enig vs. enep(() pg) under bump metallization for lead ......metallization for lead-free wl-csp...

ENIG vs. ENEP(G) Under Bump ( ) pMetallization for Lead-free WL-CSP Solder

Bumps – a Comparison of IntermetallicProperties Using High Speed Pull Test

IMAPS International Conference on Device PackagingIMAPS International Conference on Device PackagingMarch, 17-20, 2008 – Scottsdale, AZ

Dr. Thorsten TeutschPac Tech-Packaging Technologies USA, Inc.

328 Martin Avenue, Santa Clara, CA 95050, USA

www.pactech-usa.com

Certified ISO 9001: 2000 & ISO TS 16949 Confidential

AbstractElectroless Nickel/Gold as a low cost under bump metallization for flip-chip andwafer-level CSP application is well established in the industry. Initial concerns ofthe technical community about reliability issues have been addressed bysuccessful implementation in several application fields such as powermanagement and protection devices, RFID and memory products, but alsomedical and automotive devices. Additionally, new technical challenges areproducing strong interest in the electroless metallization process driven more byperformance than only cost aspects.

Electroless Palladium, deposited as an additive metal layer of a few hundrednanometers on top of the Nickel layer, is investigated as a potential improvementof intermetallics formation with lead-free solders especially under high temperatureconditions. The different intermetallic phases observed on Ni/Au versus Ni/Pd/Aucan also change the brittleness and therefore, the stress compliance of the Sn-NiIMC.

This paper is presenting newest test results based on a comparison of 3 differentUBM configurations: Ni/Au, Ni/Pd/Au and Ni/Pd. Cross section and FIB analysis ofthe UBM has been performed to characterize the integrity of the UBM metal. Ahigh speed pull test set up has been utilized as a tool to monitor IMC properties ofthe UBM-solder interface with regard to mechanical and thermal stress


compensation.

Outline

• Company Profile• Overview on the use of electroless Ni/Au for FC WLCSP• Overview on the use of electroless Ni/Au for FC, WLCSP

and wire bond applications• Electroless Ni/Au and Ni/Pd/Au interface for Au wire

bonding• Ni/Au and Ni/Pd/Au IMC growth analysis for lead-free

soldersolder• Ni/Au vs. Ni/Pd UBM high speed pull test

performance for different solder alloysp y• Summary


Pac Tech Corporate Profile

• Established 1995 in Berlin-Germany

• Worldwide locations:Nauen, Germany; Santa Clara, CA; Himeji, Japan and Penang, Malaysia

• Shareholder Structure: NAGASE & CO., Ltd. Pac Tech GmbH GermanyFounders

• Business Units:

Advanced Packaging Equipment(Solder Jetting, Laser Flip-Chip Bonder, electroless Plating Equipment)

Pac Tech USA Inc.

Wafer Bumping Service

Chemistry


Pac Tech Asia Sdn. Bhd.

Advantages of electroless Ni/Au UBM1) Low Capital Investment Cost

Sputtering / ElectroPlating: > 10 Mio US$Electroless UBM: 1-2 Mio US$

2) High Throughput300.000 wafers per year minimal guaranteed throughput with PACLINE 300

3) Maskless ProcessNo tooling required

4) Low UBM Process Cost compared to ElectroplatingLowest in house cost process ~10 US$ for 8 inch in in high volume)Lowest in house cost process 10 US$ for 8 inch in in high volume)

5) 300 mm compatibility no additional invest (PACLINE 300)

6) Proven Reliability

7) Compatibility with all FC-Assembly processesSolderingACAConductive Adhesive

8) Suitable for Al and Cu pad metallization

9) Compatibility with Wire BondingRevolution: one pad metallization for wire bonding and Flip Chip


B k id C ti

Electroless Ni/Pd/Au Bumping on Al Electroless Ni/Pd/Au Bumping on Al

Backside Coating

Aluminum Cleaning / Cu Cleaning

Zincate / Palladium Pretreatment

Electroless Nickel

Electroless Palladium

Immersion Gold

Coating Removal


g

Process Flow - Electroless Ni/Au Bumping

Al Pad Cu Pad

Zinkating Pd Seed

Ni Plating

Flash Au Flash Au

Thick AuFlash Au

Pd Barrier

Thick AuFlash Au

FC & WLCSP Wire Bonding,FC & WLCSP Wire Bonding


Au Wire Bonding on Electroless Ni/Au Layer

• Surface Treatment• Surface Treatment– Ni/Au UBM– Needs Ar Plasma Cleaning/Activation– Lowest process cost– Sufficient bond window– Wire bond process needs to be optimized for thin Au layer

• Thick Au Finish– Ni/Au UBM– High chemistry cost– Longer processing time

Broad bond window– Broad bond window– Good reliability

• Pd LayerNi/Pd/Au UBM– Ni/Pd/Au UBM

– Broad bond window– Excellent cost / reliability ratio


Wire Bonding on Ni/Au for Power MOSFET / Power Switch Application

Challenges: Wire bondability of Ni/Au on gate pad after high temperature solder reflow

MOSFET / Power Switch Application

after high temperature solder reflow

Other Solutions: Bonding on Ni/Au> i d=> narrow process window

Ar plasma cleaning of Ni/Au=> process flow/ timing, p g,

availability of Ar plasma process

Best Solution: Wire bondable, T-resistend Interface=> electroless Ni/Pd/Au


=> electroless Ni/Pd/Au

Ni/Pd/Au MetallizationNi/Pd/Au Metallization

PdAu

Ni


Ni/Pd/A l lNi/Pd/Au+ plasma clean

Ni/AuNi/Pd/Au

From Jedec specs EIA/JESD22-B116


p

Still optimization by Pd thickness adjustment possible!

