Preliminary Ideas: PTFE-Based Microwave Laminates and Making Prototypes

Transcription

1 Appendix I Preliminary Ideas: PTFE-Based Microwave Laminates and Making Prototypes A1.1 PTFE Laminates PTFE is a popular abbreviation representing a very useful high frequency material, whose chemical nomenclature is polytetrafluoroethylene. It shows some unique properties such as very low loss-tangent (<0.002), high chemical resistance, good tensile strength (6,240 psi), low weight, excellent thermal stability ( 55 to 260 C), thermal expansion coefficient identical to that of copper, and a high melting point (330 C). All these are suitable for using the material at microwave frequencies. PTFE sheets having a thickness varying over a wide range from to , are used as microwave substrate. These are available with 1 / 4 oz to 2oz electrodeposited copper cladding on the both sides. Some popular commercial products commonly used in laboratories and industries are provided in Table A1.1. For detailed technical and commercial information, readers can consult the following websites: Dielectric-Constant-of-RT-duroid Glass-Microfiber-PTFE.aspx 3. Ceramic-Laminates.aspx Laminates Comparison Chart.pdf Microstrip and Printed Antennas: New Trends, Techniques and Applications 2011 John Wiley & Sons, Ltd. ISBN: Edited by Debatosh Guha and Yahia M.M. Antar

2 464 Appendix I Table A1.1 Some commercial PTFE composite laminates commonly used for making prototypes (1 18 GHz) and laboratory measurements Product e r at 10 GHz Tand Comment RT/duroid 5870* Glass microfiber reinforced PTFE RT/duroid 5880* composites, developed in the 1960s. Ideal for high moisture environments as its water absorption is extremely low. RT/duroid 6006* Ceramic-PTFE composites, RT/duroid 6010* developed in late 1970s. High e r to provide option to reduce the size of the circuit elements. RT/duroid 6002* Introduced in the late 1980s RT/duroid 6202* Ideal for phased array/gps antennas. RT/duroid 6202PR* laminate Suitable for planar and nonplanar PR stands for laminate 2.98 antennas and multilayer Palanar Resistor circuits laminate / laminate 2.9 TLY** Fiberglass/PTFE resin laminate TLX** Notes: *Manufactured by Rogers Corporation ( **Manufactured by Taconic ( A1.2 Making Prototype Boards Any prototype of a printed antenna or circuit is created by etching. This means removal of selected unwanted conductor from the copper-clad microwave substrates. Two standard techniques are usually adopted: chemical etching, and mechanical etching. They are briefly described below: A1.2.1 Chemical Etching The basic principle is to cover the wanted metallic part by a chemical coating and then the unwanted part is exposed to a corrosive liquid. The liquid dissolves the unwanted copper, leaving the wanted pattern on the substrate as the prototype printed antenna or circuit element. The basic steps usually followed in an in-house process in a laboratory are described below:. Copper surface should be cleaned and ungreased.. The surface is coated with a photoresist. Photoresist is a chemical paste (synthetic polymer of methyl methacrylate or phenol formaldehyde resin) which is sensitive to ultraviolet (UV) light and reacts to it in two ways. One type of photoresist, on being exposed to UV light, gets hard and does not dissolve in the photoresist developer. This is known as negative

