Screen Printing of Multilayered Hybrid Printed Circuit Boards on

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Abstract— This paper reports on the successful fabrication of a multilayered hybrid printed circuit board (PCB) for applications in the consumer electronics ...
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Screen Printing of Multilayered Hybrid Printed Circuit Boards on Different Substrates Ali Eshkeiti, Avuthu S. G. Reddy, Sepehr Emamian, Binu B. Narakathu, Michael Joyce, Margaret Joyce, Paul D. Fleming, Bradley J. Bazuin, Member, IEEE, and Massood Z. Atashbar, Senior Member, IEEE Abstract— This paper reports on the successful fabrication of a multilayered hybrid printed circuit board (PCB) for applications in the consumer electronics products, medical technologies, and military equipment. The PCB was fabricated by screen-printing silver (Ag) flake ink, as metallization layer, and UV acrylic-based ink, as dielectric layer, on different substrates such as paper, polyethylene terephthalate, and glass. Traditional electronic components were attached onto the printed pads to create the multilayered hybrid PCB. The feasibility of the hybrid PCB was demonstrated by integrating an embedded microcontroller to drive an liquid-crystal display (160 × 100 pixels). In addition, the amount of the ink spreading after printing, the effect of bending on the printed lines, and the effect of the roughness of the substrates on the resistance of the printed lines was investigated. It was observed that the resistance of the lines increased by ≈1.8%, after 10 000 cycles of bending, and the lowest resistance of 1.06  was measured for the 600 μm printed lines on paper, which had a roughness of 0.175 μm. The advantage of fabricating PCBs on flexible substrates is the ability to fold and place the boards on nearly any platform or to conform to any irregular surface, whereas the additive properties of printing processes allow for a faster fabrication process, while simultaneously producing less material waste in comparison with the traditional subtractive processes. The results obtained show the promising potential of employing screen printing process for the fabrication of flexible and light-weight hybrid PCBs. Index Terms— Flexible printed circuit boards (PCBs), hybrid PCBs, printed electronics (PEs), screen printing.

I. I NTRODUCTION

A

S EMBEDDED electronics have become common in an ever widening array of products and applications, the form factors, flexibility, and materials used to construct circuit boards must evolve to support emerging applications. As a result, there has been an increasing demand for further development and understanding of the characteristic traits pertaining to the printing of flexible electronic circuits

Manuscript received August 4, 2014; revised December 19, 2014; accepted December 30, 2014. Recommended for publication by Associate Editor R. N. Das upon evaluation of reviewers’ comments. A. Eshkeiti, A. S. G. Reddy, S. Emamian, B. B. Narakathu, B. Bazuin, and M. Z. Atashbar are with the Department of Electrical and Computer Engineering, Western Michigan University, Kalamazoo, MI 49008 USA (e-mail: [email protected]; [email protected]; sepehr. [email protected]; [email protected]; brad.bazuin@ wmich.edu; [email protected]). M. Joyce, M. Joyce, and P. D. Fleming are with the Department of Chemical Engineering, Western Michigan University, Kalamazoo, MI 49008 USA (e-mail: [email protected]; [email protected]; [email protected]). Color versions of one or more of the figures in this paper are available online at http://ieeexplore.ieee.org. Digital Object Identifier 10.1109/TCPMT.2015.2391012