Intermetallic Formation of Lead-Free Solder Bumps on electroless Ni

Challenges for WLCSP’s:Lead free solder alloys show high failure rate in non linear

Bumps on electroless Ni

Lead-free solder alloys show high failure rate in non linear stress situation (drop test) due to brittle IMC interface compared to eutectic PbSn alloy

Root Causes:- Ni diffusion in bulk solder during ball attach reflow triggers formation of brittle Ni-Sn intermetallics

- some intermetallic formations are more brittle than others

P ibl O ti i tiPossible Optimization:Pd as diffusion barrier

=> electroless Ni/Pd/(Au) UBM


Comparison of Ni/Au and Ni/Pd/Au UBM after 1x and 3x solder reflow (SAC 405)3x solder reflow (SAC 405)

Reflow Ni/Au Ni/Pd/Au

1 x

3 x


IMC Interface Characterization

• Cross section comparison between Ni/Au and Ni/Pd/Au does not show significant difference in IMC layer thickness

• Brittleness of solder intermetallic area critical for reliability• Brittleness of solder intermetallic area critical for reliability performance of WL-CSP’s (Drop Test)

• Avoid brittle (Cu,Ni)3Sn4 IMC !

• Different IMC growth obtained in metallurgical analysis of lead-free solder for Ni/Au and Ni/Pd UBM systems

⇒ electroless Ni / immersion Au (ENIG) or el. Ni / el. Pd (ENEP)?

• Drop Test does not provide an in line monitor during wafer bumping

• Test for IMC quality (brittle or ductile) needed

⇒ Cold Ball High Speed Pull Test


Cold Ball High Speed Pull TestExperiment Setup

Test Parameters

pe e t Setup

Pull Test Speed Rising table pressure Jaw pressure Jaw Opening

50 mm/sec 0.025 Mpa 0.8 bars 350um

100 / 0 040 M 0 8 b 350

Test Parameters




1000 mm/sec 0.055 Mpa 0.8 bars 350um

Dage HS Pull Tester000 /sec 0 055 pa 0 8 ba s 350u

Cold Bump Pull

Tools and MaterialsCold Bump Pull

Jaw PCB Adhesive Dage CBP JAW

300Pactech PCB

Board Pattex / Henkel


Cold Ball High Speed Pull TestTest Matrix

Sample UBM Solder AlloyNA1 NiAu SAC105NP1 NiPd SAC105

est at

Ni thickness: 5 µmPd thickness: 0 2 µmNP1 NiPd SAC105

NA3 NiAu SAC305NP3 NiPd SAC305NA4 NiAu SAC405NP4 NiPd SAC405

Pd thickness: 0.2 µmAu thickness: 0.05 µm

Failure Mode AnalysisO ti l MiOptical Microscope

ductile quasi-ductile quasi-brittle brittleq q


Cold Ball High Speed Pull TestResults50 mm/sec esu ts50 mm/sec


Cold Ball High Speed Pull TestResults500 mm/sec esu ts500 mm/sec


Cold Ball High Speed Pull TestResult Overviewesu t O e e

Ni/Au and Ni/Pd Comparison Using SAC305 on Different Pull Speed


70 00%

80.00%

90.00%

100.00%

110.00%

Ni/AuNi/Pd

re M

ode)

80.00%

90.00%

100.00%

110.00%

Ni/AuNi/Pd

Mod

e)

20 00%

30.00%

40.00%

50.00%

60.00%

70.00%

% D

uctil

e (F

ailu

r

30 00%

40.00%

50.00%

60.00%

70.00%

Duc

tile

(Fai

lure

M

0.00%

10.00%

20.00%

50mm/sec 100mm/sec 300mm/sec 500mm/sec 1000mm/secPull Speed (mm/sec)

0.00%

10.00%

20.00%

30.00%

50mm/sec 100mm/sec 300mm/sec 500mm/sec 1000mm/secPull Speed (mm/sec)

% D


Pull Speed (mm/sec)

Cold Ball High Speed Pull TestResult Overviewesu t O e e


70 00%

80.00%

90.00%

100.00%

110.00%

Ni/Au

Ni/Pd

de)

30 00%

40.00%

50.00%

60.00%

70.00%

ctile

(Fai

lure

Mod

0.00%

10.00%

20.00%

30.00%

50mm/sec 100mm/sec 300mm/sec 500mm/sec 1000mm/sec

% D

uc

Pull Speed (mm/sec)

• Ni/Au approaches earlier brittle fracture as the pull speed becomes higher compared to Ni/Pd• For higher pull speed the failure mode approaches in more brittle fracture


• For higher pull speed the failure mode approaches in more brittle fracture

Conclusions

• High Speed Pull test is a suitable tool to measure IMC performance on WLCSP devices (≥ 250µ ball size)

• Pd layer after Ni reduces slightly the IMC growth of Sn-Ni intermetallics. However, no significant difference detected after 3 reflows!

• Low Ag solder alloy (SAC105) outperforms high Ag content solder materials (SAC305 and SAC405)SAC105 is most ductile solder alloy

• Ni/Pd UBM shows ~15% better pull test performance than Ni/Au in case of SAC 105 solder material

• WLCSP’s with SAC105 (300µm spheres) on electroless Ni/Au are


already fully qualified (including drop test) in volume production

Next Steps

• Low speed shear test correlation

• Influence of ball attach reflow profile and reflow methodep

• Include more low Ag solder alloys

• Variation of Pd thickness

• Influence of Al pad structure (geometry & thickness)Influence of Al pad structure (geometry & thickness)

• Characterization of Au interface: Is there a correlation between Au wire bondability and IMC growth formation?wire bondability and IMC growth formation?


enig vs. enep(() pg) under bump metallization for lead ......metallization for lead-free wl-csp...

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