Mounting Hole & Pattern Specifications

Owned by Daniel Puratich
Last updated: 2024-09-28 by Ryan Scomazzon
4 min read

Context

This page was created by Daniel Puratich to standardize how custom hardware is designed with respect to mounting hole dimensions and patterns during the 2023 competition cycle.

This document is valid for at least 2023 System Architecture & 2024 System Architecture, and directly refers to Connector Standards for implementation. This standard was initially discussed as a part of 2023-02-11 - EE Hardware Mounting Hole Specification in the RFC Forum as a collaboration between WARG’s electrical and mechanical sub-teams.

This document is being revised over time. When creating new board designs, check the recommended specifications below before beginning. Both mounting hole size and spacing are standardized and must be followed for WARG hardware.

Common Mounting Holes Sizes

Going forward, general custom EE flight PCBAs will follow the mechanical mounting hole and pattern specification listed below. These mounting holes can be found in our Altium library.

Larger is welcome if it is (1) designed with collaboration with the mechanical team and (2) generally intended for a specific application. You can do anything as long as it is well backed up with engineering information. For example, LED Board Rev 3 uses M4 mounting holes and is intended to share bolts with a mechanical system.

Sizes

These mounting holes can be found in our Altium library.

  • M3

    • 3.40 mm hole diameter

    • 0.10 mm hole diameter tolerance

    • 6.00 mm annular ring diameter

    • Advised on any PCBA exceeding a board area of 900 mm^2

      • a vast majority of WARG boards fall in this category, if you aren’t sure, use these M3 holes.

  • M2.5

    • Not recommended

  • M2

    • 2.40 mm hole diameter

    • 0.10 mm hole diameter tolerance

    • 4.20 mm annular ring diameter

  • No Mounting Holes

    • Do this for any smaller boards because running holes smaller than M2 is not really practical as per past conversation .

    • Used on any PCBA with area less than 750mm^2

    • Mounting will be done with double sided sticky tape or Velcro and the board should be designed to facilitate this.

Mounting Hole Parameters

All Mounting Holes follow the following parameters:

  • Size

  • Electrically/Galvanically not connected to the PCBA GND Plane

  • Eight vias should be placed evenly in the annular ring around the drill hole

    • Via’s should be plated drills in whichever drill size is used on the board

    • This is done for mechanical stiffness.

    • Altium doesn’t like this, it thinks the vias are shorted to to mounting holes (their net is "No Net") and it keeps complaining about it in DRC. I have to go into design rules → unrouted nets and then set the custom query to “InAnyNet” . Because “No Net” is not actually a net in Altium, use “InAnyNet” to return objects that are actually assigned to a proper net.

    • via dimensions should be reasonable, up to designer, preferably use the same via size as you use elsewhere on the board.

  • Proper clearance from components

    • Electrical component soldering pads should be at least 1mm away from the annular ring

    • This is done to avoid damaging the components when mechanically securing the board.

      • Board may flex under mechanical load and we do not want to crack components (especially ceramics like big capacitors).

Mounting Hole Pattern

The mounting hole pattern will be 30.5x30.5mm for most PCBAs. When measuring the centre-to-centre distance of your mounting holes, you should get 30.5mm for both the X and Y axes. Boards of this size will use M3 holes specified above.

This means that if you are building a board, try to fit it in this pattern. If it cannot fit, then expand the board size while keeping the holes within a grid that falls within 30.5x30.5mm pattern. For example, a larger board could have a pattern of 30.5x61mm.

This is the general recommendation for stand alone PCBAs. For hats or boards intended for specific mechanical requirements you are encouraged to make educated engineering decisions about how you want to do hole patterns. An example is RPI Interface Rev B which uses its own mounting pattern to fit the Raspberry Pi.

Mounting Hole Pattern Philosophy

This 30.5x30.5mm pattern is commonly used in COTS world for 5” propeller quadcopters so we’re picking that up on WARG as well. Larger quads basically have no patterns because it’s assumed a fully custom application specific airframe.

There are other standards for quads that are smaller than 5” props including (but not limited to) 25.5x25.5mm, but at our class of airframe we’ve never built any aircraft small enough to warrant this pattern. Most our aircraft are quite large. Generally 5” quads are designed around 30.5x30.5mm pattern with M3 clearance holes while smaller drones with say 3” props are built around smaller patterns with M2 and M2.5 holes.

In the past we made a mistake and actually used 30x30mm. See conversation around mounting hole mistake discussion (it’s 30.5mm bc that’s 1.2” so yea …). In theory 30x30mm will work its just really tight.

Historical WARG Hardware Patterns

  • Project Pegasus uses a 30x30mm mounting pattern in it’s frame.

    • This holds for Pegasus 1 and Pegasus 2

  • Servo module 12S first rev uses 30x30 mounting pattern.

  • ZP3 32.5x32.5

  • TODO: add more

COTS Common Patterns

  • Pixhawk is not 30.5x30.5mm

  • TODO: add more COTS items for reference, some that meet the specs, some that dont, links appreciated

Mounting Hole Standard Specifications Exceptions

Exceptions can be made to this in some cases. This is meant to be the general rule. If a WARG board is violating these rules please ensure proper engineering justification is given!

Common Dimensions

Try to fit board dimensions into these common configurations

  • Standard 30.5x30.5mm M3

    • TODO give dimms & link examples

  • Standard 30.5x61mm M3

    • TODO give dimms & link examples

  • Standard 61x61mm M3

    • TODO give dimms & link examples