TM
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he Energy Efficient Solutions logo, mobileGT, PowerQUICC, QorIQ, StarCore and Symphony
are trademarks of Freescale Semiconductor, Inc., Reg. U.S. Pat. & Tm. Off. BeeKit, BeeStack,
ColdFire+, CoreNet, Flexis, Kinetis, MXC, Platform in a Package, Processor Expert, QorIQ
Qonverge, Qorivva, QUICC Engine, SMARTMOS, TurboLink, VortiQa and Xtrinsic are trademarks
of Freescale Semiconductor, Inc. All other product or service names are the property
of their respective owners. © 2011 Freescale Semiconductor, Inc.
Confidential and Proprietary
Andrew Mawer, Dr. Stephen
Lee, Tu Anh Tran and Les
Postlethwait
Freescale Semiconductor
Austin, Texas USA Feb. 5, 2013
TM
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All other product or service names are the property of their respective owners. © 2011 Freescale Semiconductor, Inc.
2
Confidential and Proprietary
• Cu Wire Cost Motivation
• Cu Wire Technical Advantages
• Cu-Al IMC Formation and Phases
• Cu Wire Challenges
• X-Ray of Cu Wire
• Qualification Stress Testing
• Wire Pull / Wire Bond Ball Shear
• Pd Coated Cu (PCC)
• Conclusions
TM
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All other product or service names are the property of their respective owners. © 2011 Freescale Semiconductor, Inc.
3
Confidential and Proprietary
• Cost was the initial motivator
for transition from Au to Cu
bond wire
• Large increase in Cu wire
shipments once Au
exceeded $800/oz in 2009
• Oct. 16, 2012 prices:
−Au = $1744.40/oz
−Cu = $0.23/oz
−~7500X difference
• Depending on wire diameter,
Au spool can cost 10 to 100X
more than Cu
Historical Au Pricing (1993 –Present)
Au and Cu Bond Wire Spools
Au
Pri
ce (
$ /
oz)
TM
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All other product or service names are the property of their respective owners. © 2011 Freescale Semiconductor, Inc.
4
Confidential and Proprietary
• For Au, bond wire cost varies by wire diameter
• Due to low raw material cost, Cu generally does not
• Leading edge wire diameter is 18 to 20 microns
• Bond wire cost based on May 2012 Au pricing:
Wire Diam. (mm) Wire Cost ($/1000 ft)
Au Cu
18 77.1 7
20 91 7
25 142.4 7
32.5 210.3 7
37.6 329 7
50 561 7
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All other product or service names are the property of their respective owners. © 2011 Freescale Semiconductor, Inc.
5
Confidential and Proprietary
• Wire bond diagram of a PBGA with 769 wires @ 20 mm diam
• Total wire length per part of 2.1 meters
• Wire cost of 30¢ for Au and only 2.3¢ for Cu
Typical Cu Ball Bond
on This Device
20 mm
Wire
Diam
Wire Bond Diagram Showing 769 Wires
Cu Ball
Bond
M/C
Si Die
Al Bond
Pad
TM
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All other product or service names are the property of their respective owners. © 2011 Freescale Semiconductor, Inc.
6
Confidential and Proprietary
Source: Tanaka Denshi Kogyo
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and Symphony are trademarks of Freescale Semiconductor, Inc., Reg. U.S. Pat. & Tm. Off. BeeKit, BeeStack, ColdFire+, CoreNet, Flexis, Kinetis, MXC, Platform in a
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All other product or service names are the property of their respective owners. © 2011 Freescale Semiconductor, Inc.
7
Confidential and Proprietary
• Compiled from major wire bond suppliers Tanaka, Heraeus,
Sumitomo Metal Mining (SMM), Nippon Micrometal (NMC), and
MK Electron (MKE)
• Make up 90% of the bonding wire market
Source:
TechSearch Intl.
*
* PCC =
Palladium
Coated Cu
TM
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All other product or service names are the property of their respective owners. © 2011 Freescale Semiconductor, Inc.
8
Confidential and Proprietary
• Besides cost, Cu wire can offer advantages in other areas:
− Thermal conductivity
− Electrical performance
− Intermetallic compound (IMC) stability at higher temperatures
Material
Property Units
Wire Type
4N Au 2N Au Cu
Purity % 99.99 99.0 99.99
Resistivity m•cm 2.2-2.4 3.0 1.7
Elastic Modulus GPa 80-95 85-100 60-90
Tensile Strength N/mm2 >240 >260 160-220
Elongation at
Break % 2-6 2-6 7-15
Load at Break cN >3 6-12 4-10
Thermal
Conductivity W/m•K 317 401
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All other product or service names are the property of their respective owners. © 2011 Freescale Semiconductor, Inc.
9
Confidential and Proprietary
• Taking into account the frequency-dependent skin effect,
resistance of a 4 mm long Cu bond wire can be ~10-20%
lower than Au depending on wire diameter
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1
1.1
1.2
0.5 0.6 0.7 0.8 0.9 1
Wire Diameter (mil)
R o
f 4
mm
Wir
e
Au (500 MHz)
Au ( 1 GHz)
Cu (500 MHz)
Cu (1 GHz)
Resistance of a 4 mm wire
TM
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All other product or service names are the property of their respective owners. © 2011 Freescale Semiconductor, Inc.
