2nd generation lead free alloys: is sac the best we can do?
DESCRIPTION
oWhy did SAC305 become the standard LF alloy? SAC was never considered an ideal replacement for eutectic SnPb, it was simply the best choice at the time. It was readily available, had a reasonable melting temperature and had the least reliability issues compared to other options. oHowever, SAC305 has weaknesses that can be overcome with newer alloys. SAC is a precipitation hardened alloy which means the microstructure and mechanical properties are significantly impacted by reflow temperature and time, cooling rate, and aging (dwell times). It is undesirable for the properties of the solder to be so dependent on the assembly conditions and the customer use environment. oThis presentation addresses the latest research and reliability results for 2nd generation lead free (LF) alloys.TRANSCRIPT
© 2004 - 2007 © 2004 - 2010 © 2004 – 2010
2nd Generation Lead Free Alloys: Is SAC the Best
We Can Do? SMTA ICSR
Toronto, Canada
May 7, 2011
Cheryl Tulkoff, ASQ CRE
DfR Solutions
Sr. Member of the Technical Staff
© 2004 - 2007 © 2004 - 2010
2nd Generation Lead Free Alloys Course Abstract
o Why did SAC305 become the standard LF alloy? SAC was never considered an ideal replacement for eutectic SnPb, it was simply the best choice at the time. It was readily available, had a reasonable melting temperature and had the least reliability issues compared to other options.
o However, SAC305 has weaknesses that can be overcome with newer alloys. SAC is a precipitation hardened alloy which means the microstructure and mechanical properties are significantly impacted by reflow temperature and time, cooling rate, and aging (dwell times). It is undesirable for the properties of the solder to be so dependent on the assembly conditions and the customer use environment.
o This workshop addresses the latest research and reliability results for 2nd generation lead free (LF) alloys.
2
© 2004 - 2007 © 2004 - 2010
Instructor Biography
o Cheryl Tulkoff has over 17 years of experience in electronics manufacturing with an emphasis on failure analysis and reliability. She has worked throughout the electronics manufacturing life cycle beginning with semiconductor fabrication processes, into printed circuit board fabrication and assembly, through functional and reliability testing, and culminating in the analysis and evaluation of field returns. She has also managed no clean and RoHS-compliant conversion programs and has developed and managed comprehensive reliability programs.
o Cheryl earned her Bachelor of Mechanical Engineering degree from Georgia Tech. She is a published author, experienced public speaker and trainer and a Senior member of both ASQ and IEEE. She holds leadership positions in the IEEE Central Texas Chapter, IEEE WIE (Women In Engineering), and IEEE ASTR (Accelerated Stress Testing and Reliability) sections. She chaired the annual IEEE ASTR workshop for four years and is also an ASQ Certified Reliability Engineer.
o She has a strong passion for pre-college STEM (Science, Technology, Engineering, and Math) outreach and volunteers with several organizations that specialize in encouraging pre-college students to pursue careers in these fields.
3
© 2004 - 2007 © 2004 - 2010
Outline
o SAC background & alternative alloys
o Shock/Drop Test Results
o SAC vs SnPb
o Results of alternative alloys
o Vibration Results
o Thermal Cycling
o SAC vs SnPb
o Results of alternative alloys
o Will there be one winner?
o Summary
o Supplementary Material
4
© 2004 - 2007 © 2004 - 2010
Why is this important to the Industry?
o Some types of equipment have been RoHS exempt to date.
o This will change with RoHS 2 which states that many
formerly exempt products must be RoHS compliant by
2014 (as defined by Directive 93/42/EEC).
o In-vitro by 2017
o Some exemptions (including lead solder in portable defibrillators)
o SnPb BGA components are being eliminated from the
supply chain (forcing LF transition or reballing).
o Good News. The industry can leverage the improvements
made since the initial LF transition.
o SAC305 has weaknesses that can be overcome with newer
alloys.
5
© 2004 - 2007 © 2004 - 2010
Refresher
o Why did SAC305 become the standard LF alloy?
o Readily available
o Reasonable melting temp
o Had the least reliability issues compared to other options
SAC was never considered an ideal replacement for
eutectic SnPb, it was simply the best choice at the time.
