5 th International & 26 th All India Manufacturing Technology, Design and Research Conference (AIMTDR 2014) December 12 th 14 th, 2014, IIT Guwahati, Assam, India Fabrication of Complex Circuit Using Electrochemical Micromachining on Printed Circuit Board (PCB) Singh Jitendra 1, Jain V.K. 2*, Ramkumar J. 3 1 Mechanical Engineering Department, IIT Kanpur - 208016, jitendr@iitk.ac.in 2* Mechanical Engineering Department, IIT Kanpur - 208016, vkjain@iitk.ac.in 3 Mechanical Engineering Department, IIT Kanpur - 208016, jrkumar@iitk.ac.in Abstract Electrochemical micromachining (ECMM) is an advanced machining process for machining of electrically conducting materials. In the present work, an experimental set-up for electrochemical micromachining (ECMM) is used to fabricate complex circuits on printed circuit board (PCB) by using masking technique. Mask is made using 50 µm transparency sheet cut by laser beam. This mask is bonded on printed circuit board of cross section 35 mm X 35 mm by a water insoluble glue Araldite (epoxy adhesive). For the analysis of the data the channel width of the circuit structure is measured with the help of photographs taken by using Dinolite capture and optical microscope. For this purpose whole structure is divided into 4 regions, which have fifty eight points in which width has been measured. For the analysis of the depth of a channel, a dial gauge having 1 µm least count is used to measure the depth at different sections (or regions) as marked in the Figure. For this purpose, the whole structure is divided into 4 regions, which have twenty seven points. After all the experiments have been completed, the circuit is compared with the main circuit which is fabricated by milling process and % error in the circuit is evaluated. Keyword: Electrochemical micromachining, Printed circuit board, micromachining 1 Introduction Due to the limited resources such as space, material and energy, their efficient utilization is an important issue. Miniaturization is becoming an important need of the days. The term micromachining considers the miniature features/structures/parts which are not easily visible with naked eyes and have dimensions smaller than 1 mm (1 µm dimension 999 µm) [1]. Recent changes in society demands have forced to introduce micro parts, or macro parts with micro features into various industries; for example, automobiles, aviation, space, electronics and computers (printed circuit boards). Making 3D structure by conventional machining is difficult as compared to advanced machining processes. Electrochemical micromachining (ECMM) process is an advanced micromachining process for machining electrically conductive materials. The process works on the principle of Faraday s laws of electrolysis, where material is liberated from the work piece surface atom by atom. ECMM process is just reverse of the electroplating process where the objective is to deposit the material. On applying potential across the tool and work piece, positively charged ions are attracted towards cathode and negatively charged ions are attracted towards anode. The inverse shape of the tool is produced on the work piece. Positive metal ions leave the work piece and they form insoluble precipitate. Following reactions take place at both the electrodes [1, 2]. At anode; In the present case, M is copper and n (= 2) is valency of the work piece material. The electrolyte accepts these electrons resulting in a reduction reaction which can be represented as At cathode; Thus, positive ions from the metal react with the negative ions in the electrolyte, forming hydroxides and thus the metal is dissolved forming a precipitate. In solution; Insoluble material and heat generated during machining are removed by flowing electrolyte. ECMM appears to be a promising micromachining technology due to its advantages that include high material removal rate (MRR), better precision and control, low machining time, and no tool wear [3]. It has been observed that the desired accuracy can be achieved at higher frequency, medium electrolyte concentration 102-1
Fabrication of Complex Circuit Using Electrochemical Micromachining on Printed Circuit Board (PCB) (20 g/l), and average machining voltage (3 V) [4-7]. Smaller parts size with better machining accuracy are produced ECMM by side insulation of tool [8]. Use of short pulse voltage and passive electrolyte, higher dimensional accuracy can be achieved [9]. ECMM process is comparatively more environmental friendly and high speed technology [10]. 2 Experimental set-up A schematic diagram of experimental set-up is shown in Fig. 1. Hyper 10 (Multi Process Micro Machine Tool by Sinergy Nano systems) machine is used to carry out experiments by electrochemical micromachining process. The maximum travel ranges of the machine are 130 mm (X) x 75 mm (Y) x 80 mm (Z). Each axis has positional accuracy of ±5 µm, repeatability of all sides ±1 µm and side straightness ±1 µm for all axes. Desired interelectrode gap (IEG) is maintained by giving feed to the motor by Z-axis controller. A perspex tray with drain is mounted on X Y table, and it also holds electrolyte and reaction products 3 Fabrication of complex circuit and performance evaluation In this work, a complex circuit structure on printed circuit board (PCB) is made by ECMM using masking technique. Fabrication is not expensive and takes less time as compared to other fabrication processes. 