Surface breakdown of printed circuit board under magnetic field with reduced pressure

Abstract

Abstract：Epoxy resin laminate onto which a pair of copper foil was printed was employed as test samples. The samples were placed in an artificial atmospheric chamber, which was vacuumed by a rotary pump from 100 kPa to 5 kPa. The magnetic field was produced by permanent magnets that were assembled to make E×B drift away from, into and parallel to the sample surface, respectively. Magnetic flux density was adjusted at 120 mT, 180 mT and 240 mT respectively. By applying a negative bias voltage between the electrodes, the time to surface breakdown was recorded. Obtained results show that when E×B is into the surface, the time to the breakdown is shortened; when E×B is away from the surface, the time to the breakdown is delayed; when E×B is parallel to the surface, the time to the breakdown remains approximately the same as the case without magnetic field. With the decrease of pressure, the time to the breakdown increases and the effect of magnetic field on breakdown appears to be strengthened. Keywords：printed circuit board; surface breakdown; magnetic field; ambient pressure Printed circuit board (PCB) is widely used in electric and electronic industry. With the development of miniaturization and high accumulation of devices, the insulation distance between conductors is getting smaller. The narrow insulation distance gives rise to very strong electric field [1] . Provided the electric field reaches the breakdown strength of air, surface discharge occurs between the electrodes. Due to the thermal energy produced by discharge, surface dielectric breakdown can be introduced to the polymer, which is characterized by the formation of a carbonized conductive path which bridges the electrodes. It causes a sudden decrease in insulating resistance and has been considered as the origin of short circuit, insulation failures and even fires in the devices [2] . Therefore, for the safe use of devices, one should take into consideration the surface breakdown behavior of PCB.Recently, the PCB is needed to work in more complex environments such as high altitude regions where the ambient pressure is low [3] and magnetic field is involved [4] , which possibly results in the alternation of discharge behavior. Heylen investigated the effect of a cross magnetic field on gas breakdown under low pressure, which revealed the dependence of breakdown voltage upon the magnetic field varying as a function of the ambient pressure [5] . Korzekwa et al reported that surface flashover on solid dielectric was difficult to occur with E×B drift away from the surface and was easy to be induced with E×B drift into the surface [6,7] . However, previous studies fell short of taking proper account of the effect on surface dielectric breakdown behavior. Our previous study, which was carried out with the purpose of investigating the resistance to tracking under low pressure with DC voltage application, showed that the comparative tracking index (CTI) of epoxy resin was not obviously changed with the decrease of the ambient pressure [8] . Additionally, by ...