Area Array Research Consortium
Lead-Free Soldering Program
Anthony Primavera - Program Manager
Michael Meilunas - Reliability
Mark Dunlap - Assembly
Universal Instruments Corporation
SMT LABORATORY
1
Consortium 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
2
Reliability 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)
Year 2001 Consortium
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
Year 2002 Consortium
Wrap up of ATC Program II
Follow on ATC Program III - Commercial & Generic BGAs
3
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
4
Glitch or Electrical Event
A 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.
Cu
Ni
Ni
Cu
5
Test 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)
Encapsulant Layer Solder Bumps
Assembled to 0.062” FR-4 Based PCBs
6
Glass Die
Generic Flip Chip PBGA (FCPBGA) Test Package
BUMPED SUBSTRATE ( 30 mil Dia Solder Sphere,
1.1”Square Substrate with 256 BGA pattern)
COPPER PAD ATTACHMENT (Copper Pads - 3 mils
thick; used to obtain an uniform standoff)
7
ENCAPSULANT DISPENSE ON SUBSTRATE
(Encapsulant used : Namics 8437-2)
Generic Flip Chip PBGA (FCPBGA) Test Package
GLASS PLACEMENT : Use of Glass Placement
Fixture to center the die on the Substrate
8
Generic Flip Chip PBGA (FCPBGA) Test Package
Unbalanced Component
Component utilized in
ATC Testing
Both In-plane and out-of-plane
deformation
Balanced Component
Component utilized in
Crack Growth Rate Study.
Predominately in-plane deformation
Samples tested every N
cycles in ATC, then subject
to dye penetration testing
Goal: correlation of crack
area with number of thermal cycles
9
Assembly 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
10
Assembly of Test Vehicles CSPTB2
0.03” Sphere
14,130 cubic mils
Approximately
80% transfer
&50%
solidified
paste volume
Vcone = 1/3h(r12 +r22 + r1r2)
11
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
23 mil circular pads
24.2 mil mean, 0.2 mil std dev. (top side)
24.6 mil mean, 0.3 mil std dev. (bottom side)
Aperture Volume = 2337 cubic mils
Solder Volume = 15,065 cubic mils
Experimental Test Matrix 1
Solder Alloy
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)
12
PCB
Finish
Cu OSP
Ni/Pd
Ni/Au
Cu OSP
Ni/Pd
Ni/Au
Cu OSP
Ni/Pd
Ni/Au
Cu OSP
Ni/Pd
Ni/Au
Sample
Size
8
8
8
8
8
8
8
8
8
8
8
8
Test Equipment
UIC THERMAL PROFILE 10/16/00
120.00
100.00
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
Temperature (Deg. C)
80.00
60.00
Series2
Series4
40.00
20.00
-20.00
13
Time (minutes)
38
36
34
32
30
28
26
24
22
20
18
16
14
12
8
10
6
4
2
0
0.00
Failure 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.
14
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.
15
Onset of Glitch vs. Confirmed Open
16
Failure Analysis
Cross Sections of Glitch Failures
Extremely fine cracks
Incomplete Cracking
Sn/Ag/Cu on Ni/Pd
3500 Cycles
17
Sn/Ag Alloy on Ni/Pd PCB
3500 Thermal Cycles
Test 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.
18
Test 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)
19
Confirmation 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
20
Characterization Experiments
High Temperature Probe
• Sample set on a hotplate (115°C)
• Standard multimeter used
• No opens located
• Resistance changes noted
Solder
Alloy
PCB
Finish
Sn/Ag/Cu
Ni/Au
Ni/Pd
Sn/Ag
Ni/Au
21
Resistance (Ohms)
Hot
Hot
Ambient
(Max) (Final)
4.05
4.71
4.71
7.72
15.26
10.58
7.55
13.38
9.55
3.00
24.10
5.71
2.82
19.23
4.93
4.63
6.20
6.20
5.62
27.89
7.24
Characterization Experiments
Oscilloscope
• 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)
22
Characterization Experiments
Resistance Recording
• Sample set on a hotplate (115°C)
• Source Meter (10mA, 40mS Freq.)
• Resistance spikes noted
Sn/Ag on Ni/Pd
25
25
Resistance vs. Time
Sn/Ag on Ni/Pd
•Measured Events
Resistance spikes as small as 5-10  20
Duration on order of nanoseconds.
