Heuiseok Kang

Silver inkjet printing with control of surface energy and substrate temperature

The characteristics of silver inkjet printing were intensively investigated with control of surface energy and substrate temperature. A fluorocarbon (FC) film was spincoated on a silicon (Si) substrate to obtain a hydrophobic surface, and an ultraviolet (UV)/ozone (O3) treatment was performed to control the surface wettability of the FC film surface. To characterize the surface changes, we performed measurements of the static and dynamic contact angles and calculated the surface energy by Wus harmonic mean model. The surface energy of the FC film increased with the UV/O3 treatment time, while the contact angles decreased. In silver inkjet printing, the hydrophobic FC film could reduce the diameter of the printed droplets. Merging of deposited droplets was observed when the substrate was kept at room temperature. Substrate heating was effective in preventing the merging phenomenon among the deposited droplets, and in reducing the width of printed lines. The merging phenomenon of deposited droplets was also prevented by increasing the UV/O3 treatment time. Continuous silver lines in the width range of 48.04–139.21 µm were successfully achieved by inkjet printing on the UV/O3-treated hydrophobic FC films at substrate temperatures below 90 °C.

Periodic fuel supply to a micro-DMFC using a piezoelectric linear actuator

A reciprocating pump using a piezoelectric linear actuator is introduced as a fuel supply module for a direct methanol fuel cell (DMFC) stack. The results show that the key advantage of the pump with the linear actuator is its ability to operate at a low frequency, which enables high flow rectification performance. At the end of this study, the pump is applied to a DMFC system to demonstrate that the output voltage of a DMFC stack remains stable with time even when the pumping pressure oscillates periodically at frequencies of a few Hz. It is also demonstrated that the maximum output power of the stack exceeds 6 W with power consuming 3.5% of the stack power for the fuel pump.

Fabrication of organic light emitting display using inkjet printing technology

In this paper, we have investigated the characteristics of polymer inkjet printing and fabricated OLED(organic light emitting diode) display using inkjet printing technology. To control the wetting condition of the droplet to the substrate, UV/ozone treatment technique is used. To make the repeated pixels on a substrate, precise vision alignment and multi-layer printing is used. With the successive printing of PEDOT/PSS as a hole injection layer and MEH-PPV as a light emitting layer, we have successfully fabricated an OLED and demonstrated basic working of OLED.

Laser sintering of inkjet-printed silver nanoparticles on glass and PET substrates

Laser sintering of inkjet-printed silver nanoparicles was studied. Conducting lines were printed on glass and PET substrates of various thicknesses by a drop-n-demand (DOD) inkjet printer and silver nanoparticle ink. Selective sintering of the printed silver nanopartilces was then performed using laser beam to enhance electrical conductivity. Focused laser beam was irradiated normally to the silver layer printed on the transparent substrates with various beam intensity and various irradiation time. Conductivity and morphology of the laser sintered silver patterns were measured and compared with the results of oven-heated sintering. Temperature elevation by laser irradiation was also calculated based on laser beam intensity, irradiation time, surface reflectivity, and thermophysical property of a substrate to estimate enhancement of the electrical conductivity by laser sintering.

Fabrication of a paper-based analytical device for multiple colorimetric analysis via inkjet-printing and paper-cutting

We introduce a process for the fabrication of a paper-based analytical device (PAD) for multiple colorimetric analysis via paper- cutting and inkjet-printing. To create the selective flow channels, hydrophobic barriers were formed by using hydrophobic polyvinyl chloride tape-cutting and ultraviolet-curable resin printing. Multiple paper channels were isolated by the hydrophobic barriers on a single chip. An assay model was created from bovine serum albumin (BSA) and tetrabromophenol blue (TBPB) to verify the feasibility of the fabricated PAD. A multiple colorimetric analysis was demonstrated a reaction with different BSA concentrations after the TBPB regent was elaborately deposited onto the circular test spot areas by inkjet printing.

