Electrochemical reliability of NTV sintered flexible substrates

Abstract

By today, die bonding on flexible circuit boards is only possible with adhesive connections or can be realized in combination by rigid-flex technology via soldering. The flex technology has a big potential with regard to new application fields such as automotive interiors (e.g. new lighting concepts, integration of electronics as functional components of the equipment), autonomous driving (new camera and sensor concepts, radar, car-to-x communication), building integration (e.g. smart home integration of functional electronic assemblies in building walls) but also medical technology and textile electronics. The application areas and the reliability of assemblies with the use of adhesive technology minimizes the application areas of flex technology. Great opportunities to new reliable polymer-based electronics could, however, be made up by transferring a low-temperature silver sintering process from power electronics to the microelectronic field. But the transfer of this bonding process also contains the risk of electrochemical failure processes (e.g. due to residual moisture in flexible polymer substrates, Ag migration, acceleration of failure modes due to mobile ions). In our studies we analyzed electrochemical corrosion and migration processes at silver sintered flexible substrates. Therefore, in a first step, we investigated technical processes to produce reliable low temperature silver sintered systems at polyimide based flexible substrates. Afterwards the silver sintered systems were tested by biased humidity testing and salt spray testing. The materials, surfaces and reaction products were investigated by high resolution microstructural and physical methods like $\mu$ XRF and SEM-EDS. The results achieved by this work give a good overview for the possibilities and limits of pressureless silver sintered systems on flexible substrates.