Statistical analysis for the manufacturing of multi-strip patterns by roll-to-roll single slot-die systems
Introduction
The roll-to-roll (R2R) printing methodology using gravure, inkjet, and slot-die printing technologies has been adapted for the manufacturing of printed electronic devices such as printed sensors, RFID tag, rectifier, OPV, EMI filter, artificial skins, etc. [1], [2], [3], [4], [5], [6], [7], [8]. Mass production is desirable to open additional markets for “ambient intelligence” by R2R printed electronics for reducing the product cost [9].
Slot-die is one of the most promising technologies for the manufacturing of printed electronics which are multi-layered and flexible, and the ability of the slot-die method to produce premetered, thin, uniform, and large-area printing has made it the target of recent research [10]. Blankenburg proposed upscaling the process for thin polymer solar cells (OPV) using a laboratory R2R slot-die printing machine [11]. Krebs proposed polymer solar cell modules manufactured by full R2R processing (comprising flexography, slot-die, and rotary screen printing), which have a best power conversion efficiency of 2.75%. He also proposed the use of slot-die printing and screen printing for an indium tin oxide (ITO)-free flexible polymer solar cell with an efficiency of 1.4% [12], [13]. Galagan analyzed the effect of process conditions on the performance of polymer solar cells that were adjusted for slot-die R2R printing [14].
In the slot-die printing, the thickness of printed pattern is determined in advance of the experiment by adjusting the flow rate of the solution that supplied to the slot-die according to the operating velocity of the moving substrate. It is known that the velocity of the moving substrate, the capillary number, and the viscosity of solution determine the thickness. However, the determination of the thickness is valid only by the applying of slot-die for whole coating or single strip printing due to its limitation of 2-dimensional mathematical model [15].
When the slot-die is applied in a multi-strip printing rather than to the whole area coating or single-strip printing, it is important to control the width of the patterned strip as well as the thickness precisely. In multi-layered printed circuits, printing errors with regard to the width of the strip can generate short-circuits or electricity leakages.
There have been several studies on the influence of the process conditions of the slot-die, including the velocity, the thickness of the coated layer, the flow rate of the solution supplied to the slot-die, and the viscosity of the solution. Romero analyzed the mechanism of the limit of minimum thickness at given conditions of substrate velocity, capillary, and inertial forces in the flow [10], [16]. Lin carried out theoretically two-dimensional numerical estimations on the operating windows of the slot die [17]. Chang investigated the minimum wet thickness of the slot-die coating in experimental studies [18]. All of these results were carried out in a two-dimensional plane which consists of thickness-axis and time-axis, under the typical assumption of no variation in the width of the slot-die coated pattern. However, the width of the patterned strip is also subject to the operating conditions, which affects the quality of registration in multilayered patterning [13], [14]. Accordingly, it is important to analyze the correlation between the process parameters and the coated thickness and width of the multi-strip. However, there has been no research with regard to variations in the width of the slot-die patterned multi-strip with changes in process conditions. In the current study, the effects of the operation parameters on the thickness and the width of the strips were analyzed. Using the full factorial design of experiment, the effects of operating conditions were examined in the range of experimental inputs. Analysis of Variance (ANOVA) tool was employed for statistical analysis of the major parameters. In addition, a mandatory strategy was proposed for fabricating multi-strip with the desired thickness and width by single slot-die system. These results can be used to tune the operating conditions for the patterning of multi-strip by the R2R slot-die systems (Fig. 1).
Section snippets
Mathematical modeling
The typical cross sectional view of slot-die lip during the coating process is depicted as shown in Fig. 2. At the downstream, the maximum pressure difference is calculated as Eq. (1) in given conditions of surface tension and gap between slot-die lip and substrate by the viscocapillary model [19].where is surface tension, H0 is gap between slot-die lip and substrate, t is thickness of coated layer.
Also, Landau and Levich [20] suggested the pressure drop along the meniscus of
Printing materials
Polyvinylcarbazole (PVK) was purchased from TCI (Tokyo Chemical Industry, Japan). The solution for the experiment was formulated using the precursor of PVK and 0.55 wt% 1,2-dichloroethane, which acted as a solvent. The viscosity of the formulated solution was 3.8 mPa s at 16.5 °C and the surface tension was 20 dyn/cm.
Primer-treated PET (polyethylene terephthalate) substrate was purchased from TOYOBO (Japan). The substrate properties were as follows: thickness of 100 μm, width of 200 mm, Young's
Experimental results
The experimental results of measured thickness and width of printed strips in treatment combinations are summarized in Table 3. For the reliability of the experiment, 5 samples were measured and averaged. Then, the experimental results were verified using the dimensionless number, T, which was suggested as follows:where, w is width (cm), t is thickness (cm), frt is supplying volume per unit length (cm3/m).
Applying Eq. (17) to Table 3, then the dimensionless number (T) was calculated
Conclusion
This paper analyzed the effects of operating conditions such as velocity, flow ratio, and gap on the width and thickness of a patterned strip in the R2R slot-die patterning system. A full factorial experiment was applied for determining the dominant parameters, and the velocity was found to be the most effective parameter on both the thickness and width of the strip. The thickness was proportional to the velocity, but the width was inversely proportional to the velocity. The flow ratio and gap
Acknowledgment
This paper resulted from the Konkuk University research support program.
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