This presentation examines the drivers for mixed alloy systems, typical voiding requirements, common ways of reducing voids and the effects of mixed alloys on voiding.

1. an Alent plc Company
The effect of mixing BGA
and solder paste alloys
on the formation of voids
September 2010
Alan Plant
Regional Applications Manager

2. HIGHLY CONFIDENTIAL AND PRIVILEGED INFORMATION an Alent plc Company
Introduction
• Reflow Profile is a known method of managing voiding in BGA’s.
– Alpha has recently uncovered the mechanism behind this and is actively
formulating lower voiding pastes.
• Since 2006 there has been a trend for BGA and CSP package
makers to supply components with lower silver alloys
• SAC 305 is still a primary lead free solder paste alloy
– Along with SAC 405 and SAC 387
• Low silver SAC alloys are known to have better drop shock
resistance
• This study also looks at voiding as a function of alloy composition

3. HIGHLY CONFIDENTIAL AND PRIVILEGED INFORMATION an Alent plc Company
Introduction
• Reflow Profile is a known method of managing
voiding in BGA’s.
– Alpha has recently uncovered the mechanism behind
this and is actively formulating lower voiding pastes.
• This study also looks at voiding as a function of
alloy composition
• Total volume of the BGA/CSP Sphere and Paste
Deposit has a significant effect on Voiding
• Stencil Aperture design also can reduce voiding

4. an Alent plc Company
BGA Voiding – Peak
Temperature Effect

5. HIGHLY CONFIDENTIAL AND PRIVILEGED INFORMATION an Alent plc Company
Introduction
It is empirically observed that:
– For the same reflow profile
• Different flux formulations have different voiding performance
– For the same flux formulation
• Different profiles will yield different voiding performance
– A lower peak temperature profile will yield better voiding
results than a higher peak temperature profile

6. HIGHLY CONFIDENTIAL AND PRIVILEGED INFORMATION an Alent plc Company
Differential Scanning Calorimetry
Differential scanning calorimetry was used to assess the
relative reaction rates of different activators with copper.
Note that there is a sharp exothermal reaction
between activator 1 and copper at 241.7º C.
Blue = Flux Weight%
Green = Sample Temp
vs. Control
Red = Sample Temp
Exothermal reaction =
Creates vapor which
contribute to voiding

7. HIGHLY CONFIDENTIAL AND PRIVILEGED INFORMATION an Alent plc Company
Voiding Performance
Void Size Distribution
BGA256
0%
10%
20%
30%
40%
50%
60%
70%
80%
90%
100%
0-4% 4-9% 9-12% 12-16% 16-20% >20%
% of Joint area
%ofJoints
OM350 (Activator 2)
HS 175C / 60 sec
soak 240C peak
CVP-380 (Activator 1)
HS 175C / 60 sec
soak 240C peak
The expected trend is observed, as with a peak reflow temperature of
240ºC, the paste containing activator 2 has better voiding
performance than the paste containing activator 1

8. HIGHLY CONFIDENTIAL AND PRIVILEGED INFORMATION an Alent plc Company
Voiding Performance
Also, as predicted, voiding performance improves measurably for
the same paste formulation reflowed with a lower peak temperature.
Void Size Distribution
BGA256
0%
10%
20%
30%
40%
50%
60%
70%
80%
90%
100%
0-4% 4-9% 9-12% 12-16% 16-20% >20%
% of Joint area
%ofJoints
CVP-380 240C peak temperature
CVP-380 230C peak temperature
Activator #1
Same Flux & alloy
Red = Peak 240ºC
Green = Peak 230ºC
Voiding
Red = Class II
Green = Class III

9. HIGHLY CONFIDENTIAL AND PRIVILEGED INFORMATION an Alent plc Company
Another Example
Void Size Distribution
BGA256 90%
0.00%
10.00%
20.00%
30.00%
40.00%
50.00%
60.00%
70.00%
80.00%
90.00%
100.00%
ZERO 0-4% 4-8% 8-12% 12-16% 16-20% >20%
%of Joint area
%ofJoints
Cerf St ramp 229Cp
Cerf 175-60 soak 245Cp
Cerf St ramp 1.5Cs 240Cp
Activator 2
Ramp vs. Soak profile
NOTE
Peak temperature has
a greater impact vs.
soak profiles

10. an Alent plc Company
BGA Voiding – Alloy
Composition Effect

11. HIGHLY CONFIDENTIAL AND PRIVILEGED INFORMATION an Alent plc Company
What effect do the package ball alloy and
the solder paste alloy have on solder joint
voids?

