Optimisation_of_Computer_aided_Screen_Pr (1)

.pdf

School

Boston University *

*We aren’t endorsed by this school

Course

MANAGERIAL

Subject

Mechanical Engineering

Date

Oct 30, 2023

Type

pdf

Pages

Uploaded by CommodoreDangerArmadillo27

Acta Polytechnica Hungarica Vol. 11, No. 8, 2014 – 29 – Optimisation of Computer-aided Screen Printing Design Eszter Horvath, Adam Torok, Peter Ficzere, Istvan Zador, Pal Racz Department of Electronics Technology, Budapest University of Technology and Economics, Egry József u 18 , H-1111 Budapest, Hungary, horvathe@ett.bme.hu; Department of Transport Technolgy and Economics, Budapest University of Technology and Economics, Sztoczek u. 2, H-1111 Budapest, Hungary, atorok@kgazd.bme.hu; Department of Vehicle Elements and Vehicle-Structure Analysis; Budapest University of Technology. and Economics; Stoczek u. 2, H-1111 Budapest, Hungary, ficzere@kge.bme.hu; Kogát Ltd ., Eperjes u. 16, H- 4400 Nyíregyháza, Hungary, istvan.zador@kogat.hu Bánki Donát Faculty of Mechanical and Safety Engineering, Óbuda University, Népszínház u. 8, H-1081 Budapest, Hungary, racz.pal@bgk.uni-obuda.hu Abstract: Computer-aided screen printing is a widely used technology in several fields like the production of textiles, decorative signs and displays and in printed electronics, including circuit board printing and thick film technology. Even though there have been many developments in the technology, it is still being improved. This paper deals with the optimisation of the screen printing process. The layer deposition and the manufacturing process parameters strongly affect the quality of the prints. During this process the paste is printed by a rubber squeegee onto the surface of the substrate through a stainless steel metal screen masked by photolithographic emulsion. The o ﬀ -contact screen printing method is considered in this paper because it allows better printing quality than the contact one. In our research a Finite Element Model (FEM) was created in ANSYS Multiphysics software to investigate the screen deformation and to reduce the stress in the screen in order to extend its life cycle. An individual deformation measuring setup was designed to validate the FEM model of the screen. By modification of the geometric parameters of the squeegee the maximal and the average stress in the screen can be reduced. Furthermore the tension of the screen is decreasing in its life cycle which results in worse printing quality. The compensation of this reducing tension and the modified shape of squeegee are described in this paper. Using this approach the life cycle of the screen could be extended by decreased mechanical stress and optimised off-contact. Keywords: screen printing; FEM; optimisation

E. Horvath et al. Optimisation of Computer-aided Screen Printing Design – 30 – 1 Introduction Screen printing is the most widespread and common additive layer deposition and patterning method because of its ability to print on many kinds of substrates with the widest range of inks and because, considering any modern print process, it can deposit the greatest thickness of ink film [1]. Although this technology has been used since the beginning of the 2 nd millennium it is still under development and there have been many innovations with the technology in the last 50 years [2]. Screen printing technology provides the most cost ef fective facility for applying and patterning the different layers for hybrid electronics industry as well, since a thick film circuit usually contains printed conductive lines, resistive and dielectric layers. Due to its simple technology and relative cheapness it is still widely used in the mass assembly of recent electronic circuits. The technology has a large application field extending to decal fabrication, balloon and cloths patterning, textile production, producing signs and displays, decorative automobile trim and truck signs and last but not least use in printed electronics [3]. Screen-printing is ideal as a manufacturing approach for microfluidic elements also used in the field of clinical, environmental or industrial analysis [4], in sensors [5] and in solar cells [6]. In general, the layers are designed using computer-assisted design (CAD) software [7]. Paste printing is carried out by a screen printer machine. The screen is strained onto an aluminium frame and the ink is pressed onto the substrate through the screen not covered by emulsion by a printing squeegee. The squeegee has constant speed and pushes the screen with a contact force. The material of the screen can be stainless steel or polymer. The design of the printed layer is realized with a negative emulsion mask on the screens. There are two main techniques of screen printing:  o ff -contact, where the screen is warped with a given tension above the substrate;  contact, where the screen is in full contact with the substrate. Contact screen-printing is less advantageous in general, because due to the lift off of the screen it often causes the damage of the high resolution pattern. In case of off-contact screen-printing, some paste is applied on top of the screen in the front of the polymer squeegee. While the squeegee is moving forward, it pushes the screen downwards until it comes into contact with the substrate beneath. The paste is pushed along in front of the squeegee and pushed through the screen not covered by emulsion pattern onto the substrate. The screen and substrate separate behind the squeegee. The off-contact screen printing process is demonstrated in Fig. 1.

