Area Array Research Consortium Lead-Free Soldering Program

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Area Array Research ConsortiumLead-Free Soldering ProgramArea Array Research ConsortiumLead-Free Soldering Program

Anthony Primavera - Program ManagerMichael Meilunas - ReliabilityMark Dunlap - AssemblyUniversal Instruments CorporationSMT LABORATORY

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Consortium Research TopicsConsortium Research Topics

Lead Free Materials Evaluation

Wetting of Lead Free Solder

Alternative PCB Pad Finish

Environmental Aging / Process & Storage Conditions

Assembly

Effects of Reflow Profile

Self Centering / Pull back / Solderballing etc.

Mixing of Alloys & Contamination

Reliability Testing

Commercially Available Component

Fabrication of Test Vehicles

Mechanical & Thermal Cycle Tests

Determination of Solder Properties & Crack Growth Rate

FEM Modeling

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Reliability Testing TimelineReliability Testing Timeline

Year 2000 Consortium

Few Commercial Components Available

Lead Free CSP/BGAs In Development

Designed & Fabricated Generic Test Vehicles (Early 2000)

Initial Testing Program I Begins (Mid 2000)

Accelerated Thermal Cycle Testing of Lead Free BGAs

Presentation of Initial Test Results for ATC (Late 2000)


Follow on ATC Program II for Generic Lead Free BGAs

Testing of Several Commercial Lead Free CSP/BGAs

Development & Test of Generic Lead Free CSPs


Wrap up of ATC Program II

Follow on ATC Program III - Commercial & Generic BGAs

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Early Results (Year 2000)Early Results (Year 2000)

Accelerated Thermal Cycle

0-100oC

20 min (5 min ramp & dwell)

Continuous event detection

Identification of electrical events

Events not consistent with Sn/Pb solder fatigue ATC event detection

Identification of failed joint difficult

Events sporadic throughout testing

Crack / Fatigue

Sn/Pb baseline components failed by typical fatigue cracking

lead-free solder joints have “unique shattered appearance”

Lead-free cracks are multidirectional and are often very fine

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Glitch or Electrical EventGlitch or Electrical EventA glitch or event is defined as a false or spurious electronic signal.

BGA devices that were continuously monitored while in thermal cycle displayed resistance “spikes” that sporadically occurred over several hundred cycles. These “spikes” were observed in Sn/Ag and Sn/Ag/Cu solder joints with nickel layers on both the component and printed circuit board pads.

CuNi

NiCu

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Test Vehicle: FCPBGATest Vehicle: FCPBGA

• 256 I/O Daisy Chain• Utilizes Electroless Ni/ Immersion

Au Pad Finish• 0.030” Diameter Spheres• 0.003” Encapsulant • 0.016” Laminate Substrate• 0.030” Glass Die• CTE of 11.1ppm/°C

(Moiré Interferometry)

Assembled to 0.062” FR-4 Based PCBs

Encapsulant Layer

Glass Die

Solder Bumps

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Generic Flip Chip PBGA (FCPBGA) Test Package

ENCAPSULANT DISPENSE ON SUBSTRATE(Encapsulant used : Namics 8437-2)

COPPER PAD ATTACHMENT (Copper Pads - 3 mils thick; used to obtain an uniform standoff)

BUMPED SUBSTRATE ( 30 mil Dia Solder Sphere, 1.1”Square Substrate with 256 BGA pattern)

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GLASS PLACEMENT : Use of Glass Placement Fixture to center the die on the Substrate

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Unbalanced Component

Balanced Component

Component utilized in ATC TestingBoth In-plane and out-of-planedeformation

Component utilized in Crack Growth Rate Study.Predominately in-plane deformationSamples tested every N cycles in ATC, then subjectto dye penetration testingGoal: correlation of crack area with number of thermal cycles

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Assembly of Test VehiclesAssembly of Test Vehicles

Consortium CSPTB2

4 Layer Construction: 2 Signal, 2 gnd

Tetrafunctional FR4 170oC Tg

Taiyo Prs4000 Mask

0.062” Nominal Thickness

2 Assembly Sites

21 Mil Circular Pads

26 Mil Mask Opening

23 Mil Circular Pads

28 Mil Mask Opening

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Assembly of Test Vehicles CSPTB2Assembly of Test Vehicles CSPTB2

Solder paste stencil

5 mil thick stainless steel

Laser cut apertures, approximately 0.5 mil taper

Circular apertures

21 mil circular pads

22.2 mil mean, 0.2 mil std dev. (top side)

22.8 mil mean, 0.2 mil std dev. (bottom side)