10
Confidential and Proprietary
• Au-Al bonds have historically been proven reliable for many
applications, including automotive under hood
• For higher operating temperatures (150 to 175C), the Cu-Al
bond may offer more stability due to lower diffusion rates:
TM
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All other product or service names are the property of their respective owners. © 2011 Freescale Semiconductor, Inc.
11
Confidential and Proprietary
• With Cu-Al, only a thin IMC (i.e., ~0.1 mm) formed initially
compared to Au-Al IMC
Cu Ball
Bond
Die Cu Metallization
Die Al Bond Pad
Cu-Al IMC
Cu Ball
Bond
TM
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All other product or service names are the property of their respective owners. © 2011 Freescale Semiconductor, Inc.
12
Confidential and Proprietary
FIB / SEM Showing Cu-Al IMC
Phases Following 150C for
504 hrs
TEM Showing Cu-Al IMC Phases
Which Is in Good Agreement with
Literature
TM
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and Symphony are trademarks of Freescale Semiconductor, Inc., Reg. U.S. Pat. & Tm. Off. BeeKit, BeeStack, ColdFire+, CoreNet, Flexis, Kinetis, MXC, Platform in a
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All other product or service names are the property of their respective owners. © 2011 Freescale Semiconductor, Inc.
13
Confidential and Proprietary
• Besides the slower IMC growth rate, Cu-Al IMC less likely to
form voids relative to Au-Al
Cu Ball
Bond
Die Cu Metallization
Al Bond
Pad
Cu-Al IMC
Au-Al IMC Formation Following
175C for 544 hrs with Extensive
Kirkendall Voids Formed
Cu-Al IMC Formation Following
175C for 2,016 hrs with No Voiding
Observed
Cu Ball
Bond Au Ball
Bond
TM
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All other product or service names are the property of their respective owners. © 2011 Freescale Semiconductor, Inc.
14
Confidential and Proprietary
• Primary Cu wire challenge is forming a reliable bond that strikes a balance between: 1) IMC coverage, 2) Al splash, 3) Al remnant and 4) does not damage die layers due to Cu being harder than Au
• Images below show potentially excessive Al splash (left) and thinner than desirable Al remnant (right)
2) Al
Splash 3) Al
Remnant
1) Cu-Al IMC Layer
4) Die Layers
TM
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All other product or service names are the property of their respective owners. © 2011 Freescale Semiconductor, Inc.
15
Confidential and Proprietary
• Au wirebonders must generally be upgraded to Cu wire capability
• Conversion cost is approx $10K, takes a day and may include: − Adding N2 or forming gas capability through the use of tubes or nozzles that
direct the gas with an accompanying flow controller/monitor
− Alternate electronics flame off (EFO) box
− Software upgrade
− Specialized capillaries
Glass
Tube
Vertical
Tube
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All other product or service names are the property of their respective owners. © 2011 Freescale Semiconductor, Inc.
16
Confidential and Proprietary
• Another challenge associated with Cu wire is corrosion of the Cu9Al4
IMC in biased accelerated moisture testing (THB, HAST)
• Can results in a volume expansion and open
• Control of the pH value and Cl- content in the mold compound is critical
Cu Ball Bond
Corrosion Layer
EDS Shows Low
Level Cl at the
Corrosion Layer
TM
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All other product or service names are the property of their respective owners. © 2011 Freescale Semiconductor, Inc.
17
Confidential and Proprietary
• Fuming nitric acid commonly used to decapsulate Au
wirebond packages for FA, wire pull and ball bond shear
• Fuming nitric known to attack Cu
• New decapsulation recipes developed for Cu wire
Cu Wires
Attacked by
Fuming Nitric
No Attack
With New
Recipe
TM
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All other product or service names are the property of their respective owners. © 2011 Freescale Semiconductor, Inc.
18
Confidential and Proprietary
• Au wires are easier to resolve in X-ray versus Cu wires
• May need a known wire composition device in order to
verify wire type for an unknown device
110kV X-Ray of Same Device Type with Au (Left) and Cu Wires (Right)
TM
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All other product or service names are the property of their respective owners. © 2011 Freescale Semiconductor, Inc.
19
Confidential and Proprietary
• Cu wire-bond products generally expected to meet same quality and reliability requirements as Au wire products
• Example of commercial qualification of a 529 pin, 19x19 mm PBGA that was converted from Au to Cu wire:
Stress Test Stress Condition Sample
Size Result
Moisture Pre-
conditioning (PC)
30C / 60%RH 192 hrs soak
+ 3X 260C peak reflow 720 0 fails
Highly Accelerated
Stress Test (HAST)
PC + 130C / 85%RH for 96
hrs. Biased in a socket. 120 0 fails
Unbiased HAST
(uHAST)
PC + 130C / 85%RH for 96
hrs. Unbiased. 120 0 fails
Temperature Cycling
(TC)
PC+500 -50 to 125C
cycles. Extended readpoint
at 1,000 cycles
240 0 fails
High Temp Storage
Life (HTSL)
150C for 504 hrs.
