2, Harbin Institute of Technology, Harbin, , China
Lead-free solders, such as eutectic SnAgCu solders as traditional lead-free solders, are widely applied because of the environmental concern; however, these kinds of solders possess high melting point, poor wettability and high cost. Other than traditional lead-free solders, electrically conductive adhesives (ECAs) are defined as another kind of alternative solders to tin-lead solders, which possess a lot of advantages with low operating temperature for fragile integrated circuits, high flexibility for future wearable electronics and many other applications. Besides used as traditional joining materials, ECAs can be easily printed to high conductive circuits. Thus, ECAs composing of polymer matrices and conductive fillers have various applications in printing flexible circuits, wearable and stretchable radio-frequency identification (FRID) devices, capacitive touch panel modules, and so on. However, the disadvantage of ECAs is their high electrical resistance, which is usually caused by some lubricant residues (mostly fatty acids) on the surface of conductive fillers during the preparation processes by milling. Although adding more conductive fillers into the ECAs can decrease the electrical resistance, this will sacrifice cost and fluidity for printing. Therefore, many significant endeavors have been focusing on how to enhance conductivity of ECAs or reduce the percolation threshold for a lower cost.
Herein, we report an innovation application of doping conjugated polypyrrole nanoparticles (PPy NPs) into ECAs to prepare low electrical resistivity interconnecting materials. PPy NPs were synthesized by a facile one-step chemical oxidative polymerization method at room temperature with the average diameter as small as 86.8 nm. Particles’ diameters and dispersity were manipulated under different polymerization conditions through the adjustment of polyvinylpyrrolidone (PVP) as the surfactants with different weight percentages and molecular weights. Results showed that higher concentrations of PVP and the longer chains of PVPs induced smaller diameters of PPy NPs. We also found a suitable portion of ethanol in the polymerization mixtures is beneficial to the better dispersity than those without ethanol. The PPy NPs were added into traditional epoxy-based and silver-flakes-filled ECAs. Resistance measurements showed that a small amount of PPy NPs can enhance the electrical conductivity, or in other word reduce resistivity, significantly. For example, the electrical resistivity of 70 wt% silver-filled ECAs was reduced from 1.6×10-3 Ω cm to 9.4×10-5 Ω cm by using only 2.5 wt% PPy NPs as the dopants. Thus our results confirmed new applications of PPy NPs in the field of ECAs for decreasing the resistance, reducing the dosage of silver in ECAs, and achieving flexible devices. Finally, flexible electrical patterns were printed on paper and polyimide substrates as conducting circuits to light LED devices.