and components. Printed electronic (PE) devices, created using traditional printing techniques such as gravure [1], [2], inkjet [3], [4], flexography [5], [6], and screen printing [7], [8] have shown the ability to produce circuits and sensors for many applications [9]–[13]. Some examples of PE devices are organic thin film transistors [14], [15], flexible displays [16], [17], flexible printed solar cells [18], [19], and substrate for surface enhancement Raman spectroscopy [20], which have shown varying levels of applicability for use in commercial applications. Although the current performance of PE devices does not match the switching speed, density, and higher current handling of traditional solid-state devices, produced using conventional manufacturing techniques, there are several advantages of using printing techniques for the fabrication of less demanding devices. These include a lower cost of manufacturing, lower-processing temperatures, and a reduction in resources used during fabrication. These advantages have facilitated the need for the production and manufacturing of PE products on a commercial scale [21], [22]. Moreover, printing techniques, which lend themselves to roll-to-roll (R2R) processes, enable electronic devices to be produced in high volumes at high speeds without the intricate process requirements associated with conventional silicon manufacturing technologies, such as complicated photolithographic patterning procedures, high temperatures, and vacuum deposition methods. Although many researchers have shown the ability to produce electronic components and devices on flexible substrates [23]–[28], there have been very few reports involving the printing of printed circuit boards (PCBs) on flexible and rigid substrates, such as paper, polyethylene terephthalate (PET), polyethylene naphthalate, polyimide, and glass. Most of the currently available flexible PCBs are fabricated using methods, such as spin coating, sputtering, and spray coating [29]. These processes are typically used for the deposition of the conductive materials onto the substrate [29]. Some of the disadvantages associated with these methods include increased production time and material waste, which can be overcome by employing printing methods. To date, there are only a few reports on the printing of PCBs; however, none of these involved the screen printing of a multilayered hybrid PCB prototype with integrated electronic components on flexible substrates [30], [31]. Both plastic and paper could play an important role in the future of light weight and flexible PCBs. Plastic offers the advantages of high smoothness, transparency, and low porosity [32], [33]. Even though plastic is not very

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TABLE I S UMMARY OF D IFFERENT C HARACTERISTICS OF S UBSTRATES I NCLUDING S URFACE E NERGY, T HICKNESS , AND

ROUGHNESS

amenable to the use of different solvents, it has been one of the main materials used in conventional flexible electronics [34], [35]. In comparison with plastic, paper is more temperature resistant, stiffer, renewable, and may be less susceptible to moisture problems [36]. Currently, paper is the material of choice for many products used on a daily basis, and therefore, the fabrication of PCBs on paper may open new market opportunities for PEs and paper. The ability to produce flexible PCBs on paper and PET enable them to be placed on and conform to different form factors and surfaces, where spacing or shape would prohibit the placement of a rigid conventional circuit board. Alternately, for some applications, such as the automotive industry, a rigid transparent PCB on glass would be beneficial to enable devices to be integrated into the windshields or mirrors of cars. Moreover, glass offers the added advantages of heat stability, transparency, and smoothness [37]. In this paper, screen printing was used to fabricate multilayered PCBs on PET, paper, and glass. Pick and place equipment was used to attach solid-state components onto the boards to produce a working hybrid PCB. The capability of the fabricated PCB, incorporating an embedded microcontroller to drive a 160 × 100 pixels liquid-crystal display (LCD) with 3.0 V power supply was demonstrated. II. E XPERIMENTAL S ECTION A. Choice of Materials 1) Substrate: Three different substrates: 1) 130-μm-thick flexible PET film (Melinex ST 506) from DuPont Teijin Films; 2) plate glass from Corning; and 3) paper (NB-RC3GR120) from Mitsubishi were used. Some of the important characteristics of the different substrates are summarized in Table I. For example, glass demonstrated a higher surface energy when compared with paper and PET. Adhesion of the silver ink employed for traces on all of these substrates was good and showed promise as substrates for the fabrication of PCBs. The roughness of paper, PET, and glass was measured to be 0.175, 0.015, and 0.005 μm, respectively, using a WYKO RST-plus optical profiler. Based on these measurements, both glass and PET have relatively smooth surfaces, making them good substrates for printing the thin conductive traces. In comparison with the glass and PET, paper is considerably rougher, although this paper is very smooth

