Development of Subsurface Magnetic Field Imaging System for Visualizing Electric Current Distribution inside Electronic Components and Storage Batteries

Abstract

Recently, miniaturization and three-dimensional integration for improving the performance of electric devices have been accelerated, and smaller storage batteries with higher energy density and shrinked electric components have been more required. Therefore, failure analysis technologies targeted for these cutting-edged devices should be developed for increasing production yields and reducing product accidents. In mass production processes, pre-shipment inspections are performed on the core parts of electronic devices. In the case of electronic parts, energization check is adopted, and in the case of storage batteries, voltage drop, that is called to be an aging test, is checked. However, many cases have been reported in which products, that have passed the inspection, cause accidents. One of the typical causes of failures related about electronic components is internal electric-short caused by foreign matters mixed into the insulation or whiskers accidentally generated from the electrodes and wiring. Similarly, storage batteries also accidentally have internal short circuits due to foreign matter contaminations and dendrite formation at the electrodes. The internal short-circuited points of electronic components and storage batteries are identified by X-ray imaging method. However, it is difficult to acquire the contrast due to the lack of the dynamic range required for visualization of short-circuited parts because the wirings and insulation areas between electrodes have been significantly miniaturized. In addition, short circuits between wirings on the surface of electronic circuit boards have been visualized by thermography, although it is difficult to apply electronic components with multi-layered electrodes. Then we have focused on the relationship between the electric current distribution inside the devices and the magnetic field generated from it and spread toward outside, we have succeeded in deriving the analysis solution of the inverse problem for the first time. Combining this novel theory and a highly-sensitive magnetic field detection based on quantum effect, we have developed a subsurface electric current imaging system capable of detecting electric short-circuited points in the deep region. In this study, we discuss about the details of the theory about the analytical solution of the inverse problem and the visualization system, and show some results of failure analysis for actual electronic components and storage batteries. [1] Yuki Mima, Noriaki Oyabu, Takeshi Inao, Noriaki Kimura, Kenjiro Kimura, “Failure analysis of electric circuit board by high resolution magnetic field microscopy” Proceedings of IEEE CPMT Symposium Japan, pp.257-260, 2013. [2] Kenjiro Kimura, Yuki Mima, Noriaki Kimura, “Local electric current reconstruction theory for nondestructive inspection inside battery cell using magnetic field measurement”, Subsurface Imaging Science & Technology, 1, 1, pp.1-16, 2017. [3] Shogo Suzuki, Hideaki Okada, Kai Yabumoto, Seiju Matsuda, Yuki Mima, Noriaki Kimura, and Kenjiro Kimura, “Non-destructive visualization of short circuits in lithium-ion batteries by a magnetic field imaging system”, Japanese Journal of Applied Physics, Vol 60, No 5, pp.056502-1-4, 2021.