12. HIGHLY CONFIDENTIAL AND PRIVILEGED INFORMATION an Alent plc Company
DOE Test Matrix
Reflow
Profile
Short
Soak
Long
Soak
Short
Soak
Long
Soak
Short
Soak
Long
Soak
SACX 0307
SAC 105
SAC 305
Sphere Alloy
PasteAlloy
SACX 0307 SAC 105 SAC 305
Voiding Measured for all 18 Combinations
Paste Alloy / Profile / Sphere alloy

13. HIGHLY CONFIDENTIAL AND PRIVILEGED INFORMATION an Alent plc Company
Voiding Test Procedure
• Test Method
– ALPHA PUT Procedure JC-PUT-0016
• Equipment- Phoenix Micromex-HLN
• Measure and report voiding area on 256 IO BGA Package

14. HIGHLY CONFIDENTIAL AND PRIVILEGED INFORMATION an Alent plc Company
Cookson Electronics Test Vehicle
2 x BGA-256

15. HIGHLY CONFIDENTIAL AND PRIVILEGED INFORMATION an Alent plc Company
Low Soak Profile
175C/60S Soak 240C Peak 60S TAL

16. HIGHLY CONFIDENTIAL AND PRIVILEGED INFORMATION an Alent plc Company
High Soak Profile
160-180ºC/120S Soak 250ºC Peak 60S TAL

17. HIGHLY CONFIDENTIAL AND PRIVILEGED INFORMATION an Alent plc Company
Results

18. HIGHLY CONFIDENTIAL AND PRIVILEGED INFORMATION an Alent plc Company
SACX® 0307 Paste
Solder paste SACX-BGA256-20mil
0
50
100
150
200
250
300
0-1 1-3 3-5 5-7 >7
Void size, %
Numberofvoids
SACX
SAC105
SAC305
SACX
SAC105
SAC305
Solid lines
•175C @ 60s Soak
•245C Peak
•60s TAL
Dotted lines
•160-180C @ 120s Soak
•250C Peak
•40s to 76s TAL

19. HIGHLY CONFIDENTIAL AND PRIVILEGED INFORMATION an Alent plc Company
SAC 105 Paste
Solder paste SAC105-BGA256-20mil
0
50
100
150
200
250
300
0-1 1-3 3-5 5-7 >7
Void size, %
Numberofvoids
SACX
SAC105
SAC305
SACX
SAC105
SAC305
Solid lines
•175C @ 60s Soak
•245C Peak
•60s TAL
Dotted lines
•160-180C @ 120s Soak
•250C Peak
•40s to 76s TAL

20. HIGHLY CONFIDENTIAL AND PRIVILEGED INFORMATION an Alent plc Company
SAC 305 Paste
Solder Paste SAC305-BGA256-20mil
0
50
100
150
200
250
300
0-1 1-3 3-5 5-7 >7
Void size, %
Numberofvoids
SACX
SAC105
SAC305
SACX
SAC105
SAC305
Solid lines
•175C @ 60s Soak
•245C Peak
•60s TAL
Dotted lines
•160-180C @ 120s Soak
•250C Peak
•40s to 76s TAL

21. HIGHLY CONFIDENTIAL AND PRIVILEGED INFORMATION an Alent plc Company
Data Analysis-Scoring Results
Solder paste SAC105-BGA256-20mil
0
50
100
150
200
250
300
0-1 1-3 3-5 5-7 >7
Void size, %
Numberofvoids
SACX
SAC105
SAC305
SACX
SAC105
SAC305
10 5 1 0

22. HIGHLY CONFIDENTIAL AND PRIVILEGED INFORMATION an Alent plc Company
Confirmation Run SACX 0807
Void Size Distribution
BGA256
0.00%
10.00%
20.00%
30.00%
40.00%
50.00%
60.00%
70.00%
80.00%
90.00%
100.00%
ZERO 0-4% 4-8% 8-12% 12-16% 16-20% >20%
%of Joint area
%ofJoints
CVP-360 CERF ST.RAMP 0.7c_s
245cp 60 TAL
CVP-360 CERF ST.RAMP 1.5c_s
245cp 60 TAL
SAC 305 Spheres, SACX 0807
Paste- Class II Voiding

