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

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 80 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

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

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

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

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

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

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 1.5x10-5 Wcm 0.8 wt% CNT in epoxy 6.4x10-4 Wcm March 6, 2006 IEEE Aerospace Conference

Mixing and DC Electrical Results Ultrasonic dispersion in EPI-CURE 3234 Manually mixed with Resin 862 - 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

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

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

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

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

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

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

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-04-2-0023) Kentucky NASA EPSCoR March 6, 2006 IEEE Aerospace Conference