6
© 2004 - 2007 © 2004 - 2010
Sn Bi
Ag
Zn
Acceptable wetting
And high strength High Melting Point
217C Strength
Weakness
Melting point is
almost the same as SnPb
Easily oxidizes, corro-
sion cracking, voids,
poor wetting
Mixing with Pb degrades
strength and fatigue
resistance
(silver)
(bismuth) (zinc) (tin)
Good wetting and
high strength
In Inadequate source
of supply & corrosion
(indium)
+ Cu
SnAgCu
became the
industry
accepted Pb-
free alloy
Lead-free Alloy Summary
7
© 2004 - 2007 © 2004 - 2010
SAC305 or SAC406?
Eutectic
3.5Ag-0.9Cu
Sn-3.9Ag-0.6Cu
Sn-3.0Ag-0.5Cu
Melting temperature is
similar.
SAC406 alloy resulted in
higher volume fraction of
Ag3Sn precipitates.
SAC406 was higher cost.
SAC305 has lower shear
stress (more compliant)
SAC305 eventually won out
as the standard
Phase Diagram Source: K-W Moon et al, J. Electronic Materials, 29 (2000) 1122-1236
8
© 2004 - 2007 © 2004 - 2010
Mechanical Properties of SAC Alloys
Ref: Yoshiharu Kariya et al. J. of Elect. Mat, 33, No. 4,
2004.
9
© 2004 - 2007 © 2004 - 2010
Effect of Cooling Rate on Microstructure
in SAC405
Cooling rate of 0.81 °C/s Cooling rate of 1.86 °C/s
Thilo Sack, Celestica
SAC is a precipitation hardened alloy.
This means the microstructure and mechanical properties are
significantly impacted by reflow temp/time, cooling rate, and aging (dwell
times).
It is undesirable for the properties of the solder to be so dependent on
the assembly conditions and the customer use environment.
SnPb
Soft Sn and Pb
phases in
eutectic solder
Hard Ag3Sn
phases form in
SAC solder.
10
© 2004 - 2007 © 2004 - 2010 11
Material Properties - Plasticity
SnPb has lower elastic modulus but the yield strength is more
impacted by the temperature.
© 2004 - 2007 © 2004 - 2010
o The creep rate of SnPb does change much after aging at
temperatures up to 125C (creep testing performed at RT).
Creep of SnPb
J.
Su
hlin
g, “M
ate
ria
l B
eh
avio
r o
f A
gin
g P
b-f
ree
So
lde
r Jo
ints
”,
Ce
nte
r fo
r A
dva
nce
d V
eh
icle
an
d E
xtr
em
e E
nviro
nm
enta
l
Ele
ctr
on
ics, A
ub
urn
U.
12
© 2004 - 2007 © 2004 - 2010
o The creep rate of SAC305 is much more dependant on
aging conditions.
SAC 305 Creep
J.
Su
hlin
g, “M
ate
ria
l B
eh
avio
r o
f A
gin
g P
b-f
ree
So
lde
r Jo
ints
”,
Cen
ter
for A
dva
nce
d V
eh
icle
an
d E
xtr
em
e E
nviro
nm
enta
l
Ele
ctr
on
ics, A
ub
urn
U.
13
© 2004 - 2007 © 2004 - 2010
o Impact of aging on creep of SAC105 is even larger.
SAC 105 Creep
SnPb creep range
14
© 2004 - 2007 © 2004 - 2010
Exposed Cu after SAC305 Paste Reflow
o High surface tension prevents flow and full wetting of Cu
features.
15
© 2004 - 2007 © 2004 - 2010
Solders: Copper Dissolution
o PTH knee is the point of
greatest plating
reduction
o Primarily a
rework/repair issue
o Celestica identified
significant risk with >1X
rework
o >0.5 mil Cu thickness at
knee after rework is a
standard requirement.