Extended readpoint at 1,008
hrs
240 0 fails
Source: http://www.freescale.com/docs/pcn_attachments/14554_I.MX51_Cu-Wire_Results.pdf
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20
Confidential and Proprietary
• Qualification stress testing also includes wire pull and
ball shear, especially for automotive
• Again, Cu must meet the same requirements at Al
Wirebond
Integrity Test Specification Details
Wire Pull
AEC-Q100 references MIL-883 Method 2011 for wire pull method
Pull required at time zero and after TC - also done as part of
CZ unmolded
Min pull is wire diameter dependent
Failure modes categorized, but no specific failure modes
disallowed
Ball Bond
Shear
AEC-Q100-001 calls out its own Method for bond shear called
001
Shear required at time zero only by AEC – also done as part
of CZ on unmolded parts
Minimum shear is ball bond diameter dependent
Failure modes categorized, but no specific mode required
Spec states that “values are applicable for Au bonds on Al
bonding surfaces” so net yet updated for Cu
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21
Confidential and Proprietary
• Min wire pull strength comes from MIL-883 Method 2011
• Min pull is wire diameter dependent
• Cu not mentioned so requirements for Au used
21 21
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22
Confidential and Proprietary
22
• Preferred / typical failure mode for wire pull is break at neck
• Cu wire may also exhibit “break at hook” like examples below
TM
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23
Confidential and Proprietary
• AEC-Q100-001 calls out its own Method for bond shear called 001
• Min shear is ball bond diameter dependent
• Spec states that “values are applicable for Au bonds on Al bonding
surfaces” – so not updated for Cu
23
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24
Confidential and Proprietary
• Preferred / typical bond shear failure mode is through the ball bond
• Due to hardness of Cu and minimal remaining Al thickness under
ball bond (i.e., remnant), shear of Cu bonds may result in almost no
Al remaining on pad
Typical Ball Bond Shear with Cu
Remaining on Al Pad
Ball Bond Shear with No Cu and
Minimal Al Remaining on Pad
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25
Confidential and Proprietary
• Most Cu wire bonding in the industry now taking place with
Pd coated Cu wire (PCC)
− ~0.08 mm layer of Pd over the Cu wire
• Potential advantages of PCC:
− Longer factory floor life (6 months versus 6 days)
− Corrosion resistance during accelerated moisture stressing
− May allow for increased 2nd bond peel strength for eplate NiAu BGA
substrates and NiPdAu pre-plated leadframes
2nd Bond
(Stitch Bond)
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26
Confidential and Proprietary
• Transition to Cu wire started in earnest in 2009 when the price of Au first
exceeded $800/oz
• Initial devices that were converted were typically low lead count, larger
wire diameter (>25 mm) leadframe based packages such as QFP, QFN
and SOICs
• >30% of bare wire shipments are now Cu
• Besides the obvious cost advantage, Cu wire can offer:
− Greater IMC stability to benefit high operating temperature applications
− Better electrical performance
− Better thermal performance
• Challenges:
− Ensuring good ball bond and IMC formation with no die circuitry damage
− Al-Cu IMC corrosion during highly accelerated moisture stressing
− Future challenges include more stringent requirements for automotive, higher
performance and high pin count products
TM
Freescale, the Freescale logo, AltiVec, C-5, CodeTEST, CodeWarrior, ColdFire, C-Ware, the Energy Efficient Solutions logo, mobileGT, PowerQUICC, QorIQ, StarCore
and Symphony are trademarks of Freescale Semiconductor, Inc., Reg. U.S. Pat. & Tm. Off. BeeKit, BeeStack, ColdFire+, CoreNet, Flexis, Kinetis, MXC, Platform in a
Package, Processor Expert, QorIQ Qonverge, Qorivva, QUICC Engine, SMARTMOS, TurboLink, VortiQa and Xtrinsic are trademarks of Freescale Semiconductor, Inc.
All other product or service names are the property of their respective owners. © 2011 Freescale Semiconductor, Inc.
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Confidential and Proprietary
• Analysis assistance:
−Anne Anderson
−Alvin Youngblood
−Roy Arldt
−Sam Subramanian
• Management support and discussions:
−Ed Hall
−Vikas Sheth
−Dr. Leo Higgins
−Trent Uehling
TM
Freescale, the Freescale logo, AltiVec, C-5, CodeTEST, CodeWarrior, ColdFire, C-Ware, the Energy Efficient Solutions logo, mobileGT, PowerQUICC, QorIQ, StarCore
and Symphony are trademarks of Freescale Semiconductor, Inc., Reg. U.S. Pat. & Tm. Off. BeeKit, BeeStack, ColdFire+, CoreNet, Flexis, Kinetis, MXC, Platform in a
Package, Processor Expert, QorIQ Qonverge, Qorivva, QUICC Engine, SMARTMOS, TurboLink, VortiQa and Xtrinsic are trademarks of Freescale Semiconductor, Inc.
All other product or service names are the property of their respective owners. © 2011 Freescale Semiconductor, Inc.
28
Confidential and Proprietary
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