EAGLE: Using the computer for circuit layout

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Rhoda Hutchinson
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1 EAGLE: Using the computer for circuit layout Introduction You ve probably noticed that these lab exercises contain some nice circuit diagrams. They were drawn using a program called EAGLE: Easily Applicable Graphical Layout Editor. EAGLE lets you create circuit diagrams, obviously, but it can also help design circuit boards, and that s what we ll be doing in this exercise. EAGLE is not the easiest program in the world to use; but it s not bad once you get used to it. There are three main editing windows in EAGLE: schematic, board, and library. They are used for editing the schematic (the circuit diagram), the board (the physical hardware), and the libraries of parts that go into the creation. We ll be using only the first two tools for this introductory exercise: a library of parts useful for this lab has been created for you already. Creating the schematic Here are the most commonly-used tools on the schematic toolbar. Display: select which layers to display. Move: Move a part, wire, or corner of a wire. Rotate: Rotate a part. Delete: Delete a part or wire segment. Add: Add a part from a library. Name: Change the name of an object. Value: Change the value of an object, such as a resistor or capacitor. Smash: Disconnect part and labels, allowing you to move or delete labels and values. Wire: Draw wires.

2 Split: Bend a wire. Junction: Add a junction to wires. Toggle between schematic and board. The exact appearance of those icons will depend on the version of EAGLE you are running: they change their icons more often than I want to change these instructions so use your intelligence and deal with it.. Start EAGLE, and open a new schematic. Tell EAGLE to use the preprepared library for this experiment by selecting Use under the Library menu, then navigating to /export/classes/phys27/eagle/ 27.lbr. 2. Build the following circuit: Red R Grn R2 U$ +VS VO 2 GND MCP9700 Blue R 2 4 S 8 4 U$ VCC GND (ADC0)PB5 (ADC2)PB4 (ADC)PB (ADC)PB2 (OCP)PB (AREF)PB TINY25/45/85-DIL08 2

3 Place each component using the Add tool. You may search for components using the dialog box that comes with the Add tool; this makes it easier to find things like resistor. The rest of the components are in the 27 library. When choosing a component, make sure you get the right package. For example, the TINY25/45/85 is an 8-pin microcontroller (more on these later in the course) that comes in either a 20SU package (surface mount) or a DIL-08 package (8-pin chip). If you select the wrong package, it complicates the board layout later. For the resistors, use the 0207/0 package: 2mm 7mm, 0mm hole spacing. The battery is a 202-H (CR-202 battery, Horizontal). The rest of the parts include an MCP9700 temperature sensor in TO-92 package, an Omron pushbutton switch, and an RGB (Red/Green/Blue) LED. The resistors, if you wish to indicate their values, should be 500Ω.. Once the components are on the schematic, add the wires and junctions. (The junctions are technically optional, but they make it easier for humans to read the schematic.) Make sure the wires are actually attached to the components at the right place: you can check this by wiggling the components with the move tool and seeing if the wires move with them. Save your schematic when completed. Designing the board In addition to most of the tools on the schematic window, the board window has some extra tools. Route: Manually route a trace on the board. Ripup: Remove a routed trace, change it back to an airwire. Ratsnest: Redraw all airwires to be as short as possible. Autoroute: Change airwires to traces in the best way the computer can figure out.. When you first go to a new board, all the components you chose will be to the left of the board in a hodgepodge pile with airwires connecting