3.1 Adhesion between work piece and tool Adhesion of the mask (Fig. 2) to the work piece plays a crucial role in fabrication of structure. If mask is not properly glued to the work piece, electrolyte flows between work piece and mask, and causes undercut. A printed circuit board sheet having the size of 35 mm X 35 mm is used as a work piece (anode) by connecting it to positive terminal of power supply and, another copper plate having dimensions of 35 mm X 35 mm X 1 mm is used as a tool (cathode) by connecting it to the negative terminal of power supply. The mask is prepared from a thin transparent sheet having thickness of 50 µm. Mask cutting is done by using engraving technique with low power laser cutting machine. Dimensions of the mask are shown in Fig. 2. Maximum width of channel cut on the mask is 385 µ m. The circuit has been divided in four different regions for characterization purposes, Fig. 3. All specified dimensions are in mm Figure 1: Schematic diagram of electrochemical micromachining process Components of multipurpose machine tool are X, Y, and Z axes feed unit, Working table, ECM power supply unit, Control unit, Electrolyte flow control unit. Table 1: ECMM parameters in Hyper 10 multipurpose machine tool Parameter Range Voltage (V) 1-24 Capacitance (pf) 33-10000 polarity Both, normal and reverse Sensitivity (%) 65-85 Figure 2: Schematic diagram of mask made on design software (Solid-work) 3.2 Performance evaluation of fabricated circuit Experimental set-up which is explained in the previous section is used for fabrication of complex circuit on PCB using ECMM process. 102-2
5 th International & 26 th All India Manufacturing Technology, Design and Research Conference (AIMTDR 2014) December 12 th 14 th, 2014, IIT Guwahati, Assam, India (c) Figure 3: Depth and width measurement at four different region 3.2.1 Channel depth Effect of voltage Fig. 4 shows the effect of voltage on depth of channel at different capacitance values for individual region. In Fig. 4, 41 µm is the minimum value in region 2 and 4 at 6 V and 33 pf capacitance, and maximum value 54 µm in region 1 for 11 V and 10000 pf capacitance. It is clear from the graph that depth increases approximately linearly from 6 V to 11 V. A dial gauge having 1 µm least count is used to measure the depth at different sections (or regions). Channel depth is measured at twenty seven points on the structure in which regions 1, 2 and 4 have six points each and region 3 has nine points. The whole structures has many sharp corners and curves. The average depth readings of different points in different regions plotted in the graphs. (d) Figure 4: Effect of voltage on channel depth at different regions with variation in capacitances Effect of capacitance Fig. 5 shows the effect of capacitance on channel depth for different voltages. As the capacitance level increases, channel depth is changing marginally in all regions. As capacitance increases energy per sparks increases hence MRR also increases. As a results, depth of micro channel also increases. Figure 5: Effect of capacitance on channel depth at different regions with variation in voltages. 102-3
Fabrication of Complex Circuit Using Electrochemical Micromachining on Printed Circuit Board (PCB) 3.2.2 Channel width Channel width can be measured by the optical microscope BX51. Whole structure is divided into four regions, in Fig. 3. Regions 1 and 2 have fourteen points each, region 3 has twenty, and region 4 has eight points. There are total fifty eight points. Image is taken on every point with 5X magnification, and channel widths are measured at 3-4 points on every image. The average readings in different regions are taken. Four images of a channel cut on a PCB are taken by the optical microscope as shown in the Fig 6 with their machining conditions used. V = 6 V, V = 7 V, Figure 7: Effect of voltage on channel width at different regions with variation in capacitances V = 10 V, (c) V = 11 V, (d) Figure 6: Variation in channel width at different voltages at 100 pf capacitance and 25 g/l electrolyte concentration Effect of capacitance Fig. 8 shows the effect of capacitance on channel width at different voltages. It is clear from that as the capacitance level (i.e. energy (= ) increases, the channel width increases up to 1000 pf beyond which it starts decreasing. In the present case, voltage varies from 6V-11 V. Effect of voltage Effect of voltage on channel width for different capacitance values is shown in Fig. 7. It is clear from the graphs that channel width increases with increase in voltage. According to Faraday s laws, with increase in voltage, current (I) increases and material removal (m) is proportional to the current flowing through IEG as given below, (1) Where, E is chemical equivalent of anode material, F is Faraday s constant. Therefore, the size of the channel is increased as the voltage increases. Other region (3, 4) have similar trend. Figure 8: Effect of capacitance on channel width at different regions with variation in voltage 102-4