16 16
S. Zweigart, Solectron
© 2004 - 2007 © 2004 - 2010
SAC is More Vulnerable to Strain
PCB deflection
Ten
sile
fo
rce
on
pad
an
d L
amin
ate
PbSn
LF
PbSn limit LF limit
Laminate Load
Bearing Capability
NEMI study showed SAC is more
Sensitive to bend stress. Sources of strain can be ICT, stuffing through-
hole components, shipping/handling, mounting
to a chassis, or shock events.
17
© 2004 - 2007 © 2004 - 2010 18
Drop Testing Performance of SAC305
Roughly a 2X - 5X
reduction in drops to
failure for ENIG
JEDEC (JESD22-
B111) standard
testing 1500 G’s, 0.5
mS pulse width
Board Level Drop Test Reliability of IC Packages
Chai TC, Sharon Quek, Hnin WY, Wong EH, Julian Chia*, Wang
YY**, Tan YM***, Lim CT****, Institute of Microelectronics
SnPb better in drop testing
ENIG is much worse than OSP
© 2004 - 2007 © 2004 - 2010 19
Shock Testing Performance with SAC – surface finish impact
Drop Test Reliability of Wafer Level Chip Scale Packages
Mikko Alajoki, Luu Nguyen(* and Jorma Kivilahti Lab. of Electronics Production Technology, Helsinki University of Technology, P.O.Box 3000, 02150 Espoo, Finland*)
National Semiconductor Corporation P.O.Box 58090, Mail Stop 19-100, Santa Clara, USA
Roughly a 5X – 10X
reduction in drops to failure
when switching from OSP to
ENIG, failures also occurred
on first drop
JEDEC (JESD22-B111)
standard testing 1500 G’s,
0.5 mS pulse width
Component Package
Qualification for Handheld
electronics
CSP metallization – Solder – PWB finish
Ni(P)/Au is ENIG
Cu/ENIG
Cu/OSP
ENIG/ENIG
ENIG/OSP
© 2004 - 2007 © 2004 - 2010
What is the Perfect Alloy? Is there one?
Desired Attribute Comment
Lower Melting Point Closer to 190C would be desirable
Lower Modulus Reduction from 51 to 40 GPa (near SnPb)
Good wetting behavior Wetting time of 0.5 sec or less
Stable behavior Preferably not precipitation hardened or at least
rapidly softens (so properties are consistent after
assembly)
Low yield strength combined with
low work hardening rate
Similar to SnPb – providing compliance without
suffering damage in fatigue.
Low Cu dissolution To prevent erosion of Cu traces
Low surface tension For covering of Cu features and wicking up PTHs
There will always be tradeoffs. So there can only be a perfect alloy
for a particular application. The following table addresses general
consumer applications:
20
© 2004 - 2007 © 2004 - 2010 21
Solder Trends
o SAC305 dominates surface mount reflow (SMT)
o SAC105 increasingly being used in area array components in mobile applications
o SNC pervasive in wave solder and HASL
o Increasing acceptance in Japan for SMT
o Intensive positioning for “X” alloys (SACX, SNCX)
K-W Moon et al, J.` Electronic Materials, 29 (2000) 1122-
1236
© 2004 - 2007 © 2004 - 2010
Likely Elements
o Tin will likely be main constituent (forms well understood
IMCs with Cu)
o Reducing Ag lowers elastic modulus (SAC105 is 11% lower
than SAC405)
o Small amounts of Ni, Co, etc. to arrest IMC formation and
reduce Cu dissolution (assuming Tm is above 220C).
o Bi to perhaps play a bigger role as Pb is eliminated from
supply chain – SnAgCuBi alloys are promising.
o Other elements to be added for improved shock resistance
- Mn, Ce show great promise.