15
Resistance (Ohms)
20
15
Ohms 10
Time (mS)
Time (sec)
23
280505
272003
250
263502
255001
246514
238013
229512
221011
200
212510
204009
195508
187007
178506
170005
150
161504
153003
144502
136001
132300
128769
110512
93510
100
102011
85009
76508
68007
59506
50
51005
42504
34003
25502
17001
0
13
0
5
8513
5
10
Characterization 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 Open
Sn/Ag on Ni/Pd
8000 Cycles
24
Lead 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
25
Failure Analysis: Dye Penetration Testing
Immerse in dye
Dry dye at 100 C for
30 minutes
26
Place in vacuum (>/= 9 in Hg) for
1 minute then allow to soak for 1 hour
Separate component
by twisting of PWB
Typical Sn/Pb Failure
Lead Free Crack Growth
Sn/Ag
Results:
• No clear growth rate measured
• Dye failed to fully penetrate cracks
• Cracking fundamentally “different”
than Sn/Pb fatigue cracks
Sn/Ag/Cu
27
Sn/Ag/Cu/Sb
Test Matrix 2 - Year 2001 follow on
A focus group was initiated to continue evaluation
of lead free BGAs in a “round robin” set of tests
UIC, IBM, Nokia, Motorola, Rockwell-Collins
Board
Ni/Au
Cu-OSP
ImmAg
Cu-OSP
Ni/Au
IBM
16
16
0
8
16
Sn/Ag
Ni/Au
Ni/Au
Ni/Au
Cu-OSP
Ni/Au
Cu-OSP
ImmAg
Cu-OSP
32
32
0
24
32
32
0
0
16
8
0
0
0
0
32
0
32
8
0
0
112
80
32
24
Sn/Ag/Cu
Ni/Au
Ni/Au
Ni/Au
Cu-OSP
Cu-OSP
Ni/Au
Cu-OSP
ImmAg
ImmAg
Ni/Au
32
32
0
0
16
32
32
0
0
0
16
8
0
0
0
0
0
32
8
0
29
8
0
0
0
109
80
32
8
16
Sn/Ag/Cu/Sb
Ni/Au
Ni/Au
Ni/Au
Cu-OSP
16
16
16
16
8
8
0
0
40
24
80
64
Total
Boards Req.
256
32
192
24
88
11
104
13
181
23
821
103
Sn/Pb
28
Component Sample Size
Nokia Motorola Rockwell Universal
16
16
0
32
16
8
0
8
0
0
32
0
0
0
0
0
0
0
0
0
Component
Ni/Au
Ni/Au
Ni/Au
Cu-OSP
Cu-OSP
Total
80
48
32
8
16
Test 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
29
Round 2
ACT Test Conditions:
Continuous Event Detection
300 Resistance Threshold, 200 nanoseconds
Universal
IBM
Motorola
Nokia
Rockwell Collins
0-100C, 20 min : 5 min ramp and dwell
0-100C,  32 minute: 7.5 min ramp, 8 min dwell
0-100C, 30 minute: 10 min ramp , 5 min dwell
-40-125C, 60 minute: 15 min ramp and dwell
-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)
30
Assembly 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
20 mil circular pads
2 sites 25 mil mask opening
31
4 sites
22 mil circular pads
26 mil mask opening
2 sites
24 mil circular pads
28 mil mask opening
Assembly of Test Vehicle CSPTB6
Solder paste stencil - Square apertures
Laser cut 5 mil thick stainless steel
0.03” Sphere
14,130 cubic mils
Approximately
90% transfer
& 50%
solidified
paste volume
Vtrap = h(l1w1 + l2w2)/2
32
20 mil circular pads
19.1 mil mean, 0.1 mil std dev. (top side)
20.1 mil mean, 0.1 mil std dev. (bottom side)
Aperture Volume = 1922 cubic mils
Solder Volume = 14,995 cubic mils
22 mil circular pads
21.2 mil mean, 0.1 mil std dev. (top side)
22.2 mil mean, 0.1 mil std dev. (bottom side)
Aperture Volume = 2356 cubic mils
Solder Volume = 15,190 cubic mils
24 mil circular pads
23.2 mil mean, 0.2 mil std dev. (top side)
24.1 mil mean, 0.2 mil std dev. (bottom side)
Aperture Volume = 2798
Solder Volume = 15,389
Assembly of Test Vehicle CSPTB6
Assembly Results
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
33
General Round 2 Information
SAC on ImmAg
Testing 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
34
SA on ImmAg
Failure 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
SnAgCu on ImmAg 1544 cycles
Sn-Ag
Micro
Voids
Cu-Sn
SnAg on ImmAg 1528 cycles
35
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
36
Other 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
37
FCCSP Test Results
Sn/Ag/Cu/Sb on ImmAg
Lead-free solder samples show
similar crack behavior to BGA testing
Mixed samples showed some failures
at Ni layer
Ag-Sn intermetallic
Cu-Sn intermetallic
Early Fail
(<2000 cycles)
Near Ni layer
38
Sn/Ag/Cu/Sb/Pb mixed on Ni/Au
Generic FCCSP Test Results
Ag-Sn
Early Failure on ImmAg
In general ATC - OSP> NiAu
Similar to BGA data on ImmAg
FCCSP Data - 7,000 cycles
Failure near and around
intermetallic layers
Presence of micro voids
Cu-Sn
Micro
Voids
Sn/Ag/Cu on ImmAg
Good joint approximately
2500 cycles
39
FCCSP ACT Results 0-100C
Predicted OSP failure with B=5
OSP Data Set
not enough fails
for accurate plot
40