Reports

Mechanical Testing Of Solder Ball ArraysMechanical Testing Of Solder Ball Arrays(continued)(continued)

BGA Solder Ball Pull TestBGA Solder Ball Pull Test

Somewhat Less Standard Test In The IndustrySomewhat Less Standard Test In The IndustryBoth Solder Ball and Ball Both Solder Ball and Ball -- Pad Interface Submitted To Tensile StressesPad Interface Submitted To Tensile StressesInterfacial Tensile Stresses Higher Than In Shear TestInterfacial Tensile Stresses Higher Than In Shear TestMixed Modes Of Failure Found In Pull TestMixed Modes Of Failure Found In Pull Test––Ductile Failure Through SolderDuctile Failure Through Solder––Laminate Solder Pad Pull Out Laminate Solder Pad Pull Out ––Interfacial Fracture Through Intermetallic CompoundInterfacial Fracture Through Intermetallic Compound

NonNon--Kirkendall Voiding MechanismKirkendall Voiding Mechanism

––Found With Higher Frequency Than Kirkendall Voiding MechanismFound With Higher Frequency Than Kirkendall Voiding Mechanism––Approximately 2/3 Of Lots Show This Type Of Embrittlement Approximately 2/3 Of Lots Show This Type Of Embrittlement –– Cu Plating Bath 1Cu Plating Bath 1––In The Absence Of Kirkendall Voiding In The Absence Of Kirkendall Voiding –– Cu Plating Bath 1Cu Plating Bath 1

0 to 5 % Interfacial Failures After Module Assembly0 to 5 % Interfacial Failures After Module Assembly5 to 10 % Interfacial Failures After 2X Additional Reflows5 to 10 % Interfacial Failures After 2X Additional ReflowsUp To 50+ % Interfacial Failures Up To 50+ % Interfacial Failures

After 72 Hr / 140 C BurnAfter 72 Hr / 140 C Burn--In + 2X Reflows At 260 C PeakIn + 2X Reflows At 260 C Peak––Brittle Interfacial Fracture Occurs Without Significant Pull ForBrittle Interfacial Fracture Occurs Without Significant Pull Force Reductionce Reduction––SnSn--Pb Eutectic Solder BallsPb Eutectic Solder Balls DO NOTDO NOT Demonstrate Interfacial FractureDemonstrate Interfacial Fracture––Ball Pull Force Levels For SAC and SnBall Pull Force Levels For SAC and Sn--Pb Eutectic Alloys Overlap. Pb Eutectic Alloys Overlap.

SAC Interfacial Intermetallic Compounds Same As In SnSAC Interfacial Intermetallic Compounds Same As In Sn--Pb SystemPb SystemIntermetallic Compound Consumption Of Cu Intermetallic Compound Consumption Of Cu -- Greater For SAC Alloy Greater For SAC Alloy

Pull Testing Of Solder Ball ArraysPull Testing Of Solder Ball Arrays

SAC SAC -- Cu Pad Solder Joint EmbrittlementCu Pad Solder Joint Embrittlement

Is It Inevitable ?Is It Inevitable ?

Some Lots Show Little Or No Embrittlement After HTS Testing Some Lots Show Little Or No Embrittlement After HTS Testing –– Cu Plating Bath 1Cu Plating Bath 1

Neither Embrittlement Mechanism Manifested Neither Embrittlement Mechanism Manifested ––Interfacial Voids Not PresentInterfacial Voids Not Present––No Interfacial Failures In Pull TestingNo Interfacial Failures In Pull Testing

NonNon--Voiding Embrittlement MechanismVoiding Embrittlement MechanismAnalytical Evaluation Of Fracture SurfacesAnalytical Evaluation Of Fracture Surfaces

Experimental Analytical / Failure Analysis ResultsExperimental Analytical / Failure Analysis Results

No Significant No Significant PreliminaryPreliminary Findings, Using TEM, EMP And Auger EMFindings, Using TEM, EMP And Auger EM––Comparison Of Voided And NonComparison Of Voided And Non--Voided Samples Voided Samples ––"Edge On" Evaluation Of Interface"Edge On" Evaluation Of Interface