Aperture Volume = 2005 cubic mils

Solder Volume = 14,932 cubic mils






0.03” Sphere14,130 cubic mils

&50% solidifiedpaste volume

Approximately80% transfer

Vcone = 1/3h(r12 +r2

2 + r1r2)

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Experimental Test Matrix 1Experimental Test Matrix 1

Solder AlloyPCB

FinishSample

SizeCu OSP 8

Ni/Pd 8Ni/Au 8

Cu OSP 8Ni/Pd 8Ni/Au 8



Sn/Ag (96.5/3.5)

Sn/Ag/Cu (95.5/3.8/0.7)

Sn/Ag/Cu/Sb (96.2/2.5/0.8/0.5)

Sn/Pb (63/37)

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Test EquipmentTest Equipment

Thermotron ATS 150

Two Zone Thermal Shock Chamber

Thermal Cycle: 0 to 100°C

5 Minute Ramps

5 Minute Dwells

Anatech 256 Event Detector

Resistance Monitor

1.2mA Current (Fixed)

300 Threshold (UIC Setting)

200nS Sample Frequency (Fixed)

PC

Data Logging

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Failure CriteriaFailure Criteria

IPC-SM-785 Section 7.8:

“Failure is defined as the first interruption of electrical continuity that is

confirmed by 9 additional interruptions within an additional 10% of the cyclic life.”

Event: • Electrical continuity interruption. Daisy chain loop resistance exceeds:• 300 , for >200nanoseconds UIC Criteria• 1000 for >1microsecond - IPC Criteria

• Maximum 15 events per cycle. The frequency of the events per cycle

increases as solder fracture increases.

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Test Results (Matrix 1)Test Results (Matrix 1)

Thermal cycling with no events until ~2700 cycles.

Lead free FCPBGA packages began to demonstrate “Failure” characteristics.

The samples were electrically continuous when tested with multimeter probe.

Samples were cross sectioned or subject to a dye penetration test.

• Fracture surfaces were minimal.

The remaining samples were returned to thermal cycling.

The electrical events were continued sporadically.

Sn/Pb packages exhibited no failures.

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Onset of Glitch vs. Confirmed OpenOnset of Glitch vs. Confirmed Open

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Failure AnalysisFailure Analysis

Cross Sections of Glitch Failures

Extremely fine cracks

Incomplete Cracking

Sn/Ag Alloy on Ni/Pd PCB3500 Thermal Cycles

Sn/Ag/Cu on Ni/Pd3500 Cycles

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Test Result VerificationTest Result Verification

Noise Control:

– New ground wires to the chamber, Anatech and

PC were installed.

– Anatech and PC wire shielding was replaced.

– Placement within the chamber was rearranged.

– Packages were firmly taped in place to minimize vibration.

– Problem persisted.

Manual Monitoring:

– Visually (continuously) monitored Anatech output.

– Noted that events occurred after basket

transitions during the ramp up phase.

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Test Result VerificationTest Result Verification

Glitches Identified in 4 Groups:• Sn/Ag/Cu on Ni/Au PCB• Sn/Ag on Ni/Au PCB • Sn/Ag on Ni/Pd PCB• Sn/Ag/Cu on Ni/Pd PCB

Apparent Trends: • All packages were bumped with Ag bearing alloys• Both PCB finishes contained Electroless Nickel• Events occurred in the ramp up phase• Sn/Pb samples showed no glitch / events• Sample assembled on Cu OSP pads showed no glitches

Samples Pulled from Test: • Sn/Ag on Ni/Pd (4)• Sn/Ag/Cu on Ni/Au (4)• Sn/Ag on Ni/Au (3)• Sn/Ag/Cu on Ni/Pd (1)

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Confirmation ExperimentsConfirmation Experiments

Reworked Samples• 29 reworked samples submitted for reliability testing• 26 packages utilized a Cu OSP pad finish• 3 utilized Sn/Ag Alloy on Ni/Pd pad finish• Glitch detected in 2 of the Sn/Ag on Ni/Pd samples• Occurred between 2000 and 2250 cycles

Tests were Conducted on Existing Samples:• High temperature Probe• Oscilloscope monitoring• Resistance recording• Circuit reliability test• Elemental analysis

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Characterization ExperimentsCharacterization Experiments

High Temperature Probe• Sample set on a hotplate (115°C)• Standard multimeter used• No opens located• Resistance changes noted

AmbientHot

(Max)Hot

(Final)4.05 4.71 4.717.72 15.26 10.587.55 13.38 9.553.00 24.10 5.712.82 19.23 4.934.63 6.20 6.205.62 27.89 7.24

Ni/PdSn/Ag

Ni/Au

Resistance (Ohms)Solder Alloy

PCB Finish

Ni/AuSn/Ag/Cu

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Characterization ExperimentsCharacterization ExperimentsOscilloscope• Sample set on a hotplate (115°C)• Constant 10mA current applied• Continuous data of Voltage vs. Time• Glitch detected

Sn/Ag on Ni/Pd (3500 Cycles)

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Characterization ExperimentsCharacterization ExperimentsResistance Recording• Sample set on a hotplate (115°C)• Source Meter (10mA, 40mS Freq.)• Resistance spikes noted

•Measured Events

Resistance spikes as small as 5-10 Duration on order of nanoseconds.

