nanofiber anisotropic conductive adhesives (acas) for ultra fine...
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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
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