22
© 2004 - 2007 © 2004 - 2010
Pb-free Alloys Investigated
SAC105 + Mn or Ce W. Liu & Ning-Cheng Lee (Indium) SMTA2006, ECTC 2009
23
© 2004 - 2007 © 2004 - 2010 24
The Current State of Lead-Free
o Component suppliers
o SAC305 still dominant, but with increasing introduction of low silver alloys (SAC205, SAC105, SAC0507)
o Solder Paste
o SAC305 still dominant
o Wave and Rework
o Sn07Cu+Ni (SN100C)
o Sn07Cu+Co (SN100e)
o Sn07Cu+Ni+Bi (K100LD)
o HASL PCB Coating
o Sn07Cu+Ni (SN100C)
© 2004 - 2007 © 2004 - 2010
LF Rework – Solder Fountain
Time required for rework
There is little to no process window for rework of through hole joints on
a thicker board with SAC solder.
Ref: C. Hamilton& M. Kelly, A Study of Copper Dissolution in LF PTH Rework”, SMTA, 2006.
25
© 2004 - 2007 © 2004 - 2010
Hole Fill Challenges with SAC and SnCu
Less than
50% hole fill
To achieve sufficient hole-fill suppliers often increase the preheat temp,
solder pot temp and dwell time (this can damage other components). 26
© 2004 - 2007 © 2004 - 2010
Mechanical Shock & Vibration
27
© 2004 - 2007 © 2004 - 2010
Laminate Cracking Leads to Trace Fracture
Bending
Force
Functional failure
will occur
Trace routed externally
28
© 2004 - 2007 © 2004 - 2010
Drop Test Results – SAC worse than SnPb
29
© 2004 - 2007 © 2004 - 2010
Drop Test Results Ref: B. Roggeman, “Comparison of Drop Reliability of SAC105
and SAC305 on OSP and ENIG Pads”, Unovis, 2007.
30
© 2004 - 2007 © 2004 - 2010
Shock Testing with SAC105
Aging causes precipitate coursening and softening of the alloy
M.D
ing a
nd A
. P
orr
as, “A
GIN
G E
FF
EC
TS
ON
DY
NA
MIC
BE
ND
TE
ST
PE
RF
OR
MA
NC
E
OF
Pb
-FR
EE
SO
LD
ER
JO
INT
S O
N N
i/A
u F
INIS
H »
, S
MT
A P
roceedin
gs, C
hic
ago, 2006
31
© 2004 - 2007 © 2004 - 2010
Drop Results for SACx Solders
W. Liu, N. Lee, “NOVEL SACX SOLDERS WITH SUPERIOR DROP TEST
PERFORMANCE”, SMTA Proceedings, Chicago, 2006.
32
© 2004 - 2007 © 2004 - 2010
Drop Testing
o 244 I/O BGA, 0.5 mm
o Electrolytic NiAu on
package substrate.
o OSP on PCB.
o JEDEC Drop Test method
used.
o 250 thermal cycles
precondition
Ref: W. Liu et al., “Achieving High
Reliability Low Cost LF SAC Solder
Joints via Mn or Ce Doping”, ECTC,
2009.
33
© 2004 - 2007 © 2004 - 2010
Mechanical Properties – SN100C
o Tensile stress-strain curves compared to SnPb
34
© 2004 - 2007 © 2004 - 2010
Mech Properties of SN100C
SN100C overlay
(-40 to 125C
range)
35
© 2004 - 2007 © 2004 - 2010
Vibration
36
© 2004 - 2007 © 2004 - 2010
Vibration Test Results
o Solder joint fatigue life for a 2512 Resistor
Vibration Strain = 2400µϵ
SnPb better than SAC
Vibration Strain = 1200µϵ
SAC better than SnPb
37
© 2004 - 2007 © 2004 - 2010 38
Vibration Results: Effect of Solder
Material Resistor
TSOP
CSP
Resistors:
Generally, SAC < SN100C < SnPb
TSOP:
At 50% failure, SN100C < SnPb < SAC
CSP
Generally, SAC << SN100C ~ SnPb
SAC: b ~1
ENIG Surface Finish,
30G vibration
© 2004 - 2007 © 2004 - 2010
Vibration Results for SACM and SACC
244 I/O BGA, 0.5 mm
Electrolytic NiAu on
package substrate.
OSP on PCB.