Acta Polytechnica Hungarica Vol. 11, No. 8, 2014 – 31 – Figure 1 The squeegee pushes the paste on the screen and presses it through the openings Since the 1960s several experiments and models of the printing process have been evaluated. The optimisation of screen printing was mainly achieved by experimental evaluation without the advantage of numerical models. The empirical optimisation method is described by Kobs and Voigt [8] in 1970 , they appointed more than 50 variables and combined the most important ones in almost 300 di ff erent ways and also compared the effects of them. These investigations offer an enormous empirical database but general rules for screen-printing cannot be created from this without models. Miller [9] has investigated the amount of paste printed on the substrate in the function of paste rheology, mesh size and line width. Others have examined the influence of squeegee angle and squeegee blade characteristic on the thickness of the deposited paste [10] and the effect of the screen on fine scale printed patterns [11]. In general, the best solution for optimising a process can be achieved by parameter optimisation based on a theoretical model [12]. The first efforts to achieve a theoretical description of the screen printing process were made by Riemer more than 20 years ago [13-15]. His mathematical models of the screen-printing process were based fundamentally on the Newtonian viscous fluid scraping model [16]. This model was extended by others [17-18], although they did not take into account the flexibility of the screen, which is a feature influencing the process essentially. Neither of these models deals with the effect of geometry in the screen printing process. The repetitive behaviour of the printing process requires taking into consideration the effect of the cyclic load. In this work, a mechanical model is presented with similar geometry to the off- contact screen printing process. In this model, instead of the paste deposition phase, the mechanical behaviour of the screen is in the focus. Furthermore, our model effectively considers the geometry of the knife. The aim of this paper is to improve the technical solutions and in increase the performances without harming reliability as defined at Morariu [26].

E. Horvath et al. Optimisation of Computer-aided Screen Printing Design – 32 – 2 Experimental Setup 2.1 Material Parameters of Screen The first step of the model construction is to define the geometries and obtain the mechanical parameters of the screen [27]. The geometric features and the initial strain – which warps it onto an aluminium frame – are realised by the manufacturing process. In order to decrease screen tension deviation during the print the screen is tightened onto the aluminium frame with the thread orientation of 45  to the printing direction. Therefore the load distribution is more homogeneous between the threads. The elastic (Young) modulus of the screen was determined using the modified Voigt expression: ) V - ·(1 E ·V ·E E f m f f c    (1) where E m = 690 MPa is the elastic modulus of the emulsion, E f = 193 GPa is the elastic modulus of the stainless steel, V f is the volume fraction of the stainless steel and η is the Krenchel efficiency factor [19], [20]. In case of  1 =  2 = 45  thread orientation in the frame: 25 . 0 cos · 5 . 0 cos · 5 . 0 2 4 1 4       (2) The Poisson ratio can be expressed as: m m f f xy V V        (3) where ν f is the Poisson´s ratio of stainless steel (0.28) and ν m is the Poisson ratio of emulsion (0.43) [21]. In our study SD75/36 stainless steel screen was utilised with the mesh number of 230 and open area of 46%. The schematic view of screen cross section is shown in Fig. 2, Figure 2 A sketch of the SD75/36 screen cross section with the main parameters

Acta Polytechnica Hungarica Vol. 11, No. 8, 2014 – 33 – where d is the diameter of the thread,  is the bending angle, x is the element length of the thread.  sin d x  (4) Using Eq.4. the volume fraction of the stainless steel can be calculated: 27 . 0 sin d · d l l · l · 4 · d · 2 V 2 f = + =   (5) Substituting Eq. 5. into Eq. 1. the elastic modulus of the screen turned out to be 13 GPa. The sizes of the screen are 298 mm in width, 328 mm in length and the thickness of it was 72 μm. These parameters were utilised in the finite element model. 2.2 Measuring the Friction Force between the Screen and the Squeegee The paste we have applied in our experiment was PC 3000 conductive adhesive paste from Heraeus. In the process of screen printing the friction force between the screen and the squeegee plays an important role. While the squeegee passes the screen due to the friction force the position of the mask shifts. The individual friction force measuring setup is shown in Fig. 3. Figure 3 Measurement setup for determining the friction force between the squeegee By this measurement the relationship between the friction force ( F f ) and the printing speed ( v ) and squeegee force ( F s ) was estimated. Every thick film paste is viscous and has a non-Newtonian rheology suitable for screen printing. The shear stress, τ , for this kind of fluids can be described by the Ostwald de Waele relationship:

E. Horvath et al. Optimisation of Computer-aided Screen Printing Design – 34 – n dy dv K           (6) where K is the flow consistency coefficient (Pa  s n ), ∂v/∂y is the shear rate or the velocity gradient perpendicular to the plane of shear (s −1 ), n is the flow behaviour index (-) [22]. Fig. 4 shows the shear stress and paste velocity during screen printing. Thick film paste is a shear-thinning fluid, thus n is positive but lower than 1. Figure 4 Appeared shear stress and the velocity of paste during screen printing. In addition the elongation of the screen – which is greater if the off-contact is greater – results in image shift as well [23]. The effect of these lateral shifts demonstrated in Fig. 5 has also to be taken into account. Figure 5 Deformed paste deposition is the result of the screen elongation The image shift was examined, where the screen tension was in the region of 2 – 3.3 N/mm, the off-contact was 0.9 – 1.5 mm, and the applied friction force was based on the measurement. The reduction of screen tension can affect the quality of the printing in other respects. The deflection force of the screen is decreasing,

Your preview ends here

Eager to read complete document? Join bartleby learn and gain access to the full version