recent advances in anisotropic conductive adhesives (acas

Nano Packaging & Interconnect Lab.

Kyung W. Paik

Korea Advanced Institute of Sci. and Tech.(KAIST) Dept. of Materials Science & Engineering


IEEE CPMT Orange Chapter6/4/2013

Recent Advances in Anisotropic Conductive Adhesives

(ACAs) Technology:Materials and Processing

Technology Features ApplicationsMarket

Size (M$)

Periods

Remark

1. ACF(Anisotropic

Conductive Film)

Thermo-curable resin+metal conductive ball

- Flat Panel Display assembly (COG, COF, FOG, FOB )- Camera module - Touch screen panel- connector replacement (FOB, FOF)- Fine pitch interconnect

1000(203)

1997~2007

-Licensed to H&S Hightech

(35 Million $/2012)

1-1. Solder ACF Polymer Resin +solder ball

2008~2013

-Funded by Nokia

-Engineering sample

1-2. Nanofiber ACF

Polymer Resin +nanofiberconductive ball

2010~2013

-2011 Licensed to OptoPac Co.

- For COG & Nokia FOF ACFs

2. 3D-TSV NCFPolymer Resin for 3D-chip stacking

-3D TSV wafer-wafer, chip-wafer stacking

652009~2013

- wafer level process developing with SEC

-NCFs developed with KCC

3. Metal Nanowire/Polymer Composite film

Polymer Resin +vertically aligned metal nanowire

- Electrical interconnection- TIM

3802012~2013

Funded by KOLON

4. Graphene/Polymer

Polymer Resin +electrically aligned

-Gas barrier film-EMI shielding film-TIM

7002012~2013

Funded by Korean government

B-Stage Polymer Films Research at KAIST NPIL Lab.


§ New ACF Processing § Ultrasonic(US) bonding (vs. thermocompression bonding)

§ Flex on Board (FOB) and Flex on Flex (FOF) combined with Solder ACFs – fast and reliable

§ Touch Screen Panel (TSP) applications – fast and no thermal damage§ Wafer-level ACFs Process

§ 3D TSV NCF Vertical Interconnection

§ New ACA Materials§ Nanofiber ACF materials for fine pitch applications

§ < 20 um ultra fine pitch for COG and COF§ < 100 um fine pitch for FOF

§ Solder ACF materials combined with US bonding - High current handling and excellent reliability

§ Summary

Contents


Anisotropic Conductive Films (ACFs)

Thermosetting epoxy resin film Conductive particles+§ ACF =

Chip or substrate 1

Substrate 2

Heat

Chip or substrate 1

Substrate 2

Pressure

ACFACF

Chip-On-Board Chip-On-Flex (COF)Chip-On-Glass (COG)Flex-On-Glass (FOG)

Flex-On-Board (FOB)

§ Applications of ACFs bondingThermal curing


ACFs bonding – Thermocompression vs. Ultrasonic Bonding

Chip or substrate 1

Substrate 2ACFHeat conduction

T/C ACFs bondingHeating tool

U/S ACFs bonding

Chip or substrate 1

Substrate 2

U/S horn

Local heat generation

Heating tool (250~350)à ACF (180)

U/S horn (R.T.) àACF (> 250)

Heat

Patented


New U/S ACFs Bonding MechanismDue to the loss modulus of ACFs,ACFs generate spontaneous heat under vertical ultrasonic vibration

Heat = f (Dε)2 E’’

2

f : Vibration frequency

Dε : Strain at the ACF layer

E’’ : loss modulus of the ACF

ACF temperatures can be controlled by DεACF temperatures can be controlled by Dε.

0 2 4 6 80

50

100

150

200

250

300

ACF

tem

epra

ture

(o C)

Bonding time (sec)

RelativeU/S vibration

10% 20% 30% 40% 50% 60%

ACF actual temperature profiles of FOB

nKorea patent issued 10-0746330nInternational Patent pending PCT/KR2006/004912nJapan, Taiwan, China, Germany patent pending


Vision systemU/S part

Stage

U/S controllerMain controller

Commercial U/S bonding machine by MicroPACK Co.

