Best positioning practices

Standard PCB templates and project cases

Best positioning practices

Postby ian » Wed Jan 18, 2012 1:36 pm

This will be a list of best placement practices (USB, sockets, LEDs) as discussed by DSM in the other thread. This is a reminder to me to assemble this data and use it on all the updates for the SOB case.
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Re: Best positioning practices

Postby dsm » Thu Jan 19, 2012 3:04 pm

minor comments

I'm not sure I have many component placement recommendations other than
  • Consider the effect that the placement of parts has on the external packaging (symmetry, board size, mounting hole locations, connector access, switch access, visibility of LEDs, heights of components, keepout areas, etc.)
  • Once you have a component placement, lock it down. Don't change the component placement for trivial reasons.
Thanks for your time.

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Re: Best positioning practices

Postby ian » Fri Jan 20, 2012 4:06 am

Thank you, Do you have a preferred grid size?
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Re: Best positioning practices

Postby dsm » Fri Jan 20, 2012 4:57 am

placement and wiring grids

Since your boards are metric, I suggest using a 0.5 mm placement grid.
The wiring grid should be somewhat smaller and also metric since most component surface-mount packages are metric.
Look at the surface-mount packages you're using for guidance about the wiring grid.

The issue is complicated by the use of 0.1" grid through-hole connectors (e.g. Bus Pirate main IO connector).
I think you're doing the right thing to place the center of these through-hole connectors on your metric placement grid
and let the actual pins locations show up where they may.

Thanks for your time.

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Last edited by dsm on Fri Jan 20, 2012 6:37 am, edited 1 time in total.
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Re: Best positioning practices

Postby ian » Fri Jan 20, 2012 6:28 am

Yes, thank you, I did mean placement grid. We will stick with that, and try to keep centers on logical values too.
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Re: Best positioning practices

Postby dsm » Thu Jan 26, 2012 4:34 am

keepout comments

After exchanging comments with arakis about the USBIRtoy design, I realize that my comments about keepout specifications were woefully inadequate. Good layout people always find ways around loose and inadequate specifications.

DFM rules often provide for various types of packaging [1] keepouts, for example...
    height [2] ~ No components taller than xxx allowed, but shorter components allowed.
    component [2] ~ No components [3] allowed, but traces and vias allowed.
    trace [2] ~ No components or traces allowed.
    via ~ No components, traces, or vias allowed.
    clear ~ Nothing allowed.
CadSoft Eagle provides
    Layer 39 tKeepout ~ Restricted areas for components (top side)
    Layer 40 bKeepout ~ Restricted areas for components (bottom side)
    Layer 41 tRestrict ~ Restricted areas for copper (top side)
    Layer 42 bRestrict ~ Restricted areas for copper (bottom side)
    Layer 43 vRestrict ~ Restricted areas for vias
In the case of the new Dangerous Prototypes boards, I used the term "keepout" for the areas around the mounting holes when I really meant to exclude everything. This would allow the mounting holes to be drilled out to support alternative mounting hardware schemes (SAE hardware instead of metric, swaged standoffs, larger metric screws, etc.).

For example, in the case of the Bus Pirate v3.5e and Bus Pirate v4c layouts, this means that the top and bottom GND fills should be moved away from the mounting holes (by tKeepout and bKeepout amount).

The rationale for this change is that people may want to use a GND fill for one side of a layout and a VDD fill for the other side of a layout using the new Dangerous Prototypes boards (or possibly inner layers of a multi-layer design). Restricting everything near the mounting holes would allow the mounting holes to be modified as needed.



I believe that most components should be at least 0.2 mm from the edge of the board, unless the component interacts with the enclosure (like the USB connector). Letting components [3] accidently hang off the edge of the board should be avoided.
If you want to allow slide-in enclosure designs, you should specify a 1.7 mm component keepout (Layer 39 tKeepout and
Layer 40 bKeepout) along the long sides of the board.

Note that this suggestion had to be adjusted. Change 1.2 mm to 1.7 mm. For more information, see (link).

Thanks for your time.

dsm

[1] And also EMC, voltage, thermal, etc.
[2] Top and bottom side versions.
[3] Either Load or NoLoad.
Last edited by dsm on Mon Feb 20, 2012 6:10 pm, edited 3 times in total.
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Re: Best positioning practices

Postby ian » Fri Jan 27, 2012 3:45 am

The rationale for this change is that people may want to use a GND fill for one side of a layout and a VDD fill for the other side of a layout using the new Dangerous Prototypes boards (or possibly inner layers of a multi-layer design). Restricting everything near the mounting holes would allow the mounting holes to be modified as needed.


Thanks DSM. This is a great point too.
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Re: Best positioning practices

Postby dsm » Thu Jun 28, 2012 11:15 am

missing edge copper keepout

ian,

I recently had some boards fabricated that follow the new Dangerous Prototypes Sick of Beige layout guidelines. The resulting boards were beautiful except for one issue. In some cases, one could see a thin trace of bare copper at the edge of the board (both sides) where the ground plane fill extended to the edge of the board and the board edge was routed accurately. This allows for corrosion of the bare copper and is not good practice. As a result, I think we should have an additional 0.5 mm copper keepout, like the area around the corner mounting holes, but along all four edges of board implemented via
    Layer 41 tRestrict ~ Restricted areas for copper (top side)
    Layer 42 bRestrict ~ Restricted areas for copper (bottom side)
Note that this change doesn't affect the solder mask which completely covers the board. Although the current BusPirate v3.6 and USBIRtoy v3 layouts will eventually have to be updated, since Seeed Studio often fabricates their boards slightly large most customers of these boards will not be immediately affected. I believe, however, that the Sick of Beige board layout guidelines are worth updating immediately.

Thanks for your time.

dsm
Last edited by dsm on Wed Jul 04, 2012 2:11 pm, edited 3 times in total.
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Re: Best positioning practices

Postby vimark » Tue Jul 03, 2012 1:12 am

Thanks dsm, I put these updates on my que.
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