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Purpose – The purpose of this paper is to present a clear picture of the key factors of blind via and through hole filling in electroplating, e.g. shape of via or hole, electroplating solution, process, as well as the developments of mechanisms and models. Design/methodology/approach – First, the paper details the development trends and challenges of via filling. Then the research status of mechanisms, electroplating solutions, including base solution and additives, numerical model and mass transfer is described. Finally, through hole filling is briefly reviewed. Findings – To achieve excellent via filling performance, the characteristics of the via or hole, the ratio of acid/copper, selection of additives and factors of mass transfer are comprehensively considered in terms of optimization of the electroplating process. It is beneficial to design vias with appropriate aspect ratios, to strengthen the adsorption of the accelerator in the via bottom, to inhibit the increase of surface copper thickness and to form butterfly-shaped copper in the centre of through holes. Optimized process parameters should be taken into consideration in superfilling. Originality/value – The paper reviews different sets of additives, mechanisms and superfilling models for state-of-the-art via filling and the developments of filling for through holes.

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Chipless RFID tag is a promising technique for future RFID systems due to its low cost and high reliability. In this paper, three chipless RFID tags are designed based on retro-reflective structures, in order to increase the reading range and enhance the tag's robustness to the direction of the interrogating signals. The three tags include two linearly polarized tags with non-symmetric and symmetric filters and a circularly polarized tag. Before introducing the tag antennas, the design guidelines on how to arrange the filters for higher detection accuracy are provided based on standing wave analysis. Following the rules, retro-radiating tags with filters are designed. Since the E fields from the retro-radiating antennas add constructively in the direction of the incoming signal automatically, the reading distance is greatly increased compared with the conventional tags with orthogonally placed antennas. The tag with symmetric filters and arrangements can further enhance the detection robustness regardless of the reader's positions. Moreover, the circularly polarized tag is insensitive to the polarization of the incoming signals. The experimental results measured in the anechoic chamber show that the retro-radiator based tag can reach a reading distance of 60 cm, which is longer than most of the existing chipless tags. In the measurement, the proposed tag was well detected even when the reader antennas were not at the boresight. INDEX TERMS Chipless RFID, tag antennas, retro-radiator, RFID systems, van-atta array.

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