Laser sintering of the printed silver ink

Because flexible electronics have a weak characteristic in high temperature, the conductive ink including nano-metal particles is printed on flexible substrate by printing methods such as screen printing, inkjet printing, and R2R. Due to high electrical resistance of printed metal ink, the process to improve the property of electrical conductivity is needed. To improve the conductivity, the furnace curing is used after printing the ink on the substrate. But this process spends much time for curing. Therefore, flexible substrate or glass substrate can be damaged due to long exposed time. The curing by using a laser is utilized to minimize the damage of flexible substrate. In this work, the specific resistance of silver ink by using furnace curing at the temperature of 250 °C after 30 minutes is 3×10−8Ωm. The specific resistance of silver ink by the laser curing at the power of 5W from 5sec to 10 sec is 2. 38×10−8Ωm. Consequently, the curing time by using the laser is significantly smaller than that of the furnace curing.

A study on driving waveform of a piezoelectric inkjet print head

Inkjet printing technology with a drop-on-demand (DOD) inkjet head has been recognized as one of the versatile and low cost manufacturing tools in the electronics industry. However, general strategy to optimize jetting stability has not been understood well, because of the inherent complex multi-physics nature in inkjet phenomena. In this paper, an experimental approach has been adopted to attack this problem. Based on the driving voltage amplitude and duration as the main and controllable parameter, the jetting map of an OLED ink has been constructed and the effect of phase matching between pressure and surface waves at the nozzle tip has been discussed.

Characterization of Inkjet-Printed Silver Patterns for Application to Printed Circuit Board (PCB)

In this paper, we describe the analysis of inkjet-printed silver (Ag) patterns on epoxycoated substrates according to several reliability evaluation test method guidelines for conventional printed circuit boards (PCB). To prepare patterns for the reliability analysis, various regular test patterns were created by Ag inkjet printing on flame retardant 4 (FR4) and polyimide (PI) substrates coated with epoxy for each test method. We coated the substrates with an epoxy primer layer to control the surface energy during printing of the patterns. The contact angle of the ink to the coated epoxy primer was 69˚, and its surface energy was 18.6 mJ/m². Also, the substrate temperature was set at 70℃. We were able to obtain continuous line patterns by inkjet printing with a droplet spacing of 60 ㎛. The reliability evaluation tests included the dielectric withstanding voltage, adhesive strength, thermal shock, pressure cooker, bending, uniformity of line-width and spacing, and high-frequency transmission loss tests.

Laser Sintering of Inkjet-Printed Silver Lines on Glass and PET Substrates

In this study, the laser sintering of inkjet-printed silver lines was evaluated. Silver-nanoparticle ink and a drop-ondemand (DOD) inkjet printer were used for printing on glass and polyethylene terephthalate (PET) substrates with various thicknesses. To sinter the printed silver nanoparticles, the silver layer printed on the transparent substrates was irradiated by focused CW laser beams that were incident normal to the substrates; the irradiation was carried out for various beam intensities and for various irradiation times. The electrical conductivity of the laser-sintered silver patterns was measured and compared with the conductivity of silver patterns sintered by using an oven. The increase in the temperature caused by laser irradiation was also calculated on the basis of the laser beam intensity, irradiation time, surface reflectivity, and thermophysical property of the substrate in order to estimate the increase in the electrical conductivity caused by laser sintering.

전도성 잉크의 레이저 열경화 공정 시 온도에 따른 비저항 연구

본 연구에서는 전도성 잉크의 레이저 열경화 공정시 은 나노입자 잉크의 레이저 열경화 공정 온도를 수치해석하였다. 유리기판 위에 잉크젯 프린팅을 이용하여 인쇄한 은 나노 입자 잉크를 532 nm 파장의 CW 레이저를 각기 다른 세기로 60 초 동안 조사하여 가열하였다. 온도계산을 위해서, 열생성항에 들어가는 반사율을 구하였고, 레이저 조사 중 실시간 은 나노입자 잉크의 비저항을 측정하였다. 온도 계산은 2 차원 열전도 방정식에 Wiedemann-Franz law 를 적용하였다. 그 결과, 레이저 조사로 인해 인쇄된 잉크의 온도가 상승할수록 비저항이 떨어지는 결과를 확인하였다.