Auger EM Evaluation Of Interfacially Failed BGA Site Auger EM Evaluation Of Interfacially Failed BGA Site

––Original Socket Failure Module => Selected For Evaluation Original Socket Failure Module => Selected For Evaluation ––BGA Solder Joint Fractured BGA Solder Joint Fractured Immediately Before Auger EM EvaluationImmediately Before Auger EM Evaluation––Carbon Found At Fracture Surface Carbon Found At Fracture Surface ---- Approximately 30 % Area FractionApproximately 30 % Area Fraction––Carbonaceous Material In Electrolytic Cu Plating Carbonaceous Material In Electrolytic Cu Plating Suspected As Origin Suspected As Origin ––"Clean" Experiment "Clean" Experiment ---- Little Chance For Experimental ArtifactsLittle Chance For Experimental Artifacts––May Be Very Significant FindingMay Be Very Significant Finding––Generality Of Finding Generality Of Finding ---- Yet, To Be DeterminedYet, To Be Determined

Auger Surface Analysis Of Fractured BGA Pad Surface Auger Surface Analysis Of Fractured BGA Pad Surface

BGA Solder Ball Removed By Fracture Prior To Auger EM EvaluationBGA Solder Ball Removed By Fracture Prior To Auger EM Evaluation

Low Magnification View Low Magnification View High Magnification ViewHigh Magnification ViewTweezer Removal Witness Mark EvidentTweezer Removal Witness Mark Evident

Auger Surface Analysis Of Fractured BGA Pad SurfaceAuger Surface Analysis Of Fractured BGA Pad SurfaceElemental Map Of Fractured Pad SurfaceElemental Map Of Fractured Pad Surface

Sn = BlueSn = BlueCu+Sn = GreenCu+Sn = GreenCarbon = RedCarbon = Red

Summary Of Findings To DateSummary Of Findings To DateTwo SAC Alloy Two SAC Alloy –– Plated Cu Pad Embrittlement MechanismsPlated Cu Pad Embrittlement Mechanisms

Kirkendall Voiding Embrittlement MechanismKirkendall Voiding Embrittlement Mechanism––Elevated Temperatures / Long TimesElevated Temperatures / Long Times––Significant Interfacial Degradation / Low Ball Pull Force Is PosSignificant Interfacial Degradation / Low Ball Pull Force Is Possiblesible––May Be Reliability Problem In Some ApplicationsMay Be Reliability Problem In Some Applications––May Be A "Controllable" PhenomenonMay Be A "Controllable" Phenomenon

Embrittlement In the Absence Of Kirkendall VoidingEmbrittlement In the Absence Of Kirkendall Voiding––Characterized By Interfacial Failures In Ball Pull TestCharacterized By Interfacial Failures In Ball Pull Test––Can Be Manifested Immediately After Solder Ball AttachmentCan Be Manifested Immediately After Solder Ball Attachment––Increase In Interfacial Failures With Thermal Aging Increase In Interfacial Failures With Thermal Aging -- TypicalTypical––No Solder Ball Pull Force Reduction, Even With Long Aging TimesNo Solder Ball Pull Force Reduction, Even With Long Aging Times––Not Yet Clear Whether This Embrittlement Mechanism Is A SignificNot Yet Clear Whether This Embrittlement Mechanism Is A Significant ant Reliability ConcernReliability Concern

––Most Plating Lots From Cu Plating Bath 1 Show Some Level Of ThiMost Plating Lots From Cu Plating Bath 1 Show Some Level Of This s Embrittlement MechanismEmbrittlement Mechanism

Some Plating Lots From Cu Plating Bath 1 Show No Or Little IndicSome Plating Lots From Cu Plating Bath 1 Show No Or Little Indication Of ation Of Embrittlement After HTS Testing Embrittlement After HTS Testing ---- As Manifested In Pull TestAs Manifested In Pull Test

on the question of sac solder alloy - cu pad solder joint fragility

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