23. HIGHLY CONFIDENTIAL AND PRIVILEGED INFORMATION an Alent plc Company
SACX 0807 Example
Void Size Distribution - SACX0807 CVP-360 Paste,
SAC105 BGA 256 (20 mil Sphere)
0%
10%
20%
30%
40%
50%
60%
70%
80%
90%
100%
ZERO 0-4% 4-8% 8-12% 12-16% 16-20% >20%
%of Joint area
%ofJoints
Ramp: 0.7 C/sec, 240 Peak,
TAL - 85 s
Ramp: 1.5 C/sec, 245 Peak,
TAL - 45 s
SAC 105 Spheres, SACX 0807 Paste-
Class III+

24. HIGHLY CONFIDENTIAL AND PRIVILEGED INFORMATION an Alent plc Company
Print Deposit Effect on Voiding
• Reducing aperture size reduces voiding
• Smaller paste/sphere volumes = reduced
voiding

25. HIGHLY CONFIDENTIAL AND PRIVILEGED INFORMATION an Alent plc Company
Reducing Aperture Size Reduces
Voiding
No Aperture Reduction
Void Size Distribution
BGA256
0%
10%
20%
30%
40%
50%
60%
70%
80%
90%
100%
ZERO 0-4% 4-8% 8-12% 12-16% 16-20% >20%
%of Joint area
%ofJoints
438_167B19 CERF HS 160_60 SOAK 240CP
438_167B19 CERF HS 175_60 240CP
438_167B19 CERF ST.RAMP 1.5c_sec 245CP 60
TAL

26. HIGHLY CONFIDENTIAL AND PRIVILEGED INFORMATION an Alent plc Company
Reducing Aperture Size Reduces
Voiding
Void Size Distribution
BGA256 90%
0%
10%
20%
30%
40%
50%
60%
70%
80%
90%
100%
ZERO 0-4% 4-8% 8-12% 12-16% 16-20% >20%
%of Joint area
%ofJoints
438_167B19 CERF HS 160_60 SOAK 240CP
438_167B19 CERF HS 175_60 240CP
438_167B19 CERF ST.RAMP 1.5c_sec 245CP 60 TAL
10% Aperture
Reduction

27. HIGHLY CONFIDENTIAL AND PRIVILEGED INFORMATION an Alent plc Company
Reducing Paste/Sphere Volumes
Reduces VoidingVoid % Distribution For BGA256 20Mil
0.00
10.00
20.00
30.00
40.00
50.00
60.00
70.00
80.00
90.00
100.00
Zero <0-4 4-9 9-12 12-16 16-20 >20
% of Void Area
%ofVoidDistribution
Void % Distribution For BGA256 15Mil
0.00
10.00
20.00
30.00
40.00
50.00
60.00
70.00
80.00
90.00
100.00
Zero <0-4 4-9 9-12 12-16 16-20 >20
% of Void Area
%ofVoidDistribution
20 Mil Circles (0.5 mm)
15 Mil Circles (0.4 mm)
Same Solder Paste and Reflow Profile

28. HIGHLY CONFIDENTIAL AND PRIVILEGED INFORMATION an Alent plc Company
Reducing Paste/Sphere Volumes
Reduces Voiding
Void % Distribution For BGA256 15Mil
0.00
10.00
20.00
30.00
40.00
50.00
60.00
70.00
80.00
90.00
100.00
Zero <0-4 4-9 9-12 12-16 16-20 >20
% of Void Area
%ofVoidDistribution
15 Mil Circles (0.4 mm)
Same Solder Paste and Reflow Profile
Void % Distribution For BGA225 12Mil
0.00
10.00
20.00
30.00
40.00
50.00
60.00
70.00
80.00
90.00
100.00
Zero <0-4 4-9 9-12 12-16 16-20 >20
% of Void Area
%ofVoidDistribution
12 Mil Circles (0.3 mm)

29. HIGHLY CONFIDENTIAL AND PRIVILEGED INFORMATION an Alent plc Company
Overall Conclusions
• Like alloys tend to produce fewer large voids than
mixed alloy combinations
– Slight difference in melting point is one possible cause.
• Peak Reflow Temperature
– Has a significant effect on voiding due to the interaction of the
flux with the Cu at key temperatures.
• Solder Paste Formulation
– Has a significant effect on voiding due to the type of chemistries
used and where they become reactive with the Cu.
• Solder Joint Volume
– Reduced volume of paste and/or sphere reduces voiding

30. HIGHLY CONFIDENTIAL AND PRIVILEGED INFORMATION an Alent plc Company
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visit:
www.Alpha.Alent.com