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BGA reballing reliability going from Lead free to Sn/Pb
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© 2004 - 2007© 2004 - 2010© 2004 – 2010
BGA REBALLING FROM
PB-FREE TO SN-PB METALLURGY
MDA PMAP Meeting
March 21, 2011 Huntsville, MD
Greg CaswellDfR Solutions, College Park, MD, [email protected]
Joelle Arnold, DFR Solutions, College Park, MD, [email protected]
© 2004 - 2007© 2004 - 2010
o Fewer native SnPb parts available as COTS
o Market share dominated by Lead-free manufacturers
o Military procures a small percentage of parts
SnPb Obsolescence
Computer
53%
Consumer
17%
Communications
15%
Industrial
9%
Auto
5%
Military
1%
Semiconductor International Association
Computer
53%
Consumer
17%
Communications
15%
Industrial
9%
Auto
5%
Military
1%
Semiconductor International Association
© 2004 - 2007© 2004 - 2010
o Maintaining SnPb metallurgy for Mil/Aero
o Demonstrated SnPb performance in harsh
environments and long term service
o Maintain system qualification
o 20 year repair and rework sustainment
infrastructure maintained
o All lead-free parts are compatible with
SnPb solder (at least so far)
o Except BGAs
o Reballing allows mil/aero time to develop
lead-free reliability and repair infrastructure
Reballing Option For Lead-free Risk Management
(See Mil/Aero Lead-free Electronics Research Phase 1 report at:
http://www.navyb2pcoe.org/b2p_news.html)
© 2004 - 2007© 2004 - 2010
o Areas of concern
o Ball pad integrity
o Ball uniformity
o Ball attach strength
o Soldermask integrity
o Top of die delamination
o Die attach adhesive integrity
o Interconnect substrate delamination
o Interconnect run and via integrity
o Part, supplier and process qualification
Reballing Challenges
© 2004 - 2007© 2004 - 2010
o Many different types of BGA
o Constructions and materials
o Different package manufacturers
o Different molding and interconnect
materials
o Difficult to obtain material properties
Reballing Challenges, cont.
© 2004 - 2007© 2004 - 2010
BGAs are complex packages
BAE Systems and DfR Solutions © 2010
SnPb reballPb-free
Molding compound
with particle filler
Silicon die
Die attach adhesive
over solder mask and
copper runs
BGA interconnect
substrate
Wire bonds
BGA interconnect
via
Ni over Cu pad
BGA solder mask
opening
Solder ball
© 2004 - 2007© 2004 - 2010
o Reballer Survey
o Reballing Customer Survey
o X-ray and Microsectioning Inspection
o Ball Shear Testing
o Thermal Cycling
o Vibration Testing
o Mechanical Shock Testing
o Test Result Analysis
SBIR Tasks
Top layer of test board
208 BGA
© 2004 - 2007© 2004 - 2010
o Focused on:
o Experience
o Technical aptitude required
o Capacity
o Inspection and Process Control
o Qualification
o Findings:
o Reballing needs are up
o Expected to rise in the near future
o Small lots, fine pitch, PBGAs
o Qualification responsibility of the customer
o If you’re not qualifying, you should be
Reballing Customer Survey
© 2004 - 2007© 2004 - 2010
Intermetallic between solder and Ni adequate
Microsection (untested BGAs)
Reballer A
Reballer B
Reballer C
Reballer D
Reballer E
Native Sn/Pb
BAE Systems and DfR Solutions © 2010
© 2004 - 2007© 2004 - 2010
One supplier (A) had x-ray anomalies – no findings on others
X-Ray Findings
Large void
Missing Ball
© 2004 - 2007© 2004 - 2010
Ball Shear
• Ball shear force
distributions
• Relatively equal
average strength
• Native SnPb balls have
greater standard