MPB-U110 : 1 head, 1 stage, manual alignment


Advantages of U/S vs. T/C ACF bonding

1. Reduced Assembly Times & Cost• Faster ACFs bonding times of U/S

• Epoxy ACFs: 3 ~ 5 sec. vs. 7 ~ 15 sec.• Acrylic ACFs: 1 ~ 3 sec vs. 5 ~ 7 sec.

2. Reduced thermal damage on boards• T/C bonding induces thermal damages to boards• No thermal damages by U/S

• No bonding tool heating• Very effective on FOB (organic PCBs) such as Touch Screen

Panel (TSP) and thermally sensitive special devices assembly 3. Eco-Processing

• Significant energy saving• Very little U/S loading cycle• No idling energy loss – digital control

Nano Packaging and Interconnect Lab.


ACFBonding area

§ Touch screen panels

§ Semiconductor packaging

Potential applications of U/S bonding method

§ Module interconnections(Flex-on-Board, Flex-on-Glass)

(Flip Chip-on-Board)

(RF-ID)

(Chip-on-Flex)

(PET panel)

Nano Packaging and Interconnect Lab (NPIL)

Recent issues in mobile device packages - Modulization

Source : Nokia

§ Current trends in mobile devices

1. High performance

2. Multi-functionalization

3. Miniaturization

Conventional Socket-type connectors

ü Advantages§ Fine pitch capability§ Reduced package size§ Process effectiveness

FOB(Flex-on-Board) & FOF (Flex) assemblyusing ACFs

ü Disadvantages§ Large volume & height§ Large pitch size400 um pitch

ACFFPCB

PCB


Why solder ACFs with ultrasonic bonding?

ACF ACF

ElectrodesOrganic rigid board

Electrodes

Flexible substrateFlexible substrate

Organic rigid board

Conventional ACF Solder ACF + US

Solderparticle

Electrode

Electrode

Adhesive

Physical point contact§ Limitations in electrical properties and

reliability

Metallurgical bonding

§ Higher current carrying capability

§ Improved reliability

Patent pending

Heat generationUp to 300 oC

Metalparticle

Adhesive

Electrode

Electrode


§ FOB substrates

ü Metal Electrode pitch : 200 μmü PCB : FR4 rigid substrate(700 μm thickness)ü FPCB : Polyimide film(30 μm thickness)

48 mm

§ Solder ACFs

Bonding area :19 mm X 2 mm

Solder ACF specifications

Resin Thermosetting acrylic

Solder particle composition

Sn-58Bi(melting temp. : 138)

SAC305(96.5Sn3.0Ag0.5Cu)(melting temp. : 217)

Solder particle size

20 μm

Solder particle content

10 wt%

Sn-58Bi

SAC305

PCB

ACFFPCB

101 mm

PCB

FPCB

ACF

700 μm

30 μm

35 μm

ThermosettingAcrylic resin

FOB Test boards and Solder ACFs


13 um225 oC

7 um amplitude110 oC

8 um130 oC

9 um150 oC

10 um170 oC

11 um190 oC

12 um210 oC

Melting

Wetting

Soldering within the ACF adhesive matrix

§ In US bonding, wetting of solder particles was successfully achieved at above 200 oC without any void formation.

§ Melting of Sn-Bi solder particles

observed at above 150 oC

§ Wetting of solder particlesincreased as the bonding temperature increased.

§ No ACF void formation

Sn-58Bi 45 um2MPa, 5 secFOB assembly using solder ACFs by US bonding


45um Sn-58Bi225 oC 2MPa 5 sec

ACF

Organic rigid boardElectrodes

Electrode Electrode Electrode

SolderSolder

Top electrodes surface after peel-off (200x)Flexible substrate

Electrode Electrode Electrode

Solderresidue

Solderresidue

Conventional ACF 20 um solder ACF 40 um solder ACF

Bottom electrodes surface after peel-off (200x)

FOB assembly using solder ACFs by US bondingFlexible substrate

Indentation markon electrode

Solder residueon electrode

Physical point contact Solder alloy bonding


Top electrodes surface after peel-off (3000x)


Adhesivematrix

Adhesivematrix

ACF

Organic rigid boardElectrode

Flexible substrate

Top electrode surface

Bottom electrode surface


Conventional T/C : 160 oC 2MPa 6 secU/S : 225 oC 2MPa 5 sec

Solder ACF joints - Contact resistance

0

2

4

6

8

10

12

Cont

act r

esis

tanc

e (m

Ohm

)

Contact resistances

Conv.ACF

20umsolderACF

45umsolderACF

§ The solder ACF joints showed 30% lower contact resistances than that of the conv. Ni ACF joints due to solder alloy bonding of solder particles.