Resistance vs. Time Sn/Ag on Ni/Pd

5

10

15

20

25

Time (sec)

0100 25050 150 200

Ohms

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Characterization ExperimentsCharacterization Experiments

Circuit Reliability Tester • Detects line reduction (Cracks)• Hotplate used to heat packages• Failures located• Corner solder joints

SEM Analysis• Cross sectioned to failure locations • Performed elemental analysis

Glitch Sample : Sn/Ag on Ni/Pd - 3500 Cycles

Full OpenSn/Ag on Ni/Pd

8000 Cycles

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Lead Free Crack Propagation TestLead Free Crack Propagation Test

• Balanced Construction• Samples Subjected to 0-100C ATC• 20 min test 5 min ramp and dwell• Samples pulled from test at fixed times• Samples subjected to dye penetration test

Goal: • Measure crack growth rate for lead free alloys

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Failure Analysis: Dye Penetration TestingFailure Analysis: Dye Penetration Testing

Immerse in dyePlace in vacuum (>/= 9 in Hg) for 1 minute then allow to soak for 1 hour

Dry dye at 100 C for30 minutes

Separate component by twisting of PWB

Typical Sn/Pb Failure

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Lead Free Crack GrowthLead Free Crack Growth

Results:

• No clear growth rate measured• Dye failed to fully penetrate cracks• Cracking fundamentally “different”

than Sn/Pb fatigue cracks

Sn/Ag

Sn/Ag/Cu Sn/Ag/Cu/Sb

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Test Matrix 2 - Year 2001 follow onTest Matrix 2 - Year 2001 follow on

Component Board IBM Nokia Motorola Rockwell Universal TotalNi/Au Ni/Au 16 16 16 0 32 80Ni/Au Cu-OSP 16 16 8 0 8 48Ni/Au ImmAg 0 0 0 32 0 32

Cu-OSP Cu-OSP 8 0 0 0 0 8Cu-OSP Ni/Au 16 0 0 0 0 16

Ni/Au Ni/Au 32 32 16 0 32 112Ni/Au Cu-OSP 32 32 8 0 8 80Ni/Au ImmAg 0 0 0 32 0 32

Cu-OSP Cu-OSP 24 0 0 0 0 24

Ni/Au Ni/Au 32 32 16 0 29 109Ni/Au Cu-OSP 32 32 8 0 8 80Ni/Au ImmAg 0 0 0 32 0 32

Cu-OSP ImmAg 0 0 0 8 0 8

Cu-OSP Ni/Au 16 0 0 0 0 16

Ni/Au Ni/Au 16 16 8 0 40 80Ni/Au Cu-OSP 16 16 8 0 24 64

Total 256 192 88 104 181 821Boards Req. 32 24 11 13 23 103

Sn/Ag

Sn/Ag/Cu

Sn/Ag/Cu/Sb

Component Sample Size

Sn/Pb

A focus group was initiated to continue evaluation of lead free BGAs in a “round robin” set of testsUIC, IBM, Nokia, Motorola, Rockwell-Collins

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Test GoalsTest Goals

Confirmation of Test 1 results

Evaluation of Lead-free BGAs under different ATC conditions

Evaluation of Alternative PCB finishes

Test Differences

Round 1

NSMD Component Pads

1 ATC cycle (0-100C 20 min)

30 mil thick glass die (more warpage?)