deviation
• May indicate that
initial balling may be
lacking sufficient
process controls
© 2004 - 2007© 2004 - 2010
Thermal CyclingOn Board Thermocouple Measurements
10
30
50
70
90
110
130
150
90000 90050 90100 90150 90200 90250 90300 90350 90400
Time (m)
T (
°C)
Reballer A
Reballer B
Reballer C
Reballer D
Reballer E
NativeSnPb
SAC305
• Reballer C is
outlier
• Low
volume,
low tech,
less
experience
• Likely poor
process
controls
© 2004 - 2007© 2004 - 2010
Vibration
• 157Hz, 85mil initial displacement• 1st mode
~171Hz• Displacement
“tuned” with elastomeric mass
• Results for highest stressed BGA group
• SAC 305 had reduced performance
© 2004 - 2007© 2004 - 2010
Mechanical Shock: No Precondition
o All have
similar
perfor-
mance
© 2004 - 2007© 2004 - 2010
Mechanical Shock: -40 to 85°C, 120 cycle Precondition
o All have
similar
perfor-
mance
© 2004 - 2007© 2004 - 2010
Mechanical Shock: 150°C 100 hr Precondition
o Native
SnPb
exhibited
reduced
perfor-
mance
o Reballed
SnPb not
affected
© 2004 - 2007© 2004 - 2010
o Five reballing suppliers evaluated
o No “special technique” proved superior
o Higher volume, more experienced reballers
performed best in all tests
o Vibration results show greater differentiation amongst
reballing suppliers
o Mechanical shock
o Isothermal high temperature aging may have less of an
effect on reballed parts subjected to mechanical shock
than native SnPb parts.
o Thermal cycling precondition shows little to no effect.
Summary
© 2004 - 2007© 2004 - 2010
BGA reballing part assessment:
Evaluation of four parts with one supplier’s process
BAE Systems- activity
Ball
Dia.
Ball
Pitch
Ball
Extents
(LxW)
Die
Size
M90 13x10x0.65 0.45 0.8 11.2 x 6.4 7.53x6.79
L256 17x17x1.25 0.5 1.0 15 x 15 4.28x3.94
F473 19x19x1.12 0.4 0.8 17.6 x 17.6 6.23x5.68
X1148 35x35x 2.8 0.6 1.0 33 x 33 19.3x13.6
(mm)
BGA
Pkg
Dims.
(LxWxT)
Reballing Process: •Flowing wave ball removal
•Convection SMT attachment
© 2004 - 2007© 2004 - 2010
o Cross-sectioning, visual and radiographic assessments
o Measure critical BGA parameters
o Die size, ball attach size, ball diameter, solder mask, etc.
o The measurements will be used for future similarity analyses
o Some parameters are very crucial to the reballing reliability
o Proper intermetallic layer between the solder ball and BGA pad
o Excessive dissolution of BGA pad is detrimental
o Nickel layer improves dissolution resistance
o Fracture of BGA soldermask and interconnect is detrimental
o Key parameters were compared between the “as-received” and “reballed” BGAs
o Visual inspection
o Scanning acoustic microscopic (SAM) examination for delamination
o Warpage
o Coefficient of thermal expansion
o Ball shear/pull
o Finite element modeling of package stress during ball removal
o Assembly level testing
Evaluations performed
© 2004 - 2007© 2004 - 2010
Reballing assessment
BAE Systems and DfR Solutions © 2010
CSAM measurements indicated
that no additional damage was
observed following the reballing
process.
Warpage measurements are
within the permissible limits
per JEITA ED-7306.
CTE measurements indicated that there were
no significant changes in CTE due to reballing
The failure mode on the reballed tin-lead
BGAs was almost exclusively ductile. Ball
shear and pull results provide confidence
that the reballing process is sufficient.
Nickel thickness not changed
during reballing.
Cross-section examination
confirmed no delamination
or cracking occurred.
