The effect of heat treatment on flexibility of silver nanopaste circuit screen-printed on polyimide

We evaluated the flexibility of screen-printed silver (Ag) circuits under various sintering temperatures. The circuits were built on a polyimide (PI) film by a screen printing technique using Ag nanopaste (73 wt%, 24 nm in diameter, Ag nanoparticles). The sintering temperature was raised from 150 to 300°C while the sintering time was fixed at 30 min. The flexibility was measured by two different kinds of methods; Massachussets Institute of Technology (MIT) type folding endurance test, and Institute for Interconnecting and Packaging Electronic Circuits (IPC) flexural resistance endurance test. In order to measure the flexibility of Ag circuits under various sintering temperatures, the changes of electrical resistance were observed while the printed Ag circuits were being folded or slid. The circuits sintered at higher temperatures needed more sliding and folding cycles to reach the critical point: while the sintering temperature increased from 150°C to 300°C, the flexibility decreased by around 90 % and 75 % after the IPC sliding process, and MIT folding process, respectively. The results showed that the flexibility of printed circuits was affected by the microstructural changes depending on the sintering temperature.