Current carrying capability


Solder ACF joints - Current carrying capability

0.0 0.5 1.0 1.50.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

4.0

Curr

ent (

A)

Bias (V)

45um solder ACF 20um solder ACF Conv. ACF

à Conv. ACF

à 20um solder ACFà 45um solder ACF >30%

§ The solder ACF joints showed >90% higher current carrying capabilitythan that of the conv. Ni ACF joints due to solder alloy bonding.

§ The PCB line burns before ACF joint failure.

Joint failureat 1.6 A

Joint failureat 2.66 A

PCB line burningat 3.0 A

Line width : 160 umLine thickness : 18 um

60%


0 20 40 60 80 100 120 140 160 180 2000

20

40

60

80

100

0 hrs 5 hrs 10 hrs

Contact resistance (mOhm)

Cum

ulat

ive

dist

ribut

ion

(%)

0 20 40 60 80 100 120 140 160 180 2000

20

40

60

80

100

0 hrs 5 hrs 10 hrs


Cum

ulat

ive

dist

ribut

ion

(%)

0 20 40 60 80 100 120 140 160 180 2000

20

40

60

80

100

0 hrs 5 hrs 10 hrs


Cum

ulat

ive

dist

ribut

ion

(%)

§ The solder ACF joints showed significantly improved reliability in an unbiased autoclave test due to solder alloy bonding of solder particles.

Rapid degradation of physical point contacts

Unbiased autoclave test results

Conventional Ni ACF 20 um solder ACF

Stable resistances Stable resistances

(121 oC, 2 atm, 100%RH, 48 hrs)


Solder ACF joints - Reliability

45 um solder ACF

Early failures


Touch panel

ATM

Mobile devices

Navigation

Tablet PC

Touch Screen Panel (TSP) ACF assembly by US bonding

Touch panel base materials

Glass PET(Polyethylene terephthalate)Low cost

light weightshock resistance

Applications

Low temperature process needed!

Weak thermal resistance! (Tg = 75 )

19

Flexible PCB

Touch Screen Panel

ACF Bonding


§ Optimization in terms of ACF joint characteristics

ACF joint characterizations – adhesion and degree of cure

1 sec 3 sec 5 sec 7 sec

120 < 100 160 90 170

140 < 100 200 290 410

160 < 100 120 600 570

180 < 100 520 610 580

Peel adhesion strength (gf/cm)

Stable resistance

Degree of cure> 90 %

20

Sufficient ACF degree of cure at 160, 5sec and 180, 5 sec


TSP ACF joints by US bonding

FPCB

Touch panel

§ No PET deformation § ACF conductive balls well captured between Ag paste and FPC electrodes.

Ag electrodes

Cu electrodes

No PET deformation

Cu electrodes of FPCBs

Ag electrodes of touch panels

Well capturedconductive ball

21


Wafer Level ACF Technology

Applications§ COF§ 3D TSV NCF Vertical Interconnection


o Conventional flip chip assembly processes using ACFs

Flip chip assembly

Wafer bumping

ACF coating

Chip singulation

ACF slitting

Remove a releasing film

ACF pre-lamination on a substrate

In the WLP processes using ACFs,Number of processing steps can be significantly reduced.

o Wafer-level flip chip assembly using pre-applied ACFs & NCFs

Chip singulation (ACF-applied wafer dicing)

