============================== Printed Circuit Board Design ============================== :Author: Ni This document should gather any useful information for PCB [1]_ design at APBTeam. .. [1] Printed Circuit Board Standards ========= Software -------- The software used currently for any PCB is Eagle from CadSoft. CadSoft distributes a freeware version running under Linux or MS Windows for non-profit usage, limited by the PCB size and the number of usable layers. In the future, we may switch to a free software solution: KiCad. But Eagle offers several advantages: all previously designed libraries and boards were using Eagle and Olimex takes directly Eagle files. Manufacturer ------------ Our boards should be suitable to be manufactured by Olimex, a low-cost prototype board manufacturer. They make double sided Europe size boards for about 30 euros, with solder mask, metallised holes and one silk screen. PCB Size -------- Most boards are a division of a Europe format board. Europe is 10x16cm, 1/2 Europe is 8x10cm and 1/4 Europe is 5x8cm. Actually, a margin must be subtracted for panelization. Some components have a standard position. This include mounting holes (MOUNT-HOLE3.3 in the holes library) and side connectors. Exact dimensions and positions: - 1/4 Europe: - board dimensions: (1.900 3.100) - mounting holes: (0.175 0.175) and (0.175 2.925) - 1/2 Europe: - board dimensions: (3.900 3.100) - mounting holes: (0.175 0.175), (0.175 2.925), (3.725 2.925), and (0.175 2.925). - 1/8 Europe: - board dimensions: (1.900 1.500) - mounting holes: (0.175 0.175) and (TBD) - smaller boards: - minimum boad dimensions: (0.800 0.800) - no standard mounting holes position. Theses dimensions are designed with Olimex design rules in mind. Connectors ---------- Here is the list of used connectors: - HE10 for signals, usually 10 pins - HE10, 6 pins for I2C - pin heads for servomotors - HE14 (TBC) for miscellaneous sensors and low pin number signals - HE14 (TBC) for digital board power - Phoenix 381 connectors for power connections - Micromatch SMD connectors for small size connections As a rule of thumb, use the pin number 1 for ground and pin number 2 for positive voltage. Check List ========== For board manufactured at Olimex use their DRC file for auto-routing and Design Rules Check! As the same errors occurs again and again, here is a list of items you should check when you design a PCB: Power connections Are all the components powered? Is there a power connector? Decoupling Use decoupling capacitors on the power and near each components. Capacitors should be the nearest possible of the components. Do not put to much decoupling or the board will drain to much current when switched on. Text size Check text size in order to be able to read it. For Olimex: vectorial font, 70 mils, 15%. Text on each layer There should be text on each circuit layer (top and bottom). This makes it easy to known if the layer is in the right direction. This is also a Olimex requirement. Board size Has the board the right size? Is there extra margin for panelization? Mounting holes Check holes types and positions. Connectors Are the connector near enough of the board edge? Some connectors, like Phoenix ones, use space lower than the board, if the connector is to far from the edge, it will not be possible to plug it. Are they at the right position ? Track size Use the largest possible tracks. Use larger tracks for power. For high power tracks, the solder mask can be removed on a particular region. Component side Are the components on the right side? This is particularly hard to see for through hole mounted components. Component placement Check than no component override each other. Check that it is possible to solder them without a master level of dexterity. For board without metallised holes, use restrict layer to forbid connection where soldering would be very hard (for example, it is not a nice thing to try to solder a track going under a HE10 connector). Always place component on the grid. Logistic Check that all components are available with the right size and package. This is particularly true for capacitors. Serial ports Double check pinout! Should TX and RX be inverted? This one is for you Pierre! Protection Use pull-up or down resistors or shorts-protection resistors. Optocouplers Use them to separate command and power. ERC and DRC Use Eagle tools to check the board. For boards manufactured by Olimex, use the file they provide for DRC. Drills Limit the use of different drill size and use standard drill size. This increase the board cost. Be careful, the metalization will reduce the hole size! Routing Prefer auto-routing as this is easier to make modifications. Silk screen Print useful information on it. Do not forget to run the Olimex provided script to fix it right before sending file to them, but do not save the result in the repository as this make editing harder.