Cyclic bend test
Preconditioning at 150C
for 250 hours
Cyclic Bend Testing (Fatigue)
In high cycle fatigue SACM
and SACC perform better
than SnPb but worse than
SAC305
Ref: W. Liu et al., “Achieving High
Reliability Low Cost LF SAC Solder Joints
via Mn or Ce Doping”, ECTC, 2009.
39
© 2004 - 2007 © 2004 - 2010 40
Alloy Comparison - Vibration
o Times to failure for all three solders at extreme test conditions varied based upon the solder joint geometry
o Why? Stiffness (SAC > SN100C > SnPb)
o For a given force / load, a stiffer solder will respond with a lower displacement / strain (elastic and plastic)
o Low-cycle fatigue (plasticity driven)
o Under displacement-driven mechanical cycling, lower stiffness solder will tend to out-perform higher stiffness (e.g., chip scale packages [CSP])
o Under load-driven mechanical cycling, higher stiffness solder will tend to out-perform lower stiffness (e.g., leads of thin scale outline packages [TSOP])
o High-cycle fatigue (elasticity driven)
o Stiffer solder (i.e., SAC and SN100C), lower strain range
© 2004 - 2007 © 2004 - 2010 41
Shock & Vibration Summary
o SAC (Ag ≥ 3%) should not be used with ENIG in high G environments, vibration testing at 30G’s yielded “random failures” on a small PCB (organic solder protection) with chip scale packages
o Performance of Pb-free solders in high cycle fatigue is the same or better than SnPb
o SAC105 and SN100C have almost identical creep behaviors and likely have very similar modulus and plastic properties, should have o Drop/shock performance o High cycle fatigue o SAC105
o Pasty range of SAC alloys increases as the silver content drops, 217 - 226C, reflow greater than 226C necessary
o Shrinkage cracks, and effect on life under vibration
o SN100C o Melting point 227C (liquidus and solidus are at 227)
© 2004 - 2007 © 2004 - 2010
Thermal Cycle Performance
42
© 2004 - 2007 © 2004 - 2010
Non underfilled flip chip – SnPb better
Ref: E. Al-Momanl and M. Mellunas, “Lead-free Thermal Cycle
Progress, Unovis, June, 2008.
Stiff Component – SnPb is better
43
© 2004 - 2007 © 2004 - 2010
TBGA – SAC better
Ref: E. Al-Momanl and M. Mellunas, “Lead-free Thermal Cycle
Progress, Unovis, June, 2008. Compliant Component – SAC is better
44
© 2004 - 2007 © 2004 - 2010 45 45
Thermal Cycling: SnPb vs. SAC
Where does SnPb outperform Pb-free?
Leadless, ceramic components
Leadless ceramic chip carriers (crystals, oscillators, resistor
networks, etc.)
SMT resistors
Ceramic BGAs
Severe temperature
cycles
-40 to 125ºC
-55 to 125ºC
Syed, Amkor
© 2004 - 2007 © 2004 - 2010 46 46
Thermal Cycling: Effect of Dwell Time
Normalized time to failure as a function of dwell time at
maximum temperature for SAC solder
• 40% to 60% drop in the number of cycles to failure as dwell is increased past 8 hours
• As the CTE mismatch decreases and the part becomes more compliant, the effect of
dwell decreases
© 2004 - 2007 © 2004 - 2010 47 47
Thermal Cycling: SnPb-SAC Transition
I. Kim, ECTC 2007
But not this simple – slope of curves and transition will
depend greatly on component type and dwell time.
