Se-Hoon Park’s research while affiliated with Korea Electronics Technology Institute and other places

This study compares the characteristics and low-temperature curing properties of pastes prepared from silver (Ag) powders synthesized by either wet powder (WP) or dry powder (DP) processing. The WP synthesis of electrode particles has the advantage of controlling the average particle size and particle size distribution but the disadvantage of producing low-purity, crystalline particles because they are synthesized through chemical reduction at less than 100 °C. Conversely, the DP synthesis of electrode particles has the advantage of producing pure, highly crystalline particles (due to synthesis at high temperatures) but the disadvantage of a high processing cost. WP and DP were used to manufacture pastes for low-temperature curing, and the physical properties of the pastes and the electrode characteristics after low-temperature curing were compared between powder types. Shear stress as a function of the shear rate shows that the WP paste is a plastic fluid, whereas the DP paste is a pseudoplastic fluid, closer to a Newtonian fluid. Screen printing the Ag pastes and curing for 30 min at 130 °C produces a nonconductive WP paste, whereas it produces a DP paste with a conductivity of 61 mΩ/sq, indicating that the highly crystalline DP paste is advantageous for low-temperature curing.

In advanced millimeter-wave packaging technology, signal loss is considered the main problem, which can be resolved using low-loss circuits for minimizing transmission losses. Herein, the radio frequency integrated circuit (RF-IC), an active element, and the antenna are assembled in one package form to realize such a low-loss circuit by utilizing the modified polyimide (MPI) material. The material analysis of MPI reveals a dielectric constant of 2.5 with a loss of 0.0025 mm-waves (at 20 GHz) and confirms the curability of the proposed film at lower temperatures through differential scanning calorimetry analysis. Next, the antenna and the RF-IC were assembled in a single package using MPI to verify their suitability for the millimeter wave package. The 1 dB gain compression point (P1dB) of the integrated module is measured to be 31 dBm, with 20.38 dBm for a single RF-IC, confirming that the proposed MPI is a suitable candidate as a low-loss circuit for modern millimeter-wave packaging technology.

Several recent studies have investigated textile-electrodes, as they can be used in various types of wearable devices in many different fields. However, if an electrode is printed directly on top of a textile, the pattern cannot spread or permeate the textile to form an electrode. To solve this problem, this study printed electrodes on stretchable and printable polyurethane substrates. They were then integrated with a textile through a vacuum-thermal-pressure (VTP) process. This process forms an electrode pattern on the textile and complements the recovery of the polyurethane substrates. Various analyses were conducted to assess the properties of these fabricated textile-electrodes compared to those of conventional TPU-electrodes. We compared the mechanical durabilities and conductivities of textile-electrodes vs. conventional TPU-electrodes. The degree of waterproofing was also increased through the encapsulation process, and the characteristics of the encapsulation-material were analyzed through environmental reliability tests and washable-durability tests to examine their suitability for future applications in wearable products.

Recently, flexible electronic device technology has evolved beyond curved devices with the development of flexible/stretchable devices that can be crumpled or stretched. Both elasticity and durability are essential for these devices, which should have high-conductivity for antennas and repeatability for sensors. In addition, electronic-skins, which can have a direct impact on the human-body, should be harmless to the human-body and should not be deformed by contact with sweat or organic matter. In this study, PDMS substrates were used to satisfy the above conditions. PDMS is used to fabricate human-friendly, flexible/stretchable substrates, and it has excellent repeat durability characteristics. To improve the adhesion of these PDMS films and electrodes, conductive paste was produced based on PDMS resins of the same properties. In addition, two types of Ag particles were selected as conductive fillers because the electrode characteristics of the antenna application requires excellent conductivity, and conductive paste were produced using flake Ag, which could affect conductivity, and Ag nanoparticles that affect stretchability and repeatability. The paste was applied using a high-efficiency printing technique. The printed electrodes were cured in a thermal oven. For higher conductivity, photonic-sintering was carried out during post-processing. As a result, 1.1117×106 (S/m) had excellent conductivity, performed well in repeated tensile-durability experiments of 30% to 100 times, and produced a bow-tie antenna for the above electrodes. As a result of tensing up to 35% through a Network-Analyzer, there was no performance change in the resonance-frequency or return-loss values, and excellent electrodes were developed that would achieve excellent performance even if they are applied in the sub-frequency area of 5G-antennas in the future.

In industry, recent research developments include flexible films and foldable films. The next step is the development of stretchable films, and studies are being intensively carried out. Research on the development of stretchable and transparent materials is also increasing greatly. Currently, polydimethylsiloxane (PDMS) is the most commonly used film in the industry. However, PDMS surfaces are hydrophobic, so their use is limited to making materials and compounds with hydrophilic properties. In this study, we developed a transparent polyurethane film that can be used for multiple purposes. A transparency comparison between the transparent polyurethane film and the general polyurethane film was used to verify their future application. The conventional polyurethane films showed a transmittance rate of 2.2 percent, but the transparent polyurethane films achieved a high transmittance rate of 85 percent. To determine whether the film can be realized, we produced a conductive paste using resin for the transparent polyurethane film. In addition, a conductive paste was made based on the material used in the transparent polyurethane film to verify the hardness and reliability of the adhesion of electrodes, and we confirmed this with thermogravimetric analysis (TGA). The transparent polyurethane based paste was made with stretchable electrodes through a screen printing method. The manufactured stretchable electrodes were demonstrated by mechanical and adhesion tests. Finally, a permittivity test was conducted to determine the suitability of the film for application to printed electrodes for antennas in the future. The genetic rate of transparent polyurethane films was better than that of conventional polyurethane films. Moreover, the adhesion of the transparent polyurethane film and stretchable electrodes was as good as that of conventional polyurethane film and stretchable electrodes, and observation by optical microscopy confirmed that the printing performance was also excellent. In addition, the conductive paste made based on the transparent polyurethane film material was cured for 1 hour at 120 °C, and TGA analysis confirmed that both the binders and curing agent responded well in the test for curing the developed stretchable electrodes and transparent polyurethane.