by paper standards. However, unlike PET and glass, paper is more absorptive, which can reduce the amount of ink spreading after printing. Since the thickness of a printed ink film depends on the amount of ink spreading and absorption, if both properties are controlled, highly conductive lines can be produced on paper. The roughness and absorptive properties of paper could also be an added advantage for ink adhesion. Hence, it is worthy of study, since it is flexible, readily available, tunable, and of low cost. 2) Metal and Dielectric: There are different materials such as silver (Ag), gold (Au), and copper (Cu) that can be employed for the fabrication of electronic circuit boards. In this paper, a Ag flake ink (Electrodag 479SS) and a UV acrylic-based ink (Electrodag PF-455B) from Henkel were used as the metallization and dielectric layers, respectively. All layers were printed using an AMI 485 semiautomatic screen printing press at room temperature. 3) Adhesive for Component Attachment: A commercially available Ag conductive epoxy (H20E), purchased from Epoxy Technologies Inc., was used to attach the surface mounted devices (SMD) to the different substrates. The epoxy was screen printed onto the pads prior to the placement of the electronic components with an automated pick and place instrument (MY100sxe) from My Data Inc. B. Design of the Circuit To demonstrate that the hybrid PCB would work once fabricated, a microcontroller-based circuit was designed to control an externally attached LCD, and a three-layer PCB layout pattern was produced. The circuit design consisted of a dc to dc convertor, microcontroller, and other necessary passive components. The design layout was done using PCB123 design software and then transferred to a multilayer Adobe Illustrator file for screen generation. The layout of the design that consists of two conducting layers and an insulating layer, which prevents shorting between the top and bottom Ag layers, is shown in Fig. 1. C. Fabrication of PCB 1) Screen Printing Process: The conductive and dielectric materials were screen printed onto the substrates using an AMI 485 semiautomatic screen printing press at room temperature. Screen printing is a process in which the image carrier (mask) is not in direct or physical contact with the substrate. Fig. 2 shows a schematic of the screen printing process. The printer consists of a screen printing plate or mesh and squeegee. Typically, the squeegees are made from rubber or polymeric raw materials. The screen printing plate consists of a frame that holds the screen mesh and stencil, which is the image carrier. A common frame material used is aluminum, which is known for its lightweight, hardiness, and ease of cleaning. The screen or mesh is composed of screen fabric, which is a veil like material composed of metal, fabric, plastic, or other type of material depending on the ink and screen’s functionality. The stencil can be made using different processes, electronically, or photographically, depending on its purpose. The screen mesh and stencil materials are highly

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Fig. 1. Layout of PCB design created in PCB123 design software. This design consists of the pads for a dc to dc convertor, a microcontroller and necessary passive components (red layer: bottom electrodes, green layer: dielectric layer, and yellow layer: top electrodes). Fig. 3. 3-D vertical scanning interferometer images of (a) silver and (b) dielectric on paper, (c) silver and (d) dielectric on glass, and (e) silver and (f) dielectric on PET.

Fig. 2.

Typical screen printing process.

dependent on the type of solvents and cleaning agents to be used during printing. The ink is deposited on top of the screen mesh and it is swept from one side of the screen to the other with a squeegee on top of the screen at a defined force and pressure. The ink passes through the screen mesh and is transferred onto the substrate as the squeegee is stroked across the mesh. This process is very fast and can be done at room temperature. Screen printable inks have a high viscosity (0.5–50 Pa · s); therefore, only the ink under pressure passes through the screen minimizing the amount of ink required. When compared with other methods of printing such as inkjet and gravure, there is relatively lesser ink spreading in

this process, which plays an important role in the prevention of electrical short circuit in the printed lines. 2) Screen Printing of PCB: A 320 mesh count screen was used for printing the materials on the various substrates. After printing the first or bottom metallization layer on all three substrates, the ink was cured for 20 min in a VWR 1320 temperature-controlled oven at 120 °C. The thickness of the ink film on paper, glass, and PET was measured to be 14.8, 10, and 12.8 μm, respectively, using a Bruker vertical scanning interferometer microscope (CounterGT) [Fig. 3(a), (c), and (e)]. All PCB lines were found to be continuous (no breaks) and conductive. Next, the dielectric layer was screen printed over the first metal layer. The printed layer was then cured using a UV fusion drying system equipped with a D bulb. Fig. 3(b), (d), and (f) shows the dielectric layer printed over the Ag layer on paper, glass, and PET, respectively. The thickness of the dielectric layer on paper, glass, and PET was measured to be 14, 9.46, and 8.62 μm, respectively. As shown, the dielectric layer was printed uniformly, thereby preventing any shorting between the top and bottom Ag layers. Finally, the top metallic layer was printed and thermally dried in an oven. A photograph of three printed layers on PET is provided in Fig. 4(a). 3) Component Attachment: The operational circuit was fabricated by attaching SMD components to defined pad areas onto the PCB. A silver epoxy was screen printed onto the pads using a 200-μm mesh screen and polymer squeegee of 80D hardness. The components were then placed onto the epoxy using an automated pick and place instrument

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Fig. 5. (a) Summary of linewidth measurement on different substrates. (b) 3-D profilometry picture. (c) Optical microscope image of printed lines for microcontroller contact pads.