3 Appendix I 465 photoresist. The other type, when exposed to light, becomes soluble to the photoresist developer while the unexposed part remains insoluble. This is called positive photoresist.. A proper mask of the prototype artwork is prepared depending on the type of photoresist to be used and is carefully aligned on the photoresist-coated surface of the PTFE board.. It is exposed to UV light for a considerable time as per specification.. The mask sheet is peeled off the exposed surface.. The photoresist image is rinsed gently. This leaves a pattern of developed photoresist which may be the exact replica of the wanted microstrip element (if a negative photoresist is used) or just its negative (in terms of photographic language) (if a positive photoresist is used).. The board is dipped in the etching solution (ferric chloride with hydrochloric acid). It is carefully observed and flipped frequently. As soon as the chemical etching is completed, the board is removed from the tray.. The board is rinsed using distilled water to remove the photoresist layer and is dried for experiments. A1.2.2 Mechanical Etching This technique involves a highly precise computer-controlled small cutting machine. It uses a special milling cutter, which moves horizontally along a linear axis (say, x or y) over a circuit board and extracts copper strips. Multi-axis horizontal movements of the board help to remove unwanted copper-clad areas leaving a fine and precise prototype geometry. The printed circuits with accurate dimensions are primarily drawn using a commercial CAD tool, like autocad. This is fed to the etching machine from a computer through proper interface. All mechanical movements of the board and cutter are controlled accordingly. The instrument is quite expensive, but does not have to undertake the many intermediate time-consuming processes involved in chemical etching. Moreover, it also does not use any toxic chemicals. These factors have made mechanical etching very popular and attractive in different settings from microwave laboratory to PCB prototyping shops. Mechanical milling needs high quality cutters or bits, which are available in different configurations. The bit diameter typically varies from to and it looks like a sharp lead pencil. Figure A1.1 shows a commercially available mechanical etching machine, which can be used for prototyping printed antennas and circuits operating up to about 26 GHz. In Figure A1.1, the horizontal platform is holding a PTFE board, on which a semi-finished printed antenna is shown. The rectangular deep shaded box is attached to a cylindrical body and it moves in a left right direction. The vertical cylinder holds the cutter and moves linearly along the edge of the black shaded box. Thus a two-dimensional motion over the PCB is achieved mechanically. The typical weight of such a machine is about kilograms. The overall process is simple and vendor-specific. Once learned, it is not a hard task to implement. Figure A1.2 shows two 2.45 GHz microstrip arrays etched in the laboratory. These are as good as chemically etched prototypes. Some basic steps are noted below:. Circuit layout is done using a PCB layout package.. A post-processor generates the boundary paths to be followed by the cutter.. A PCB board, cut to the proper size, is mounted on the machine.

4 466 Appendix I Figure A1.1 Commercial PCB-making machine using the mechanical etching technique Figure A1.2 Printed prototypes etched on PTFE substrates using the machine shown in Figure A1.1 (a) sample 1; (b) sample 2. Vertical drilling, needed to insert a coaxial probe or interconnects, is done first.. The etching bit is chosen and fitted to the holder.. The cutting depth is adjusted so that just the copper foil is pilled, not the substrate. The tip angle of the cutter plays an important role and proper knowledge is required prior to using it.. The machine is allowed to operate and the process is a bit slow. It may take considerable time to finish.. The finished board is immediately ready for experiments. A1.3 Making Simple Printed Antennas or Circuits Using a Laser Printer A simple in-house technique may be tried by the students for their laboratory experiments. Its basic principle and process are described below:

5 Appendix I 467. A laser printer does not use any ink, we know it is simply a plastic toner, that is, a type of black powder. This plastic toner is resistant to etching solutions. Therefore, if we can print a circuit on a copper-clad substrate, the rest follows the same process as chemical etching to make a prototype board.. The question is: How to take a printout on copper board? The intelligent technique of tonertransfer is to be employed. On applying heat, laser toner melts to a sticky paste, like most plastics. So a normal print-out taken on paper may be placed with the front face-down on the copper board. Application of mild heat and uniform pressure on the paper using a household iron will make the toner melt to be glued to the copper surface.. Now if the paper is removed safely, the printed layout is transferred onto the PCB. This process is easy if a clever technique is applied. A glossy photo paper or magazine paper is an ideal choice. The toner hardly penetrates the glossy layer and thus the toner transfer is efficient. Also the glossy coatings dissolve in water easily and thus water helps to remove the paper from the PCB surface.. Once the toner-transfer is done properly, we get the circuit layout on the board and the exposed metals are removed to get the final circuit.. Like chemical etching, ferric chloride solution may safely be used to dissolve the exposed copper.. The board is rinsed in water and finally the toner is removed using a thinner. Nail polish remover serves the purpose very well.. The circuit is now ready for measurements. Readers may consult the following websites for detailed information: