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. 2023 Sep 14;14:5701. doi: 10.1038/s41467-023-41498-x

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© The Author(s) 2023

Open Access This article is licensed under a Creative Commons Attribution 4.0 International License, which permits use, sharing, adaptation, distribution and reproduction in any medium or format, as long as you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons licence, and indicate if changes were made. The images or other third party material in this article are included in the article’s Creative Commons licence, unless indicated otherwise in a credit line to the material. If material is not included in the article’s Creative Commons licence and your intended use is not permitted by statutory regulation or exceeds the permitted use, you will need to obtain permission directly from the copyright holder. To view a copy of this licence, visit http://creativecommons.org/licenses/by/4.0/.

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Fig. 2 — a 3D OCT image of the multilayer structure in a top-to-bottom view. b 3D OCT image of the multilayer structure in a cross-sectional view. c Diagram of the multilayer structure of a micro-SD card, including an insulating layer, PCB and plastic substrate. d En face OCT images at three different depths: Top—surface (depth = 0 μm), Middle —PCB pinout side (depth = 23 μm), Bottom—PCB dice side (depth = 273 μm). e Averaged A-scans at two different positions, P1 and P2, marked as white rectangles in (d). S1 corresponds to the surface of the micro-SD card. S2 represents the boundary between the insulating layer and the PCB. The distance between S1 and S2 represents the thickness of the insulating layer, estimated to be ~23 μm. S3 indicates the reflection of light from the copper conductive layer on the dice side of the PCB, suggesting the thickness of the PCB to be ~250 μm.