ACF

Si chip

ACF lamination on a wafer

Remove a releasing film

ACF

Silicon wafer

ACF

Releasing film

Silicon wafer

• Korea patent : 361640

• US patent : 6,518,097

Wafer level ACFs/NCFs or ACP/NCP


6 inches test wafer

Test chip (8X8 mm) pattern

Au plated bumps

Wafer level ACF COB packages DemonstrationoACF width : 8cm

oACF thickness : ~ 50um thickness

oConductive particles : Ni/Au coated polymer ball (5um diameter)

oSupplied by H&S HighTech Inc. (Korea)

50um

• Cross-section of ACFs • 5um diameter conductive particles

Propertiesa1

(ppm/oC)a2

(ppm/oC)Tg G' at R.T G' above Tg

Value 48 324 133.4 oC 2.75 GPa 0.0723 GPa

Test PCB design

• Test boards (PCBs)

- Size : 2.5mm (w) x 2.5mm (l) x 1mm (t)

- PCB finishes : Electroless Ni/Au pads on Cu lines

• Bonding force : 20N, 40N, 60N, and 80N

• Bonding temperature : 180oC (chip), 80oC (PCB)

• Bonding time : 25sec


v Void-free ACFs lamination on Au bumped wafers.

v Si Particle-free wafer dicing

v 0.5~0.7wt% of moisture absorption

– No problem of subsequent assembly and reliability

¼ of 6” wafer with pre-applied ACF (after dicing)

Kerf

Al line

Au bumps

Magnified image near scribing line

ACF

Wafer level ACF COB demonstration

ACF

o Void-free ACF lamination and Defect-free wafer dicing

An ACF-WL Package


COB Assembly Properties using Wafer level ACFs

Chip

PCB

40N Pressure Assembly

40N 60N 80N

0

5

10

15

Bump contact resistance (m

W)

Bonding force

Comparison of contact resistanceConventional flip chip assemblyWafer-level flip chip assembly

v Stable bump contact resistance at > 40N

v Similar contact resistance as conventional T/C ACF bonding

0

50

100

150

200

Wafer-level flip chip assembly

Die shear strength (kgf/cm2)

Conventional flip chip assembly

Die adhesion strength measurement

v Silicon chip fracture during a die shear test

v Similar die adhesion strength as conventional T/C ACF bonding

chip

PCB


o ACF-WLP assembly o Conventional ACF assembly

Reliability of COB Assembly using ACF-WLP

Contact resistance (m W)0 25 50 75 100 125 150

0

20

40

60

80

100

Cumulative distribution (%)

85 oC/85% RH reliability 0 hr

160 hrs335 hrs580 hrs

0 25 50 75 100 125 150

0

20

40

60

80

100


0 hr160 hrs335 hrs580 hrs

85 oC/85% RH reliability

0 25 50 75 100 125 150

0

20

40

60

80

100

Contact resistance (m W)


T/C reliability (-40oC, 15min ~ +125oC, 15min)

0 cycle150 cycles270 cycles390 cycles550 cycles700 cycles820 cycles

0 25 50 75 100 125 150

0

20

40

60

80

100


(-40oC, 15min ~ +125oC, 15min)0 cycle

150 cycles270 cycles390 cycles550 cycles700 cycles820 cycles

T/C reliability



No reliability difference between conventional TC and US ACF bonding


Current 3-D Packaging Trend & TSVØ Current 3-D Packaging

Source : Samsung

< 2Gb NAND flash dies >

Substrate

Chip 1Chip2 TSV

< Application processor & memory chip >

< System on Chip (SoC) >

Source : Tohoku Univ.

< Through Silicon Via (TSV) >Source : P. Leduc, LETI, D43D, 2010

Mobile chip & module

NAND & DRAM Memory System on Chip (SoC)

TSV is the core technology of current 3-D chip stacking technology.


Cu diffusion bonding Cu/SnAg double bump bonding

Image

General process

condition

• Vacuum process (10 -3 torr)• High temperature (< 300 oC)• Long time (>30 min.)

• Normal pressure process (1 atm)• Low temperature (~250 oC)• Short time (< 1 min.)

