Nano Packaging & Interconnect Lab.

Kyoung-Lim Suk* and Kyung-Wook Paik

Department of Materials Science & Engineering

KAIST

Nanofiber Anisotropic Conductive Adhesives (ACAs)

for

Ultra Fine Pitch Chip-on-Film (COF) Packaging

Nano Packaging & Interconnect Lab.

1. Introduction: Electronic Packaging Trend

- http://www.samsung.com

- http://www.apple.com

- http://www.fujitsu.com

LCD

ACAs

Chip-on-Glass

(COG)

Chip-on-Flim

(COF) Fine pitch capability (~ 30 μm)

Simple process (no underfill)

Low process temperature (150~200 oC)

Green process (lead-free, flux-free)

ACAs (Anisotropic conductive adhesives)

: thermo-curable polymer + conductive particles

Z - Conduction

X, Y - Insulation

Nano Packaging & Interconnect Lab.

Limitation of conventional ACAs

Open circuit / high resistance bumps:

very small number or none conductive

particles are not captured

→ higher joint resistance

→ poor reliability

1. Introduction: Issues on Fine Pitch ACAs Interconnection

Short circuit

Open circuit

Short circuit: agglomerated conductive

particles

→ electrical conduction in X-Y axes

After bonding

process

Nano Packaging & Interconnect Lab.

1. Introduction: Why nanotechnology? - Nanofiber ACFs

Nanofiber ACFs for ultra fine pitch ACAs interconnection

Reduce particle flow by

entangled nanofiber

Type-A nanofiber ACF

Nanofibers Nanofibers coupled with particles

Type-B nanofiber ACF

1. Fundamentally prevent conductive particles flow

2. Insulate particles one by

one

Nano Packaging & Interconnect Lab.

1. Introduction: Why nanotechnology? - Nanofiber ACFs

Requirements for nanofiber ACFs

Korea patent pending

:10-2009-0092623

:10-2010-0001674

:10-2010-0090520

:10-2011-0022041

PCT patent pending

:PCT/KR2010/006623

USA patent pending

:13/075,147

Japan paten pending

:2011-072488

Heat & pressure

Joint resistance Insulation resistance

Thermo-plastic

: Not cured at joint area Melting temp. (Tm) > 200~250 oC

: Maintaining fiber structure after

bonding process Glass transition temp. (Tg)

< 100~150 oC

: squeezed out from bonding area

Polyacrylonitrile (PAN), polystyrene (PS), polycarbonate (PC), ect.

Type-A nanofiber ACF Type-B nanofiber ACF

Heat & pressure

Chip bump

Substrate pad

Conductive

particle Squeezed out

nanofiber

layer

Nano Packaging & Interconnect Lab.

2. Research motivations

Integration of nanotechnology to ACFs

Demonstration of nanofiber ACFs for ultra

fine pitch COF packaging

7 μm space

Nanofiber

Type-A nanofiber ACF

Nanofiber coupled

with particle

Type-B nanofiber ACF

Nano Packaging & Interconnect Lab.

Nanofiber formation by an electrospinning process

A: Syringe

B: Needle

C: High voltage power supply

D: Nano-fibers

E: Collector units

H: working distance

L: length of capillary tube

R: radius of capillary tube

Γ: surface tension of fluid

Charge accumulation Molecular repulsion (Vc) Nanofiber formatioon

Polymer solution

3. Experiments: Nanofiber Formation

[Electrospinning set-up]

Needle Polymer droplet (Vc: critical voltage)

Nano Packaging & Interconnect Lab.

Uniform PAN nanofibers coupled with conductive particles were made by an

electro-spinning process.

- Fiber diameter: 457+48 nm.

Nanofibers coupled with conductive particles

4. Results & Discussion: Type-B Nanofiber ACFs

Nanofiber coupled with

conductive particle

Nano Packaging & Interconnect Lab.

5. Conclusion

Nanofiber ACFs

combined the nano technology with ACFs

technology were successfully demonstrated

in ultra fine pitch COF.

Excellent 100 % electrical insulation at 7 μm bump space of COF

- No electrical short

Stable bump joint resistance

- Below 5 mΩ (similar to that of conventional ACF)

- Stable joint shapes