PCB pad finish - OSP, Ni/Au, Ni/Pd

Round 2

SMD Component Pads

Different Thermal Cycles

40 mil thick glass die

PCB pad finish - OSP, Ni/Au, Imm Ag

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Round 2Round 2

ACT Test Conditions:

Continuous Event Detection

300 Resistance Threshold, 200 nanoseconds

Universal 0-100C, 20 min : 5 min ramp and dwell

IBM 0-100C, 32 minute: 7.5 min ramp, 8 min dwell

Motorola 0-100C, 30 minute: 10 min ramp , 5 min dwell

Nokia -40-125C, 60 minute: 15 min ramp and dwell

Rockwell Collins -55-125C, 71 minute: approximately 15 min dwell*

Assembly:

UIC fabricated all test components & provided solder ball bumping

UIC supplied PCBs, components and solder paste to each participant

Each participant assembled the component to PCB (UIC TB6)

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Assembly of Test VehiclesAssembly of Test Vehicles

Consortium CSPTB6

4 Layer construction: 2 Signal, 2 gnd

Tetrafunctional FR4 175oC Tg

Taiyo Prs4000 mask

0.062” nominal thickness

Assembly locations


2 sites 25 mil mask opening





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Assembly of Test Vehicle CSPTB6Assembly of Test Vehicle CSPTB6Solder paste stencil - Square apertures

Laser cut 5 mil thick stainless steel














Aperture Volume = 2798

Solder Volume = 15,389

0.03” Sphere14,130 cubic mils

& 50% solidifiedpaste volume

Approximately90% transfer

Vtrap = h(l1w1 + l2w2)/2

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Assembly of Test Vehicle CSPTB6Assembly of Test Vehicle CSPTB6

UIC •Several components showed underfill to die

delamination, due to insufficient pre-bake.•Several “Ni/Au” component early failures

Nokia•Voids observed in Sn/Ag/Cu/Sb samples•Several “Ni/Au” component pad early failures

Rockwell-Collins •Sn/Ag/Cu paste utilized in all combinations.

IBM •Several “Ni/Au” component pad early failures

Motorola•Several “Ni/Au” component pad early failures

Assembly Results

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General Round 2 InformationGeneral Round 2 InformationTesting completed “substantial”

number of cycles at each location

UIC > 16,700 cycles

Nokia > 7,000

Motorola > 13,700

Rockwell > 2,000

IBM > 13,500

Cracking - Fine in appearance,

multiple crack fronts and paths

Failure time - earlier on Ni/Au

than OSP for some test cells.

“Glitch behavior” noted in

several lead free BGAs with Ni/Au pads

SAC on ImmAg

SA on ImmAg

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Failure on ImmAg - Rockwell CollinsFailure on ImmAg - Rockwell Collins

-55 to 125C ATC

Failure near Cu-Sn intermetallic layer

High amounts of Sn-Ag near cracks

Cracks “go around” Sn-Ag plates

Micro-voiding observed near crack

SnAg on ImmAg 1528 cycles

SnAgCu on ImmAg 1544 cycles

Cu-Sn

Sn-Ag

MicroVoids

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Confirmation of Glitch Behavior?Confirmation of Glitch Behavior?“Glitch behavior” - Sporadic events for >100’s of cycles

Motorola

2 samples Sn/Ag/Cu on Ni/Au pads

UIC (note most samples pulled from test after first events)

2 samples Sn/Ag/Cu on Ni/Au pads

1 sample Sn/Ag/Cu/Sb on Ni/Au

Nokia

1 sample Sn/Ag/Cu on Ni/Au pads

1 sample Sn/Ag on Ni/Au pads

1 sample Sn/Ag on OSP pads

2 samples Sn/Ag/Cu/Sb on Ni/Au pads

1 sample Sn/Ag/Cu/Sb on OSP

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Other Lead Free Testing - Generic FCCSPOther Lead Free Testing - Generic FCCSP

Generic CSP size package

0.8 mm pitch 64 I/O

Similar construction to generic BGA

0.02” lead free & Sn/Pb solder bumps

Assembled on OSP, Ni/Au and ImmAg PCBs

0-100C ATC testing

Mixed assembly (ie. SAC bump & Sn/Pb paste)

Samples through 8,000 cycles

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FCCSP Test ResultsFCCSP Test Results Lead-free solder samples show

similar crack behavior to BGA testing

Mixed samples showed some failures

at Ni layer

Sn/Ag/Cu/Sb on ImmAg

Cu-Sn intermetallic

Ag-Sn intermetallic

Sn/Ag/Cu/Sb/Pb mixed on Ni/Au

Early Fail (<2000 cycles)Near Ni layer

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Generic FCCSP Test ResultsGeneric FCCSP Test Results

Sn/Ag/Cu on ImmAgGood joint approximately

2500 cycles

Cu-Sn

Ag-Sn

MicroVoids

Early Failure on ImmAgIn general ATC - OSP> NiAuSimilar to BGA data on ImmAgFCCSP Data - 7,000 cyclesFailure near and aroundintermetallic layersPresence of micro voids

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FCCSP ACT Results 0-100CFCCSP ACT Results 0-100C

Predicted OSP failure with B=5

OSP Data Setnot enough failsfor accurate plot

Area Array Research Consortium Lead-Free Soldering Program

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