Dangerous Prototypes best practices for PCB and schematic design

in how-to, tutorials by DP | 10 comments

For future projects we recently updated our best practices for designing schematics and PCBs. We compiled them to a wiki page, as well as some tips for using Cadsoft Eagle. Check them out below.

Feedback and suggestions are welcome.

When designing the PCB

  • Put all components that could interact with the casing on 0.5mm grid, e.g. USB connector, headers, LEDs, sensors, switches.
  • It is wise to set the grid to mm first when placing the above components and later set back to mil for the parts that doesn’t need to “interact” on the enclosure. e.g. ICs and passive components

Schematic components

Common sizes and width:

  • Main border size: 20mil, if you want to group each circuit.
  • Partition inside the main boarder: 6mil
  • Text size: 70mil part name and value
  • Text ratio: 8% part name and value
  • Label text size: 100mil for labels, this will be the name of that group/bordered circuit

Board components

Common sizes and width:

  • Signal: 12mil
  • Power: 16, 24mil
  • Via Signal: drill 20mil
  • Via Power: drill 35mil

Parts creation – Symbols

  • Name text size: 70mil
  • Name text ratio: 8%
  • Name text size: 70mil
  • Name text ratio: 8%

Parts creation – Package

  • Name text size: 50mil, this is an initial size, we can smash and resize this one when we want to during board design process.
  • Name text ratio: 10%
  • Value: 50mil (Do we still need a value text on the footprint? If not we can disregard this field.
  • Art Lines width: 16mil

Eagle Tips

These are pretty handy when you want to be precise on the location of your components

Command line definitions:

  • > right mouse button click
  • A Alt key
  • C Ctrl key
  • P Polar coordinates (relative to the mark, x = radius, y = angle in degrees, counterclockwise)
  • R Relative coordinates (relative to the mark)
  • S Shift key
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Comments

  1. arhi says:

    mm grid – why? if you design box using imperial units what’s the reasoning for mm’s ? G-Code for routing / cutting / printing boxes accepts both inches and mm’s .. I do use mm grid to setup all “interfacing” objects ’cause I do everything in mm’s (design, cut ..) but why’s that “best practice” for someone in US ?

    • Squonk says:

      From 1974 – 1991 Inch units were used for PCB layout, just because all chips were TH and manufactured in the US using a 0.1″ pitch, and anyway it was difficult to get trace smaller than that.

      From 1991 – 2001 Mil units were used, as PCBs got finer and finer traces, and people started to route signals between 0.1″ pitch pads.

      But since 2001, we should all use millimeter units, since now SMT components are a common place, and all of these are using metric units (both package and pitch).

      So the only reasons left to use Imperial units are 1) habits 2) PCB size imposed by manufacturers 3) case/screw/nuts/bolts size as you mentioned above.

      I suggest to check this very informative blog (19 part !) about PCB design, by one of the expert at Mentor (also member of some IPC committees):
      http://blogs.mentor.com/tom-hausherr/blog/2010/07/08/metric-vs-imperial-measurement-systems/

      Beside this Imperial/metric open debate, this series of articles is a must-read to learn the schematic/layout best practices.

      • arhi says:

        The layout of the components should IMO be related only to the “case manufacturing” limitations irrelevant to pitch of the components (metric or imperial). If you are making your case in US you will be asked to provide data in imperial units, if you are making it in Europe they will “don’t care” and take both metric and imperial… As I said, I use metric always ’cause it’s easier (and no sane person want to think in 1/12th and similar units) but I don’t see how a guy who’s going to cnc his box in US want to use metric

      • Squonk says:

        I don’t agree: if you are not using the same unit as the most common pitch, you will have to adapt constantly your traces to each component pin, wasting a lot of space in “round-offs” and creating unnecessary turns.

        As opposed to this, if you use the some unit as the majority of your components, you will have straight traces (see http://blogs.mentor.com/tom-hausherr/blog/2010/10/08/pcb-design-perfection-starts-in-the-cad-library-part-4/) and only one single round-off for the external board dimensions/holes/connectors/case…

      • arhi says:

        It really very much depends on your PCB application. I never experienced any of those issues with neither altium nor ares

    • AndThen says:

      Some producers use mils then for no reason require border setback in mm on the requirements. It’s some sneaking tactic to get us to us the metric system..

  2. Dan says:

    If you are using a weird part with its own layout, like a SIM card holder from molex, you want to put the part number in the part description! That way you can find it again when your initial stock runs out.

    Did that just yesterday..!

  3. R3tikus says:

    Thankss very usefull!!

  4. rsdio says:

    Is there a snap-to-grid command for Eagle? I usually edit the position to zero out any fraction, then all movements land on the grid. But, if you place a part first on mil spacing, then switch to mm grid, it will not always land on the mm grid (unless there is something I’ve missed). The nice thing about the behavior that I see is that if I have to move all parts 5.345″ on a board, then I can set the grid to 5.345″ and move all the parts by that exact amount, keeping their relative spacing.

    If your parts are all SOIC, then mils make a better grid. My designs typically only have one case-relative component – the USB jack. Everything else is unlimited.

    Keep in mind that the Eagle autorouter (if you license it), works on a grid. The coarser this grid, the faster the autorouter runs. If you can stick to SOIC parts and place on a 100 mil grid, then you can route at 50 mil and still run traces between pins, and it will solve your layout super fast.

    • vimark says:

      actually yes! there’s a snap to grid command when moving objects. you just hit the Ctrl on your keyboard when clicking that object!

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