© 2004 - 2007 © 2004 - 2010 48
Test
Spec
48
Thermal Cycling: When is a Failure Not a Failure? D
Tem
pera
ture
Time to Failure
Field Condition
Test
SnPb Pb-Free
Life R
equirem
ent
Understanding the acceleration factor is very important
© 2004 - 2007 © 2004 - 2010
SnCuNi Data at Different ΔT
The same was
done for resistor
and TSOP
components
49
© 2004 - 2007 © 2004 - 2010
SnCuNi – Acceleration Factor
y = 1E+07x-2.153
y = 2E+07x-2.011
y = 2E+07x-2.175 0
500
1000
1500
2000
2500
3000
3500
40 60 80 100 120 140 160 180
Cycl
es
to 1
% F
ailure
Delta Temperature C
SN100C Thermal Cycle Results
Resistor (2512)
TSOP 44IO Alloy 42
CSP 96IO, 7mm, 0.5mm
Power (Resistor (2512))
Power (TSOP 44IO Alloy 42)
Power (CSP 96IO, 7mm, 0.5mm)
n ~ 2.1 for
SN100C
AF = ΔTt
ΔTf ( )
n
50
© 2004 - 2007 © 2004 - 2010
Typical Pb-Free Thermal Cycle Results
Stiff Component More Compliant Component
Raw data from Qi et al., U. Toronto, Microelectronics
Reliability, 2005 Raw data from Ahmer Syed, Amkor
Thermal Cycle Analysis
27mm PBGA
1000
2000
3000
4000
5000
6000
7000
8000
50 70 90 110 130 150 170 190
Delta Temp
Cy
cle
s t
o 1
% F
ailu
re
SnPb
SAC
SnPb (n=1.55)
SAC (n=1.75)
Large/stiff components typically perform worse with Pb-free solder.
Acceleration factor is different – typically higher for Pb-free solder
Theoretical ► N1%App = N1%test(∆Ttest/ ∆TApp)n
y = 4E+06x-1.731
y = 8E+06x-1.895 300
500
700
900
1100
1300
1500
1700
1900
60 80 100 120 140 160 180
Cycl
es
to 1
% F
ailure
Delta T
Resistor 2512 - SnPb
Resistor 2512 - SAC
Power (Resistor 2512 -SnPb)
51
© 2004 - 2007 © 2004 - 2010
Testing of SAC, SnPb, and SN100C
Similar results when testing
from -40/125C (large delta T)
52
© 2004 - 2007 © 2004 - 2010
Expected Field Failure Time (1%)
0
2000
4000
6000
8000
10000
12000
14000
0 20 40 60 80 100 120 140 160 180
Cycl
es
to 1
% F
ailure
Ra
te
Temperature Change in Application
Extrapolation for a CSP (compliant component)
Sn100C
SnPb
SAC
n = 2.18
n = 1.75
n = 1.55
53
© 2004 - 2007 © 2004 - 2010
Estimation for 1% Field Failure Rate
0
2000
4000
6000
8000
10000
12000
14000
0 50 100 150 200
Cycl
es
to 1
% F
ailure
Temperature Change in Application
Extrapolation for a Resistor (stiff component)
Sn100C: n=2.15
SnPb: n=1.73
SAC: n=1.90
54
© 2004 - 2007 © 2004 - 2010
Thermal Cycle Requirements of New Alloys
o Test to failure with two or more delta T values.
o Create Weibull plots and calculate N1%.
o Using best fit, calculate n value in acceleration factor.
o Use n value to perform extrapolation to your desired
field use conditions.
Results in temp cycling (example -40/125C) are not
meaningful without the acceleration factor for that solder.
55
© 2004 - 2007 © 2004 - 2010
Thermal Cycle Results for SACM and SACC
Thermal Cycle (-
40/125C)
244 I/O BGA, 0.5 mm
Electrolytic NiAu on
package substrate.
OSP on PCB.
JEDEC Drop Test method
used.
Preconditioning at 150C.
Characteristic Life in Thermal Cycle Testing
SACM and SACC perform
similar to SAC305 in ATC
but are much better in
shock.
Ref: W. Liu et al., “Achieving High
Reliability Low Cost LF SAC Solder
Joints via Mn or Ce Doping”, ECTC,
2009.
56
© 2004 - 2007 © 2004 - 2010
SAC 105 vs SAC 305 Thermal Cycle
o Testing of a BGA memory device with SAC305 passed thermal
cycle requirements of 0/100C for 1000 cycles with no cracks
shown after dye and pry.
o The samples with SAC105 failed this testing with 30% of the
samples having cracks over 50% of the solder joint area.
57
© 2004 - 2007 © 2004 - 2010
SAC105 +Ni +Cr
o Addition of both elements improved shock performance.
Ran
jit S
Pa
nd
her,
Rob
ert
Hea
ley,
“R
elia
bili
ty o
f P
b-F
ree
So
lde
r
Allo
ys in
Dem
an
din
g B
GA
an
d C
SP
Ap
plic
atio
ns,”
Pro
ce
ed
ings
58
th E
lectr
on
ic C
om
po
nents
an
d P
acka
gin
g T
ech
nolo
gy
(EC
TC
), O
rla
nd
o, M
ay 2
7-3
0, 2
00
8.
58
© 2004 - 2007 © 2004 - 2010
SAC0307+Bi
o SACX (Cookson) is a version of this alloy.
o Thermal cycle results are said to approach that of SAC305.
Ran
jit S
Pa
nd
her,
Rob
ert
Hea
ley,
“R
elia
bili
ty o
f P
b-F
ree
So
lde
r
Allo
ys in
Dem
an
din
g B
GA
an
d C
SP
Ap
plic
atio
ns,”
Pro
ce
ed
ings
58
th E
lectr
on
ic C
om
po
nents
an
d P
acka
gin
g T
ech
nolo
gy
(EC
TC
), O
rla
nd
o, M
ay 2
7-3
0, 2
00
8.
59
© 2004 - 2007 © 2004 - 2010
Creep Results after Aging at 100C
Ref: J. Suhing, “Material Behavior of
Aging LF Solder Joints”, CAVE, Auburn
U, 2009 60
© 2004 - 2007 © 2004 - 2010 61
Divergence in Solder Selection
o Considerations include
o PRICE!
o Insufficient performance
o Newly identified failure mechanisms
o Market still unsteady; proliferation and evolution of material sets
o Solder seeing the fastest increase in market share?
o SnCu+Ni (SNC)
SAC405
SAC305
SAC105
SACX
SNC
SnAg
SNCX
SnCu SnAgCu
??
© 2004 - 2007 © 2004 - 2010
o It was easiest to widely adopt when the LF transition was
required.
o Its high strength provides better thermal cycle behavior for
compliant packages (BGAs, CSPs, QFPs).
o Its high yield strength enables better high cycle fatigue
performance (low amplitude vibration).
o The wetting properties are sufficient for surface mount
components (although head & pillow defects are more
common and lack of flow results in exposed Cu).
o Its higher creep resistance enables higher operating
conditions.
Advantages of SAC305
62
© 2004 - 2007 © 2004 - 2010
o Its high liquidus temp requires up to 260C processing
(higher energy usage – not really “green”, stresses the PCB
and components).
o It is a precipitation hardened alloy so the mechanical
properties change dramatically depending on processing
and aging conditions.
o Its marginal wetting behavior (high surface tension), Cu
dissolution, and cost are not ideal.
o Its high modulus results in pad cratering as a common
failure mode (under dynamic strain).
o The thermal cycle reliability is worse for “stiff” components
such as resistors and capacitors.
o Shock performance is much lower than SnPb.
Weaknesses of SAC305
63
© 2004 - 2007 © 2004 - 2010
SAC105
o Improved drop and shock performance over SAC305
o Thermal cycle life is less than SAC305
o Creep rate is very high
o Lower copper dissolution rates in SMT joints
o Reduced intermetallic compounds and occurrences of silver
tin platelets
o Is greatly improved with additions of Mn or Ce (more
data may prove these to be winners as a ball alloy)
64
© 2004 - 2007 © 2004 - 2010
Wave Solder Alloys
o SnAgCu – moderate wetting, much dross, excessive Cu
dissoluton, expensive.
o SnCu – poor wetting, lower cost, higher pot temp
required.
o SnCuNi (or SnCuX) – good wetting, moderate cost,
bath control required (to keep Cu & Ni ratio in spec).
65
© 2004 - 2007 © 2004 - 2010
SN100C
o Proving promising for wave solder and HASL coating.
o Careful control of Cu & Ni levels are required.
o Promising data for surface mount (ATC, vibration, shock).
o Smooth surface reduces crack initiation sites.
o Higher reflow temp required.