4 them. Use the move and rotate tools to arrange the components on the board. Components should be laid out primarily so as to minimize the complexity of the paths between attached points, and secondarily to put all components in a smallish area in the bottom left corner of the board. It may be helpful to occasionally hit the ratsnest button so you can better see how you re doing. 2. Once you have a reasonable component layout, hit the autoroute button. In the ensuing dialog box, set top to N/A and bottom to * : this will tell the autorouter to keep the top of the board empty and put wires any direction on the bottom. Click Ok to continue: the autorouter will do its best.. You ll immediately realize that the autorouter s best is not very good. Look for pairs of wires that, if switched, would decrease the complexity of the circuit. Use the ripup tool to rip those wires up, then either re-autoroute or manually route the wires. You may find that rotating or moving parts helps, too. Wires must not overlap each other, or overlap soldering pads. Components must not overlap. Wires should be at least one wire width away from each other, and generally two 45 corners is better than one 90 corner. Acute angles on traces are undesirable: the corrosive etchant tends to get stuck in the angle and gradually eats the junction. 4. Eventually (And don t kill too much time on this) you ll get a board that will work. Move the boundaries of the board to fit neatly around your circuit, which should be no more than inches in size. Add your name on layer bottom (it ll be backwards) and save your work. What to turn in for this part: Print three things:. Your schematic, at an appropriate scale factor. 2. The board, showing the default layers (top, bottom, pads, vias, unrouted, dimension, t/b place, origins, names, and values). Print at a scale factor of 2.. The board, showing only layers bottom, pads, vias, and dimension. Print in black, scale factor. 4

5 Creating the board Creating the board is a multi-step process. We first print the board design with a laser printer on a special transfer paper. Next, we use heat and pressure to transfer the toner from the transfer paper to a copper-clad board. Once on the board, the toner forms an acid-resistant mask. The next step is to etch the unprotected copper off the board, leaving only the desired electrical traces. Finally, we wash the toner off the traces, and then drill and solder our board.. print your board at a scale factor of, showing only the layers in the circuit layout that should be reproduced in copper on the bottom of the board. Give some thought as to whether this should be printed mirrored or not, remembering that it s the toner you are transferring to the copper-clad board! Print on a quarter-sheet of blue transfer paper, on the coated side. This paper is expensive, so feel free to testprint on a quarter-sheet or two of scratch paper at first to ensure that you will print where you want to print! 2. Clean your board. It must be completely free of fingerprints, smudges, etc nothing but copper!. Cut out your design, leaving 5 0mm clearance around the traces. Tape this design (toner side down) on the clean copper side of the board. For best results, just tape one edge, so that the rest of the paper can smooth out over the copper without wrinkling. Run the board/paper through the laminator 5 0 times, going different directions and with different sides up. When the board is uncomfortable to handle, place it in a tray of water to remove the paper. 4. Carefully inspect the toner design on the copper. Is it oriented correctly? If not, deduct one point from your lab grade, then go back to step and print mirrored. Or Unmirrored. Whichever you didn t do before! Is the pattern mostly intact? Small breaks in traces or other minor defects can be touched up with a fine-tip Sharpie pen. Large defects or missing traces will require that you go back to step one again. 5. (Optional) Cut a piece of green transfer-protection film large enough to cover your circuit, then laminate this film to your toner. (Dull side should be on the toner.) After two passes through the laminator, let 5

6 the board cool gently, then peel the film off. The green layer acts to further protect the copper during the etch process, but it s not strictly necessary. 6. Once you re satisfied with the quality of your print job, put the board in a ziplock bag with about 50ml of etchant. Seal the bag, and put it in a second bag. Seal the second bag also, then gently squish the liquid around over the board until etching looks complete. Etch for another couple minutes, then pour the used etchant into the disposal container and rinse the board under copious quantities of water. Note! The FeCl etchant will stain everything it touches, including clothing, skin, and stainless steel. Use caution, and wear disposable nitrile gloves. 7. Scrub the toner off the board, and take a minute or two to admire your handiwork. Next, drill the pads on the board, using the 0.75mm and.0mm carbide drill bits provided. (Most of the components will fit fine in 0.75mm holes, but the battery and switch will require.0mm.) 8. Finally... solder the provided components into the board. Be particularly careful with the orientation of the parts: it s quite possible to install everything backwards (other than the resistors) and it won t work if anything is backwards. If done right, you will have a chromatic theromometer. Enjoy! 6

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