5 th International & 26 th All India Manufacturing Technology, Design and Research Conference (AIMTDR 2014) December 12 th 14 th, 2014, IIT Guwahati, Assam, India 4 Inspection of circuit After fabricating the complex circuit, it was checked using a microscope for continuity, and some defects were found as explained below. For testing the circuit, fifty eight points are selected on every circuit. In some points, the channel was not cut properly up to the required depth, and at some points of PCB, substrate reacted with electrolyte and depth increased. At other points, the overcut was greater than the channel width on the mask because of improper adhesion of mask on the work piece. Because of this, the gap between two channels at some points is decreasing. This is clearly seen in Fig. 9,. Fig. 10 shows a discontinuity in the circuit, which is present due to mask design. These discontinuities can be removed by taking an additional selected cutting. There is no contact between tool and work piece so no mechanical defects are present in the circuit. Figure 10: Discontinuity present in circuits due to mask design in ECMM process 5 Comparisons between Circuit fabricated by ECMM process and Milling process Fig. 11 shows a complex circuit fabricated by ECMM process, and milling process. Width and depth of the channels are compared to each other and % error is found, (Table 2). Figure 9: Uncut channel with at volt=5, C= 33pF, concentration = 20g/l volt = 11V, Cap. = 1000pF, concentration = 25g/l Figure 11: Circuits fabricated on on ECMM (Voltage =7 V, capacitance = 1000 pf, NaCl concentration =25 g/l of water, IEG= 1000 µm and, on LPKF Protomat 955/II (milling) Fig. 12 and Fig. 13 show a comparison of width and depth of the machined channels by ECMM and micromilling. In some regions, there is substantial difference (33%) between ECMM d work piece and micro milled work piece. Table 2: Comparisons between circuits fabricated by ECMM process and micromilling process Region Average width (µm) Average Depth (µm) By ECMM By Milling % error By ECMM By Milling % error 1 479.48 309.5 54.92 51 38.5 32.46 2 476.92 271.0 54.34 49 38 28.54 3 493.87 283.33 43.01 49 40 22.5 4 482.97 348 38.78 48 45.5 5.5 AV. 483.3 302.96 59.52 49.25 40.5 21.60 102-5
Fabrication of Complex Circuit Using Electrochemical Micromachining on Printed Circuit Board (PCB) Figure 12: Caparisons of channel width at different regions Figure 13: Caparisons of channel depth at different regions 4) B. Bhattacharyya, S. Mitra, and A. K. Boro, Electrochemical machining: new possibilities for micromachining, Robotics and Computer Integrated Manufacturing, vol. 18, pp. 283 289, 2002. 5) B. Bhattacharyya, S. Mitra, and A. K. Boro, Electrochemical machining: new possibilities for micromachining, Robotics and Computer Integrated Manufacturing, vol. 18, pp. 283 289, 2002. 6) A. S. Chauhan, Micro tool fabrication using electro chemical micromachining, Master s thesis, Indian Institute of Technology Kanpur, UP, India, 2009. 7) B. Bhattacharyya, J. Munda, Experimental investigation into electrochemical micromachining (EMM) process, Journal of Materials Processing Technology, vol. 140, pp. 287 291, 2003. 8) L. Yong, D. Zhu, Y. Zeng, Huang Shaofu, Yu Hongbing, Experimental Investigation on Complex Structures Machining by Electrochemical Micromachining Technology, Chinese Journal of Aeronautics, vol. 23, pp. 578-584, 2010. 9) M. Datta, L.T. Romankiw, Application of chemical and electrochemical micromachining in electronic industries, J. Electrochem. Soc. 136 (6) (1989) 285C 292C. 10) M. Datta and D. Harris, Electrochemical micromachining: An environmentally friendly, high speed processing technology, Electrochimica Acta., vol. 42, pp. 3007 3013, 1997. 6 Conclusions From the present work, following conclusions are made: 1. A circuit of complex structure is made by electrochemical micromachining. It is capable to successfully cut micro feature such as circle, rectangle and zigzag. 2. Minimum average channel width on complex circuit is 345 µm at 7 V, 25 g/l concentration, and 10000 pf capacitance. 7 References 1) B. Bhattacharyya and J. Munda, Experimental study on electrochemical micromachining, Journal of Materials Processing Technology, vol. 169, pp. 485 492, 2005. 2) V. K. Jain, Advanced machining processes, Electrochemical Machining (ECM), pp. 232 279. New Delhi: Allied publishers, 2002. 3) K. P. Rajurkar, D. Zhu, and B. Wei, Minimization of machining allowance in electrochemical Machining, Annals of the CIRP, vol. 47, pp. 165 168, 1998. 102-6