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Freescale, the Freescale logo, AltiVec, C-5, CodeTEST, CodeWarrior, ColdFire, C-Ware, t

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

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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• 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

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• 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)

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• 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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• 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

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Source: Tanaka Denshi Kogyo

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• 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

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• 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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• 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

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• 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:

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• 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

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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

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• 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

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• 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

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• 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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• 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

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• 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

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• 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)

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• 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

http://www.freescale.com/docs/pcn_attachments/14554_I.MX51_Cu-Wire_Results.pdf



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• 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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• 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

• Preferred / typical failure mode for wire pull is break at neck

• Cu wire may also exhibit “break at hook” like examples below

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• 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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• 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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• 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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• 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

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• Analysis assistance:

−Anne Anderson

−Alvin Youngblood

−Roy Arldt

−Sam Subramanian

• Management support and discussions:

−Ed Hall

−Vikas Sheth

−Dr. Leo Higgins

−Trent Uehling

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Thanks!!!! Questions?

[email protected]

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