Space Charge Formation in Polyimide Film under High DC Voltage at High Temperatures

Abstract

Since polyimide (PI) film shows a good performance as a polymeric insulating material even at high temperatures, it is used for a flexible print-circuit board (PCB). Recently, the PCB is required to be reduced in its thickness for downsizing of the electronic devices. When the thickness of insulating layer becomes much thinner, however, the applied electric field to the layer becomes much higher, and then the possibility of electrical breakdown in the layer becomes higher. Therefore we need to investigate the breakdown characteristics of thin PI film. To investigate the breakdown mechanism in polyimide film under dc stress, we have focused on space charge accumulation. It is well-known that the space charge accumulation in some polymeric materials strongly affects the breakdown characteristics under dc stress. Especially in this study, we focused on the relationship between the moisture of the sample and the breakdown strength. Through some experimental results, it was found that a certain kind of space charge always accumulates in the bulk of the sample before the electric breakdown under dc high electric field. Furthermore, it was also found that the breakdown strength and the time-to-breakdown are strongly affected by the moisture in the sample.