66
© 2004 - 2007 © 2004 - 2010
Supplementary Material
67
© 2004 - 2007 © 2004 - 2010 68 68
Thermal Cycling: Stress Relaxation Pb-free alloys demonstrate higher creep resistance
Results in greater durability under accelerated testing (fast ramps, short dwells)
Exception: Very high temperatures (>125oC), high stress loadings (leadless, ceramic)
When will Pb-free be less reliable in the field?
Time
Str
es
s
SAC
SnPb
Temperature
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Thermal Cycling: Effect of Dwell Time
0
2
4
6
8
10
0 100 200 300 400 500
SA
C life /
SnP
b life
Dwell Time (min)
Ceramic BGA on FR4
0 to 100C (experimental data, Bartello, 2001)
2512 Resistor on FR4
25 to 80C (modeling, Blattau, 2005)
Based on creep laws developed by Schubert
and damage model developed by Syed
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Copper Dissolution by Alloy
Initial Thickness = 1.7 mil
Ref: C. Hamilton& M. Kelly, A Study of Copper Dissolution in LF PTH Rework”, SMTA, 2006.
SnCuNi is similar to SnPb with respect to Cu dissolution rate.
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Intermetallic Growth
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Intermetallic Growth
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Intermetallic Growth
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Reflow of SAC105
The narrow thermal margin between the
liquidus temperature of the low silver
spheres and the peak temperature of the
assembly raises concerns about
incomplete ball collapse and incomplete
mixing of the solder alloy with the sphere
material, resulting in non-homogenous
solder joints. This head-in-pillow solder
joint was formed under temperatures high
enough to melt the SAC 305 solder, but too
low to melt the SAC105 sphere.
REFERENCE:
LOW-SILVER BGA ASSEMBLY PHASE I – REFLOW
CONSIDERATIONS AND JOINT HOMOGENEITY
SECOND REPORT: SAC105 SPHERES WITH TIN-LEAD PASTE
Chrys Shea
Ranjit Pandher
Cookson Electronics
South Plainfield, NJ, USA
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Dwell Time Impact on SAC351
Ref: E. Al-Momanl and M. Mellunas, “Lead-free Thermal Cycle
Progress, Unovis, June, 2008.
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Testing of SAC, SnPb, and SN100C
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SAC Mechanical Properties
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Auburn Drop Results
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Failure Analysis
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Evaluation of Lead-less Resistor Reliability
FEA results and calibration of model predictions with experimental
results. SnPb performs significantly better under these conditions.
Bend cycling ENIG
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SnPb Aging Effects
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SAC 405 Aging Effects
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o Tin and copper bond to form intermetallics of Cu3Sn and Cu6Sn5
o Irreversible
o Occurs rapidly in the liquid state, but rate still appreciable in solid
state (even at room temperature)
o Total intermetallic thickness after all assembly and rework should be
between 1 to 4 um
o Elements
o Bi is in solid solution in the tin-rich phase or precipitates out (>1%)
o In will form binary intermetallic species with Ag and Cu and ternary
intermetallic species SnAgIn and SnCuIn
o Co seems to display similar behavior to Ni
Intermetallic Basics
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Zn additions can cause SCC Mechanism
Suganuma, et.al, JIEP project paper,
Soldertec/IPC conference, Brussels, 2003 85
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Pb Contamination Results
Sn-Ag-Bi alloys have attractive mechanical properties but if mixed
with a small amount of Pb severe degradation occurs.
These hold promise as Pb is eliminated from the supply chain.
T. Woodrow, Boeing
Company, IPC Pb-free
conference proceedings,
San Jose, 2003.
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Indium Containing Pb-Free Alloys
Indium has attractive low melting temperature properties.
The primary issue is source of supply.
o Yearly amount of Pb solder used in electronics = 60,000 tons.
o Yearly world wide production of In = 100 tons.
Max wt% Indium allowable in a complete Pb-free replacement alloy = 0.20% (to
use up world wide supply and drive cost up).
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