Fig. 4. (a) Photograph of three layers of printed PCB on PET. (b) Photograph of the completed PCB on paper.

from My Data Inc. The samples were then initially cured at 130 °C for 10 min followed by a second cure at 120 °C for 5 min. A photograph of the completed PCB on paper is shown in Fig. 4(b). III. R ESULTS AND D ISCUSSION A. Analysis of Printed Lines The widths of the printed lines were measured with an ImageXpert (KDY Inc.) image analyzer. The results are shown in Fig. 5(a). The patterned 300 and 600 μm lines on paper printed as 263 and 566 μm, which correlate to a −13.3% and −5.6% loss, respectively. The negative gains are

attributed to ink absorption by the paper. On PET, the line widths are measured as 323 and 625 μm, which correlate to a gain of 7.6% and 4.16%, respectively. The increase in linewidth is attributed to the wetting and spreading of the ink on the PET film. For glass, the line widths were measured to be 295 and 608 μm, producing a negative gain of −1.6% and positive gain of 1.3%. Fig. 5(b) shows the vertical scanning interferometer images of the printed lines for the microcontroller contact pads. It was observed that these lines were printed uniformly, with complete separation from one another. The picture of the pads for the microcontroller with 50 μm spacing is shown in Fig. 5(c). The separation of the pads can be attributed to the lack of ink spreading. Fig. 6 shows a comparison of the effect of substrate roughness on the resistance of the printed lines. For example, it was seen that the lowest resistance (1.06 ) was obtained on paper, for the 600 μm lines. This is because paper absorbs the ink, thereby keeping the particles in close contact with one another, and hence the lines are more conductive. The printed lines on the glass substrate exhibited lower resistance (1.13 ) than the printed lines on PET (1.16 ) due to the lower roughness of glass creating a more consistently larger cross-sectional area for the line. It was observed that higher roughness affected the uniformity of printed lines and decreased the conductivity due to variations in ink film thickness at some points. Fig. 6 shows that the printed lines on the PET are slightly higher in resistance when compared with those on glass. This is due to the higher roughness of the PET producing a less consistent and effectively smaller cross-sectional area.

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Fig. 8. Photogragh of powered, operating microcontroller, and passive electronics on the printed glass substrate driving an LCD display.

Fig. 6.

Effect of roughness on resistivity of printed lines.

Fig. 7. SEM images of the printed lines (a) and (b) before bending and (c) and (d) after bending.

images of the printed lines were acquired using a Hitachi S-4500 model SEM, with Quantax 200 software package before and after bending Fig. 7. Fig. 7(a)–(d) shows the SEM images of the printed lines before and after bending, respectively. It was observed that even though there were some small cracks, the printed lines were conductive after bending. 3) LCD Operation: The printed PCB circuit was then energized and used to drive an LCD display. The codes for the microcontroller, a low-power Texas Instruments (TI) MSP430 processor, were written using TI’s Code Composer Studio. Fig. 8 shows the performance of the printed PCB when 3 V was applied to it. The preloaded software executing on the microcontroller produced a graphic message (CAPE), as shown, on the LCD. This effectively demonstrated that the converter and embedded microcontroller driving an attached device suggest numerous other small battery-operated applications. This includes low cost R2R electronic circuit boards for consumer electronics (e.g., in packaging industry) and medical wearable devices that can be attached to the human skin, postal security systems (printed directly to packages and envelopes), and very-light-weight circuit boards for aircrafts or automobiles. IV. C ONCLUSION