Characteristics• Low electrical resistivity• High electro migration resistance• Need of surface flatness &

cleanness

• High throughput yield• Cost effective• Electro migration & solder oxide

3D-TSV Vertical Interconnection Methods

Ø Limitation of current methodsü Additional processes for fluxing, cleaning, and SAM coating

ü Solder bridging problem at finer pitch

ü Difficulty in underfill process for narrower gap & multiple stacked chips

Koji Fujimoto, et. al, Solid-state Sensors, Actuators and Microsystems Confernece,2009

Cu/Cu bonded interface

TSV Cu

Sn

TSV

Rahul Agarwal, et. al, ECTC 2010Cu


Problems in Cu/SnAg Double Bump Bonding

New TSV interconnection method without flux & underfill needed!

Ø Residual Flux

Ø Underfill voids< Flux residue between micro bumps >

J. Hwang, et. al, ECTC 2010

< Delamination due to the trapped void >

Underfill

Si chip

Yue Ying Ong, et. al, EPTC 2009

• Fine bump pitch (< 20 µm)• Small gap between chips (< 10 µm)

Residue

Void

Ø Electrical Shortage btw bumps

Ref> Low temperature bonding of 30um pitch micro bump interconnection for 3D IC stacking using Non-conductive adhesive, IMPACT, 2011

v Test vehicle (Without TSV)

NCF Cu/SnAg double bump Vertical Interconnection

v NCFsü Thermal curable adhesive

10 mm

10 mm

Observed area

7 mm

5 mm

Observed area

ü Thickness : 20 µm

ü Bump type :

• Number : 2026 • Cu/Sn2.5Ag bump• Height : 10/10 µm• Diameter : 20 µm• Pitch : 40 µm

40 um

10 um

20 um

5 um10 um

TCA

Silicon waferSi chip with

TCA

Dicing of NCF-applied wafer

Pressure & temperature

TCASi chip

Si wafer

TSV

*Patent issued : US6518097

TCA coated on releasing film

*Wafer-level lamination of NCF on a wafer

Bumped wafer

Chip to wafer bonding using NCF/solder joint

v Schematic NCF bonding processes


Test Vehicles

Ø Micro-bumped chip

ü Substrate chip design

ü Bonding chip design

Height : 10/10 μm

Diameter : 40 μm

Material : Cu/SnAg2.5

• Dimension : 13 mm X 13 mm

• Pads : 4 joint resistance circuits

Pitch : 80 μm

• Dimension : 6 mm X 6 mm

• CuSn Bump ; Reflowed

A single bump joint resistanceat the corner by 4 point probe method

Cu pillar (10 μm)

SnAg bump (10 μm)

Daisy Chain resistance

NCF Bonding methodsv Test vehicles and processes

§ Cu + Au electroplated§ Pitch : 40 um§ Bump diameter : 20 um§ Bump height : 5um Cu

Ø Substrate bump

§ Material : Cu pillar + SnAg 2.5wt%§ Pitch : 40 um

Bump diameter : 20 um§ Bump height : 10 um Cu, 10 um Sn bump

Ø Chip bump

40 um

10 um

10 um

20 um

5 um

Si Chip

Si Substrate

NCFs Joint Interconnection Mechanism

<Before bonding>

<Increase of the contact area>

<Metallurgical bonding>

<Physical contact >

v NCF viscosity effects on bump joint resistance during NCF bonding

20

40

60

80

100

50 100 150 200 25010

100

1000

10000

100000

1000000

Vis

cosi

ty (|h|

* )

Temperature (oC)

Dai

sy re

sist

ance

(W)

Bonding conditionPressure : 16.32 MPaTemperature : 250 oCTime : 30 s

No bump interconnection

(1) (2)(3) (4)

(1)

(2)

(3)

(4)NCF joint interconnection sequences1. NCF flow at low temperature2. Cu-SnAg bump contact to electrode by applied pressure3. SnAg solder melting at high temperature 4. NCF curing at the same time

Effect of epoxy resin viscosities on the Joint formation - low & high viscosity NCFs

120C150C

200C 250CTemperature

Time

Low viscosity NCFLow viscosity NCF

High viscosity NCFHigh viscosity NCF

v Joint observation at different temperature

Example: 40 µm Fine Pitch Cu/SnAg Bump Interconnection

v SEM images of the bump joints

Cu padCu pillar SnAgCu6Sn5

AuSn


Nanofiber Anisotropic Conductive Adhesives(ACAs) for Ultra Fine Electronic Packages

A success of Nanotechnology integration in electronic packaging materials

Nanofiber coupled with conductive particles

Patented


*Reference panel size:3.5’’

Trends of Display Electronicsr Requirement for higher resolution of display electronics

Resolution of mobile devices

Higher resolution(< 20 µm)

960 x 640800 x 480400 x 234

Market share

Source: http://www.samsungtomorrow.com,Display Search, resolution screens of mobile devices continues to grow, 2011

(pixels)


Display chip

§ Fine pitch capability (~ 30 μm)

§ Simple process (no underfill)

§ Low process temperature (150~200 oC)

r ACFs (Anisotropic conductive films): thermo-curable polymer + conductive particles (CPs)

Substrate

CP

Z - Conduction

X - Insulation Adhesion

Display panel

Touch screen panel

Issues for Fine Pitch Interconnection Using ACFs

Finer pitch < 20 μm

ACFs

Chip-on-Glass (COG)

Chip-on-Flim (COF)

Display panelDisplay chip

Source: samsung


r Unstable electrical performances on finer pitch

§ Short circuit: agglomerated CPs→ electrical conduction in X-axes

§ Open circuit / high resistance: Non-captured or small # of CPs in Z-axis → higher joint resistance→ poor reliability

Short circuit Open circuitFiner pitch

Issues for Fine Pitch Interconnection Using ACFs

How can we suppress conductive particle movements?

Increasing viscosity?


Suppressing CP movement → No short circuit & open circuit

v Korea patent :10-1115686 (issued):10-1146351 (issued):10-1160971 (issued):10-2011-0022041

Why Nanofiber ACFs?

v Japan paten pending:2011-072488:2012-532008

v China paten pending:201110111458.0

Nanofiber ACF

CP incorporated nanofiber (CPIN)

Adhesive resin

X-Insulation Z-ConductionChip

Substrate

v PCT patent pending:PCT/KR2010/006623

v USA patent pending:13/075,147

Ref) Kyoung-Lim Suk, Chang-Kyu Chung and Kyung-Wook Paik, 61st ECTC, Florida, USA, May 31-June 3, 2011.

Conductive particles incorporated nanofiber

(CPIN)


Nanofiber ACFs Fabrication

A: Syringe, B: Needle, C: High voltage power supply, D: Nano-fibers, E: Collector units

Polymer solutionr Nanofiber formation by electro-spinning

§ NCF lamination on Conductive particle coupled nanofiber


Nanofiber coupled with conductive particle

Nanofiber coupled with conductive particles


(a) SEM image of CPIN

(b) Cross-sectional SEM image of CPIN

(c) TEM image and elemental analysis on the surface of CP

~ 80 nm thickness

a

b c

r Conductive particle incorporated nanofiber (CPIN) structure

v CPIN structure was successfully fabricated by the electrospinning process.v Average diameter of nanofiber : ~280 nm

CP

Nylon 6 nanofiber

CPIN

Formation of Nanofiber ACFs


8 particles/bump (1024 µm2)

97% 77%

26.8% 26.8%

v Nanofiber ACF suppressed the particle movement compared to the conventional ACF during resin flow.

r Conductive particle movement during the bonding process

Conventional ACFNanofiber ACF

Initial ACF state

CPIN capturing Nanofiber melting & resin curing

Particle Capture Rate of Nanofiber ACFs

Pre bonding Main bonding


r X-axis insulation resistance r Short circuit rate at 20 µm pitch

v Nanofiber ACF successfully suppressed the conducive particle movement resulting in zero short circuit rate at 20 µm pitch.

ACF Nanofiber ACFConventional ACF

Nanofiber ACFACF Nanofiber ACFConventional ACF

Nanofiber ACF

CPs

Short Circuit Rate of Nanofiber ACFs


Fine Pitch COF (Chip-on-Film) Applications

r Nanofiber ACFs for fine pitch COF (bump to bump spacing = 7 um)

§ Bump Joint resistance § Insulated circuit ratio

ACF Nanofiber ACF ACF Nanofiber ACF

v Nanofiber ACFs showed stable bump joint resistance and excellent 100% bump to bump insulation property at 7 m bump space.

