1. IEEE Aerospace Conference
Electrical and Mechanical Characterization of Carbon Nanotube Filled Conductive Adhesive
Jing Li, PhD Candidate
Dr. Janet Lumpp, Associate Professor
Electrical and Computer Engineering Department
University of Kentucky
March 6, 2006
IEEE Aerospace Conference

2. Tensile Strength (GPa) IEEE Aerospace Conference
Approach
Sn-Pb solder alloys are being replaced with lead free alloys or electrically conductive adhesives.
Typical isotropic conductive adhesives contain up to wt% metal filler with decreased mechanical strength.
Carbon nanotubes in epoxy are conductive at loadings < 5 wt% while maintaining strength of polymer matrix.
CNT properties
Electrical conductivity: 104 S/cm
Current Density: 1013 A/cm2
Thermal conductivity: 6,600 W/mK
Material
Young's modulus (GPa)
Tensile Strength (GPa)
SWNT
1000 30
MWNT
1280
Steel
208
0.4
Epoxy
3.5
0.005
March 6, 2006
IEEE Aerospace Conference

3. IEEE Aerospace Conference
Conductive Adhesives
One or two part polymer matrix + conductive filler (e.g. epoxy + Ag)
Lead free, no-clean, low cure temperature (typically less than 150°C) compared with solder joints
High volume loading (up 80 wt%) ensures electrical continuity but degrades mechanical properties
Isotropic and anisotropic conductive adhesives
March 6, 2006
IEEE Aerospace Conference

4. Isotropic vs. Anisotropic Conductive Adhesives
Particle to particle contact
Flakes, spheres, CNT
Single particle contact
Metal or metal coated polymer spheres
Fundamentals of Microsystems Packaging,
R.R. Tummala, 2001.
March 6, 2006
IEEE Aerospace Conference

5. MWNT and Epoxy Materials
UK Center for Applied Energy Research
Continuous CVD reactor for growing CNT
2.5 kg/day production rate of bulk MWNT
Substrates as large as 1400 cm2
100 m2/day capacity for arrays
Miller-Stephenson epoxies
Epon Resins 862, 815C
EPI-CURE 3234
March 6, 2006
IEEE Aerospace Conference

6. Ultrasonic Dispersion of CNT
Low loadings can be dispersed in epoxy resin by ultrasonication
Viscosity is too high for loadings > 1 wt%
Without ultrasonication With ultrasonication
March 6, 2006
IEEE Aerospace Conference

7. Mixing CNT Filled Resins
Viscosity increases dramatically with CNT loading
Difficult to disperse ultrasonically or mix manually
Air entrained in mixture, non-uniform batches
Planetary mixing - high g, high shear
Mixes higher loadings easily, homogenously
Mixes thoroughly without breaking CNT
De-airs batches, loaded syringes
Screen print or pressure dispense
Test patterns
Bond pads for zero ohm resistors
Area for four point probe
Bond pads for high frequency
Die attach pads
March 6, 2006
IEEE Aerospace Conference

8. IEEE Aerospace Conference
DC Electrical Testing
Daisy chain test pattern
FR-4 printed circuit board
1 ounce copper, tin plated
1206 size, 0 W resistors
Average contact resistance
8 resistors per chain
8 chains per board
Four point probe
Large printed area
Volume Resistivity
Solder x10-5 Wcm
0.8 wt% CNT in epoxy 6.4x10-4 Wcm
March 6, 2006
IEEE Aerospace Conference

9. Mixing and DC Electrical Results
Ultrasonic dispersion in EPI-CURE 3234
Manually mixed with Resin too viscous for ultrasonication
Resin:Curing agent ratio = 100:15.4
Without Ultrasonic Dispersion
With Ultrasonic Dispersion
Loading
0.4 wt%
1 wt%
0.2 wt%
Solder
bump
Average
Contact
Resistance
6.31 KΩ
605 Ω
36.2 Ω
1.34 Ω
0.2 Ω
March 6, 2006
IEEE Aerospace Conference

10. Mixing and DC Electrical Results
Ultrasonic dispersion in Resin 815C
Manually mixed with EPI-CURE 3234
Resin:Curing agent ratio = 100:12
Loading limited to 0.8 wt%
Same preparation
Planetary mixing
Thinner added
CNT
Loading
Ave. Contact
Resistance
Standar
Dev.
0.4 wt%
1.34 W
1.41
0.8 wt%
0.82 W
0.67
CNT
Loading
Ave. Contact
Resistance
Standar
Dev.
1 wt%
0.42 W
1.02
2 wt%
0.16 W
0.66
3 wt%
0.11 W
0.34
4 wt%
0.08 W
0.14
5 wt%
0.09 W
0.15
March 6, 2006
IEEE Aerospace Conference

11. IEEE Aerospace Conference
Mechanical Testing
Lap shear testing with FR-4 strips
Shear area
force
adhesive
CNT
Loading
Ave. Shear
Strength
(psi)
Test
Standard
0 wt%
1835
ASTM-1002
0.8 wt%
1479
March 6, 2006
IEEE Aerospace Conference

12. AC Electrical Properties
RT/duroid 5880 Cu clad substrate
Zero ohm resistor
Solder vs. 3 wt% CNT/epoxy
Up to 3 GHz
S11 (dB)
S21 (dB)
March 6, 2006
IEEE Aerospace Conference

13. Laser Flash Thermal Analysis
1 - 5 wt% CNT in Resin 815C epoxy
Molded in cylindrical die, cured, sliced
Thermal Diffusivity (mm2/sec)
Temperature (C)
March 6, 2006
IEEE Aerospace Conference

14. IEEE Aerospace Conference
Observations
Screen printing leaves a rough surface
Stencil printing and pressure dispensing smoother
Four point probe measurements unreliable
Rough printed surface and excess resin on surface
Measure cut surfaces from laser flash samples
Viscosity and pot life dependent on resin, curing agent and loading
Add solvent (thinner) to resin
Slow cure agent - higher viscosity
March 6, 2006
IEEE Aerospace Conference

15. IEEE Aerospace Conference
Conclusions
Contact resistance is decreased by
Ultrasonic dispersion
Increased CNT loading
Planetary mixing of higher loadings
AC characteristics comparable to solder
85% retention of epoxy mechanical strength
Thermal conductivity of epoxy increases with CNT wt%
March 6, 2006
IEEE Aerospace Conference

16. IEEE Aerospace Conference
On-going Research
Evaluate cross-linking modification of CNTs
Evaluate full loading range to develop high strength to weight ratio, electrically and thermally conductive polymer materials
Reliability testing of CNT filled isotropic conductive adhesive for microelectronics
Sponsors
Army Research Laboratory (W911NF )
Kentucky NASA EPSCoR
March 6, 2006
IEEE Aerospace Conference