B. Electrical Analysis 1) Line Resistivity: The effect of the resistivity of the lines on the performance of the circuit was also investigated. The LCD was driven with a series of conventional resistances, and it was observed that the LCD worked with line resistances of up to 3.7 k. Since the resistance of all the printed lines was measured to be below 10 , it was determined that the performance of the LCD would not be affected by the resistance of the printed lines. 2) Effect of Bending: The effect of bending of the substrate on the resistance of the lines was also analyzed using a force gauge (Mark-10). The PCB was mounted on the support of the 3-point test fixture and subjected to 10 000 cycles of 5-mm elongation. At each point of elongation, a 3.4% decrease in the resistance was observed. After 10 000 cycles of bending, a 1.8% increase in the base resistance was observed. This is negligible for the given above stated tolerance of the line resistance. The scanning electron microscope (SEM)

Screen-printed multilayered PCBs using PE-deposited materials on three distinct substrates were successfully fabricated. The different characteristics of PET, paper, and glass as the substrates for PCBs were analyzed. A method for populating electronic components onto the printed PCB pads was established and demonstrated. The capability of the printed hybrid PCB circuit to operate correctly and to drive an LCD was shown. The fabrication of PCBs on flexible substrates such as paper and PET opens numerous opportunities for the development of low-cost and light-weight circuit boards for embedded electronic devices and applications that may include the requirement for conformal shapes or surfaces. At the same time, the fabrication of PCBs on flexible or rigid glass may lead to new designs for automotive and aerospace applications. R EFERENCES [1] J. Noh et al., “Scalability of roll-to-roll gravure-printed electrodes on plastic foils,” IEEE Trans. Electron. Packag. Manuf., vol. 33, no. 4, pp. 275–283, Oct. 2010.

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Ali Eshkeiti was born in Iran. He received the B.E. degree in electronics engineering from Azad University, Tehran, Iran, and the M.Sc. degree in electrical engineering from Western Michigan University, Kalamazoo, MI, USA, where he is currently pursuing the Ph.D. degree in electrical and computer engineering. His current research interests include design, fabrication, and characterization of printed sensor structures, wearable sensing systems, transistors, biochemical sensing systems, solar cells, energy storage devices, and lab-on-a-chip sensing systems.

Avuthu S. G. Reddy received the B.Tech. degree in electronics and computer engineering from Jawaharlal Nehru Technological University, Hyderabad, India, and the M.Sc. degree in electrical engineering from Western Michigan University, Kalamazoo, MI, USA, in 2011, where he is currently pursuing the Ph.D. degree with the Department of Electrical and Computer Engineering. His current research interests include design, fabrication, and characterization of printed sensor structures, transistors, microfluidics, and lab-on-a-chip sensing systems.

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Sepehr Emamian received the B.Sc. degree in electrical engineering from the Isfahan University of Technology, Isfahan, Iran, in 2010, and the M.Sc. degree in electrical engineering from the Sharif University of Technology, Tehran, Iran, in 2012. He is currently pursuing the Ph.D. degree in electrical engineering with Western Michigan University, Kalamazoo, MI, USA. His current research interests include printed electronics, design, fabrication, and characterization of printed sensor structures, piezoelectric based pressure sensors, and wearable sensors.

Binu B. Narakathu received the B.E. degree in electronics and communication from Visvesvaraya Technological University, Bangalore, India, the M.Sc. degree in computer engineering from Western Michigan University, Kalamazoo, MI, USA, in 2009, and the Ph.D. degree from the Department of Electrical and Computer Engineering, Western Michigan University, in 2014. He is currently a Post-Doctoral Fellow with the Center for Advanced Smart Sensors and Structures, Department of Electrical and Computer Engineering, Western Michigan University. His current research interests include all aspects of design, fabrication, and characterization of high performance sensing systems, microfluidic devices, lab-on-a-chip for point-of-care testing, biosensors, bioelectronics, printed electronic devices, and biomedical (or biological) microelectromechanical systems devices for applications in the biomedical, environmental, and defense industries.

