Electrostatic field analysis for clarifying the detachment mechanism of Ta-capacitors from wasted PCBs by electrical disintegration

Many kinds of valuable metals such as minor rare metals, which could not be recovered in non-ferrous smelting process, are existent in the print circuit boards (PCBs) of e-scraps. Our past research works indicated that the Ta-capacitors and sintered Ta were well detached and liberated by applying two-stages electrical disintegration(ED). In this process, Ta-capacitors were first detached from PCBs almost in nondestructive form, and sintered Ta in the capacitors was secondary liberated from covered epoxy resin. In this paper, we tried to clarify the above detachment mechanisms of Ta-capacitors from PCBs, by using FEM simulation of electrostatic field analysis. In the ED, plasma column, called streamer that occurs as a pre-breakdown phenomenon, develops from anode tip toward cathode, generating a current path along a line of electric force. Therefore, the maximum electric field point in Ta-capacitors, where the density of the lines is the highest, is expected to be the aiming point of streamer development. Based on this streamer property, the aiming points were specified considering the movement of Ta-capacitors in the ED. Specifically, under the conditions in various position of Ta-capacitors that modeled the movement of the capacitors in water by applying voltage successively, the maximum electric field point was specified by calculating electric field in the capacitors. As a result, four points in Ta-capacitors were the candidates as the maximum electric field points. We also calculated the probability of streamer developing toward each point, when the above conditions represented all the possible movement of Ta-capacitors in the ED. In this calculation, the probability was assumed to be the same as the ratio of electric field value at each point to the sum of the four values. Since two of the four points were located inside Ta-capacitors and the rest two of them were on the outside, two detachment mechanisms of the capacitors from PCBs were assumed by 59 % possibility of streamer development to the inside Ta-capacitors and 41 % of that to the outside.