Mechanical-Electrical Behaviour of Conductive Polymer / Metal Composite for Flexible Interconnect

Abstract

Flexible electronics such as foldable smartphones and electronic paper are expected to develop the market. Conductive wires for sending electric signals are necessary for electronic devices, and the requirements for them are electronic conductivity, mechanical flexibility and adhesiveness to a substrate. In addition, an efficient process is required for fabrication of the interconnect. Although metals such as copper and silver have been used for the interconnect of the conventional electronic devices which are mostly rigid, the metal interconnect might be broken and peeled off from the plastic substrate when they are bent and unbent repeatedly. As compared to metal, organic polymers have excellent mechanical properties while their conductivity are comparatively low. Therefore, composites of polymer and metal have been studied in order to achieve both electrical conductivity and mechanical flexibility. So far, the following research results were reported; that conducive polymer/metal composite showed 2×10-4 Ω-1·cm-1 of conductivity two orders of magnitude higher than the commercially available conductive polymer, more than 90% of adhesiveness onto various plastic materials, and 104times of repeated bending with keeping electrical conduction. The electrical and mechanical behavior of the composite, however, are not clear when the composite is in motion. The purpose of this research was to clarify dynamic electrical and mechanical characteristic of the conductive polymer/metal composite film. A mixture of AgBF4 and C4H5N in acetonitrile was used as the reaction solution. It was dropped in the linear shape on a polyimide (PI) sheet and polydimethylsiloxane (PDMS) sheet as the substrate. UV light was irradiated on the reaction solution for 5 minutes and composite of polypyrrole doped with BF4and metal silver was synthesized. Mechanical property of the composite on polyimide was evaluated by a tensile test according to the Japanese Industrial Standards (JIS 7127) with a load cell of 9.8 N and at a tensile speed of 10 mm·min-1. During the tensile test, the electric resistance between two ends of the specimens was measured. The tested specimens were observed by microscopies. From changes in applied force with tensile time of the polypyrrole/silver composite formed on the PI substrate, a breaking of the specimen, i.e. corresponding to the substrate was observed at 63 sec under the test load of 321 N while the specimen with PDMS substrate was broken at 168 sec under 181 N. From changes in electric resistance with tensile time of polypyrrole/silver composite formed on the PI substrate, the electric resistance was increased with the time, i.e. the strain, and it exceeded the upper limit of measurement at 63 sec after starting of the tensile test. The former behaviour was explained by decreasing in contact area between the silver particles of the composite and the latter was due to breaking of both the composite and the substrate at the same time. In the case of the specimen with the PDMS substrate, the electric resistance was almost constant from the tensile start to 55 sec and then increased gradually until 83 sec, jumped up to the upper limit of measurement at 89 sec through up and down behaviour. Considerable difference of change in electric resistance between the substrates until breaking of the composite might be caused by actual tensile stress applied to the composite formed on the substrate with different shearing stress depending on adhesive status between the composite and the substrate. In addition, it is noted that the composite can be used until the polyimide substrate is broken in flexible electronic devices while the mechanical behaviour of the composite until the composite is broken can be evaluated by using PDMS as the substrate. The load stress and the strain at the breaking of the composite were calculated 158 N·mm-2 and 17.4% for the PI substrate and 113 N·mm-2and 24.7% for the PDMS substrate, respectively. From these values, it was estimated that the composite was stretched 1.74 mm and 2.71 mm for the PI and PDMS substrates, respectively at the breaking point. From the elongation of metallic silver as one of the wiring materials is 23%, it is supposed that the composite material used in this study can exhibit improvement up to 27.1% higher than that of metallic silver alone in terms of conductive elongation. This research succeeded in evaluating both mechanical and electrical properties of conductive polymer/metal composite simultaneously for the first time. Dynamic mechanical and electrical characteristic of the conductive polymer/metal composite was clarified; the composite is superior to metallic silver as the interconnect of the flexible devices and can be used until the polyimide substrate is broken. Figure 1