Michael Joyce received the B.S. degree in psychology and M.S. degree in paper and printing science from Western Michigan University, Kalamazoo, MI, USA, in 2012 and 2014, respectively. He is currently pursuing the Ph.D. degree in paper and printing sciences with Western Michigan University, Kalamazoo, MI, USA. He is currently a Teaching Assistant with the Department of Chemical and Paper Engineering. He has been involved in various client-supported projects for industrial clients through the Center for the Advancement of Printed Electronics and the Center for Ink and Printability. He has also served as a Teaching Assistant for courses regarding digital printing processes, ink and color production, and digital graphics. He has performed projects utilizing screen, flexo, gravure, and inkjet print methods, and has created a novel processing method, enabling the creation of stand-alone printed sensors. His doctoral work is focused on the study of metallic ink interactions with poly-di-methyl-siloxane substrates for creation of biocompatible sensors utilizing various printing processes.

Margaret Joyce received the Ph.D. degree in paper engineering and science from North Carolina State University, Raleigh, NC, USA. She was a Technical Director with the industry for 11 years for a specialty chemical company serving the paper, textiles, and printing industries. She is currently a Professor with the Department of Paper Chemical and Paper Engineering, Western Michigan University, Kalamazoo, MI, USA, where she is the Director of the Center for the Advancement of Printed Electronics and the Center for Coating Research and Development. She is an expert in coatings, substrates, rheology, and surface chemistry. She has authored over 100 referred publications and presentations, and holds three U.S. patents. Her current research interests include coating and ink rheology, water soluble polymers, surface chemistry, paper and ink interactions, and paper and printing processes.

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Paul D. Fleming received the B.Sc. degree in physics from Ohio State University, Columbus, OH, USA, in 1964, and the master’s degree in physics and the Ph.D. degree in chemical physics from Harvard University, Cambridge, MA, USA, in 1966 and 1971, respectively. He is currently a Professor with the Department of Chemical and Paper Engineering, Western Michigan University, Kalamazoo, MI, USA. He has been involved in industrial research and development with Phillips Petroleum, Bartlesville, OK, USA, and GenCorp Inc., Rancho Cordova, CA, USA. His current research interests include printed electronics, digital printing, color management, and paper coatings. Dr. Fleming is a member of the Imaging Science and Technology, the Australian Psychological Society, and the Technical Association of the Graphic Arts.

Bradley J. Bazuin (M’81) received the B.S. degree in electrical engineering from Yale University, New Haven, CT, USA, in 1980, and the M.S. and Ph.D. degrees in electrical engineering from Stanford University, Stanford, CA, USA, in 1982 and 1989, respectively. He was a Research Assistant with the Center for Integrated Electronics in Medicine associated with the Integrates Circuits Laboratory, Center for Integrated Systems, Stanford University, from 1981 to 1988, a part-time MTS and System Engineer from 1981 to 1989, a Principal Engineer with ARGOSystems, Sunnyvale, CA, USA, from 1989 to 1991, and a Senior Systems Engineer with Radix Technologies, Mountain View, CA, USA, from 1991 to 2000. He has been a Term Appointed Assistant Professor and an Assistant Professor since 2000, and is currently an Associate Professor of Electrical and Computer Engineering with Western Michigan University, Kalamazoo, MI, USA. His current research interests include printed electronics, electronic and printed circuit board circuit design and fabrication, custom integrated circuit design, embedded signal processing, wireless communication, software defined radios, and advanced digital signal processing algorithms for physical layer communication systems. Dr. Bazuin is a member of the American Society for Engineering Education and the Institute of Navigation.

Massood Z. Atashbar (SM’02) received the B.Sc. degree in electrical engineering from the Isfahan University of Technology, Isfahan, Iran, the M.Sc. degree in electrical engineering from the Sharif University of Technology, Tehran, Iran, and the Ph.D. degree from the Department of Communication and Electronic Engineering, Royal Melbourne Institute of Technology University, Melbourne, Australia, in 1998. He was a Post-Doctoral Fellow with the Center for Electronic Engineering and Acoustic Materials, Pennsylvania State University, University Park, PA, USA, from 1998 to 1999. He is currently a Professor with the Department of Electrical and Computer Engineering, Western Michigan University, Kalamazoo, MI, USA. His research interests include physical and chemical microsensor development, wireless sensors, and applications of nanotechnology in sensors, digital electronics, and printed electronic devices. He has authored over 160 articles in refereed journals and refereed conference proceedings. Dr. Atashbar is the Editorial Board Member and an Associate Editor of the IEEE S ENSORS J OURNAL.