COF


Nanofiber ACFs anchoring effects: Particle densityr Conductive particle distribution before and after bonding process

ACF

Nanofiber ACF-1

2 X particle density

Nanofiber ACF-2

Nano Packaging & Interconnects Lab.

Interconnection trends at Portable Electronics

§ Small volume & height

§ Fine electrode pitch (≤ 100 μm)

§ High reparability

FOF(Flex-on-Flex) using ACFs in electronic devices

Source : NOKIA lumina

Source : NOKIA

Source : NOKIA lumina

Socket type in electronic devices

§ Large volume & height

§ Wide electrode pitch (≥ 300 μm)

§ Difficult to attach connector & board

through SMT for fine pitch

Current demands for portable electronic devices

1. High performance2. Multi-functionalization3. Miniaturization


Material preparation for Nanofiber Solder ACF for FOF assemblyØ Conductive particles

Ø Nano-fiber solder ACFs

Adhesive films B-stage NCF

Spacers No Spacer

Polymer material Polyvinylidene fluoride (PVDF)

Polymer wt.% 18 wt%

Solvent DMAC, Acetone

Solder content (polymer:solder) 1:1

ACF type Nanofiber/solder ACF

Conventional ACF

Conductive particle type

Sn58Bi solder ball(42%Sn / 58%Bi)

Ni ball

Melting temperature

138°C -

Conductive particle content 15 wt.% 30 wt.%

Ball size 5~15 μm 8 μm

Ø Flexible substrate

Top substrate Bottom substrate•Electrode pitch : 100 μm•Electrode height : 12 μm•OSP finish thickness : 100 nm

NCF: non conductive film


Magnified image

Fabrication of nanofiber/solder ACF

PVDF Nano-fiber/solder layer

Polymer is well wrapped around the solder particle !

Finalized PVDF nanofiber/solder ACF

Ø Solder incorporated nanofiber

Ø Nanofiber/solder ACF

Heat

Heat

B-stage NCF Solder ball


100 μm fine pitch FOF assembly using Nanofiber solder ACFs with vertical ultrasonic bonding

Ø Stable metallurgical solder joint forms when applying optimized vertical

ultrasonic bonding. – due to the removal of native solder oxide

Ø Successfully demonstrated 100 μm fine pitch FOF assembly using

nanofiber/solder ACF & vertical ultrasonic bonding.

Solder joint

Contact resistance: 18.4 mΩ ± 0.50 mΩ


Nanofiber effect - Solder ball movement analysis

Ø Due to the solder movement suppression capability of nanofiber,

nanofiber/solder ACF showed 34% higher capture rate for 100 μm fine

pitch FOF assembly.

34 % higher


Nanofiber effect - Insulation property

Excellent 100% insulation

Ø Due to the solder movement suppression along with the insulation

coating around the solder ball, nanofiber/solder ACF showed 100 %

insulation for 100 μm pitch assembly.

16 measurement points

Electrical bridging No Electrical bridging

Solder ball


Nanofiber solder ACF joint formation & electrical property

Ø Due to the stable metallurgical solder

joint & large joint area, nanofiber/solder

ACF showed lower contact resistance.

Conventional ACF Nanofiber/solder ACF

Ni Ball

Bottom electrode Bottom electrode

Solder joint

Ø ACF joint formation

Ø Electrical property§ Contact resistance


Side keys DisplayTouch screenpanel

Sensors Camera High power LED backlit

Battery

SummaryMulti-functional mobile devices Large size panel display

Source : SamsungSource : NOKIA Source : Samsung

New nanofiber and solder ACFs combined with fast and less thermal damage ultrasonic bonding methods are providing

High power and high reliability module (FOB, FOF) assembly and ultra fine-pitch chip (COG, COF) assembly!


Thank you for your attention !

recent advances in anisotropic conductive adhesives (acas

Documents