Assembly level thermal cycling:
current results indicate acceptable
performance – testing continuing
© 2004 - 2007© 2004 - 2010
Warpage Results
L256
-150
-125
-100
-75
-50
-25
0
25
50
75
100
125
150
25 100 125 150 175 183 200 220 230 245 230 220 200 183 175 150 100 25
Temperature (°C)
Warp
age
(m
icro
ns)
#1 As Received#5 As Recevied#4 Reballed#6 Reballed
M90
-150
-125
-100
-75
-50
-25
0
25
50
75
100
125
150
25 100 125 150 175 183 200 220 230 245 230 220 200 183 175 150 100 25
Temperature (°C)
Wa
rpag
e (
mic
rons
)
#1 As Received#5 As Recevied#6 Reballed#8 Reballed
F473
-150
-125
-100
-75
-50
-25
0
25
50
75
100
125
150
25 100 125 150 175 183 200 220 230 245 230 220 200 183 175 150 100 25
Temperature (°C)
Wa
rpa
ge
(m
icro
ns)
#1 As Received#7 As Recevied#6 Reballed#8 Reballed
X1148
-150
-125
-100
-75
-50
-25
0
25
50
75
100
125
150
25 100 125 150 175 183 200 220 230 245 230 220 200 183 175 150 100 25
Temperature (°C)
Warp
ag
e (
mic
rons)
#27 As Received#31 As Recevied#21Reballed#22 Reballed
Higher warpageafter reballing may
be due to increased molding compound curing
Postive
PostiveNegative
Postive
PostiveNegative
NegativeNegativePositiveNegativeNegativePositive
© 2004 - 2007© 2004 - 2010
Analytical modeling of reballing process
o Parametric finite-element model
o Input measured and datasheet
dimensions
o Calibrate to warpage and CTE
o Transient thermal solution
o Based on ball removal process
o Static stress solution
o Ball removal stress compared to
SMT reflow stress
-150
-100
-50
0
50
100
150
0 50 100 150 200 250
Temperature (deg C)
Wa
rpa
ge (
mic
ron
)
Pb-Free #1 Pb-Free #7
Model (95% filled, 100C Tg) Model (94% filled, 100C Tg)
Predicted and measured warpage agree
© 2004 - 2007© 2004 - 2010
Analytical modeling results
0%
20%
40%
60%
80%
100%
120%
0 1 2 3 4 5 6 7
Time (min)
Ma
xim
um
Re
lati
ve
Str
es
s
0
40
80
120
160
200
240
Te
mp
era
ture
(d
eg
C)
SMT Reflow Stress Actual Ball Removal Profile Stress
Higher Temp. Ball Removal Stress SMT Reflow Die Temp.
Actual Ball Removal Profile Die Temp. Higher Temp. Ball Removal Die Temp.
Actual ball removal stress is
less than SMT reflow stress
(right)Max stresses occurred around die
Temperature ramp rate and max temperature influence stress
© 2004 - 2007© 2004 - 2010
BGA reballing substantiation strategy
Add part to altered item drawing
© 2004 - 2007© 2004 - 2010
o Assessment of evaluation methods
o Cross-section: valuable for FEM modeling and can supplement the
delamination assessment, although coverage not a robust as SAM
o Radiography: helpful for understanding package construction
o Visual inspection: very useful after ball removal and after ball
attachment
o Scanning Acoustic Microscopy: valuable if packages can be scanned
o Warpage: valuable for adjusting properties FEM modeling and may be a
promising metric for package integrity changes during reballing
o CTE: valuable for adjusting properties in FEM modeling
o Finite element modeling: adds insight to the complex interaction
between the maximum temperature and the temperature ramp rate
o Thermal cycling test: useful especially if real parts are electrically
monitored
Summary
© 2004 - 2007© 2004 - 2010
o Multiple BGA reballing suppliers evaluated
o Reballing suppliers with high volume and experience
are the most reliable
o Re-balling is a viable means of managing the lead-
free risk for qualified tin-lead systems
o Reballing may no longer be needed when experience with
lead-free in high reliability complex applications is proven
and sustainment infrastructure becomes established
Conclusions
