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Main / Elec
Glossary
Development CycleTypically use 3 phases of HW development:
With some "revisions on a theme" projects that were iterations on a previous design, we may go straight to production because the HW was mature. The CAD process is complex, time consuming, tedious and you'd often rather pay a specialist that's not necessarily an engineer a lower rate to do it. Often the design engineer owns the schematic, and then hands it off to a layout expert that's intimately familiar with the tools. May be comfortable passing ownership of the schematic as well. Consider a two-step change process where the first file has all the stuff to be removed deleted, and the second file has all the new stuff to be added in, so you don't have a single step where double changes are happening and confusion can creep in. Board Design Best PracticesTip: I look at a lot of eval boards from various vendors. They mostly have one nasty thing in common: the lack of a big ground pin. It's common to connect an instrument to these boards, like a scope, but where will you clip the probe's ground lead? Some have ground available in a 0.1 x 0.1 header, which is great for a logic analyzer connection, but less so for an alligator clip. When designing a board provide at least a nice through-hole via where one can solder a wire (I use an RC07 resistor) to provide a nice big ground. - Ganssle EDA ToolsKiCAD is an open source tool heavily supported by CERN. Eagle is a simple, limited-use tool in its free form now developed by Autodesk and now has ransomware attributes. LinksIn 2020, I finally found the tutorial for ver 5.1: https://forum.kicad.info/t/tutorial-introduction-to-pcb-design-with-kicad-version-5-1-getting-started/20600/3 DigiKey's v4.0.7 YT Tutorial: https://www.youtube.com/watch?v=Ghv0bGiZFL8 DigiKey KiCAD v4.0.7 libraries snapshot: https://codeload.github.com/Digi-Key/digikey-kicad-library/zip/1fcb4bbab6e0cfa7658bcccbf4e7f6c643c3b9e5 (note that the current github repo is for newer versions of KiCAD only) KiCAD 4.0.7 Process OutlineDesign circuit -> Create symbols (library editor) -> Schematic capture (eeschema) -> Create footprints (footprint editor) -> Generate netlist (cvpcb) -> Board layout (pcbnew) -> Production (GerbView) -> Generate BoM Using KiCADSchematic shortcut keys: Layout shortcut keys that differ from above: 50 mils is recommended grid pitch Design RulesDefine the design rules, manufacturing and electrical. Usually this is done before the layout is created. In Pcbnew, use File->Board Setup. What is "restring" measure? This is a component of the through-hole diameter, "minimum copper restring". The via diameter is computed by: ```via drill size + 2 * restring``` Name Equivalency Chart
minimum mask land width = solder mask minimum width Basic WorkflowThe schematic is done with the EESchema tool.
The layout is done next with the Pcbnew tool.
Prepare the Gerber files In Pcbnew, use File->Plot to use the Plot action and then generate drill files. Labels
Flags are a way to connect things together without needing to show wires specifically. Nodes sharing flag names are automatically connected. It's not totally clear how flags relate to labels. The down-arrow GND symbol is an example of a flag, or VCC power. Footprint
LayoutGrid origin (0,0) is in top left corner. Use Edge.Cuts layer to define board boundary. Right-click and select Flip to put parts on the other side of the board. The space bar will set an invisible temporary marker by zeroing out the dx and dy values shown on the bottom margin. You can use "Add Graphic Lines" to create a PCB border. For a single layer two-sided board, make sure you are only drawing traces on the top or bottom (F.Cu, B.Cu) of the board. When putting down traces, a default of 10mils width is typically fine. For power circuits, you may need more. Refer to the circuit simulation waveform peak current. You can go to a website trace width calculator and enter the max current, using the typical manufacturing standard of trace thickness of 1oz/SF, to find out the minimum width needed. This is about current, not voltage. Voltage is considered for distance between traces for the concern of arcing. Typically you don't need to worry about higher currents until you get to hundreds of mA, approaching that 1 amp territory. For simple low power projects, you may go up to a 30 mil width for the power circuit only. A copper pour is an area of the PCB that's filled with copper. We typically connect it to a common rail like ground or a DC voltage. In high frequency circuits, this area of shared voltage potential can help reduce radio emissions from neighboring parts of the circuit. It acts as a shield, lowers impedance for return currents, and minimizes signal loops, reducing unwanted noise. Also heat dissipation. When you add text to the bottom layer, remember that you need to mirror it for it to appear correctly. ManufacturingSome houses like OSH Park will let you send them your KiCAD project files, but others require creation of Gerber files. You'll have a Gerber for each layer of your board, including top copper, solder mask, silkscreen, etc. The file with drill locations isn't really Gerber file format but is normally bundled with the Gerber files anyway. Do this step with the KiCAD Plot operation. Board houses might provide detailed instructions on generating and providing needed files, such as this for example: Board Manufacture with OSH Park When plotting, it may be recommended by the board house to use Protel filename extensions. A basic board will likely need the front and back copper layers, front and back silkscreen layers, front and back mask, and edge cuts. Don't forget to generate drill files as well. Note that if using GerbView to inspect the layers, the order in which they are loaded (i.e. opened) matters for how they are drawn. When inspection is complete, zip up the entire Gerber file folder. AssemblyYou can generate a CSV file BoM from the Pcbnew tool, or use a script (like bom2grouped_csv.xsl) generation option from the EEschema schematic tool. Note that in the latter case at least, the file generated does NOT have a .csv extension, it is simply called "project_name"). Open the CSV in spreadsheet/calc tool and save as a spreadsheet file instead of CSV. Edit the columns and rows as needed, including links to ordering landing sites. Soldering suggestions: 700F temp, lead-free solder with water-soluble flux core. Start soldering in the middle of the board, with the shorter leg components, moving out and to longer lead components. Solder the leads at exit point, like the back side of the board, and ensure iron tip is touching both the copper ring and the lead. Good idea to clean an assembled board with alcohol and a brush, as the flux could be conductive. Files
Component NamingThe IPC is the organization the governs naming standards for soldering, cables, etc. TroubleshootingIf the pcbnew backdrop is all white with no parts shown, try going to View menu and changing to Switch Canvas to OpenGL. Test Point OptionsTODO - place links here Board ManufacturingThe colors of the PCB are known as solder mask colors. Always send back and front mask layers even for a one-layer board, so manufacturer doesn't get confused. The inner layers are numbered, but the top and bottom aren't. For example, a four-layer board would be a sandwich like:
ConnectorsSMA (SubMiniature version A) connectors are semi-precision coaxial RF connectors developed in the 1960s as a minimal connector interface for coaxial cable with a screw-type coupling mechanism. The connector has a 50 Ω impedance. PartsThe longer lead of an LED is positive, the anode. The shorter is the cathode. |