Context

This page was created by Daniel Puratich to standardize how custom hardware is designed with respect to mounting hole dimensions and mounting patterns during the 2023 competition cycle and is being improved over time. This document is valid for at least 2023 System Architecture & 2024 System Architecture . This document directly refers to Connector Standards as well for implementation. This standard was initially discussed as a part of 2023-02-11 - EE Hardware Mounting Hole Specification in RFC Forum as a collaboration between WARG’s electrical and mechanical subteams.

Mounting Holes Sizes

For general custom EE flight PCBAs going forward we will follow this mechanical mounting hole specification:

• “Large” Boards:

  • Preferred dimensions for PCBAs

  • M3

  • Used on any PCBA with area exceeding 2500 mm^2

  • 3.40 mm hole diameter

  • 0.10 mm hole diameter tolerance

  • 6.00 mm annular ring diameter

• “Medium” Boards:

  • M2

  • Used on any PCBA with area exceeding 750mm^2 and less than 2500mm^2

  • 2.40 mm hole diameter

  • 0.10 mm hole diameter tolerance

  • 4.20 mm annular ring diameter

• “Small” Boards:

  • No Mounting Holes

    • This is because Running holes smaller than M2 is not really practical as per past conversation

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

  • No mounting holes will be present on board of this size.

  • Mounting will be done with double sided sticky tape or Velcro

Mounting Hole Parameters

All Mounting Holes follow the following parameters:

• electrically Electrically/Galvanically not connected to the PCBA GND Plane

  • This is because we want our potentially conductive drone frame to be at floating potential in accordance with General System Level Electrical Placement & Routing Guidelines

  • Can be connected to an CGND/ESDGND for ESD purposes though if proper analysis is done.

• 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.

• 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 Patterns

Mounting Hole pattern will be: 30.5x30.5mm

This commonly used in COTS world so we’re picking that up on WARG as well. 25.5x25.5mm is also standard for smaller drones, but at our class of airframe we’ve never built any aircraft small enough to warrant this pattern.

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

Historical

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

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

• ZP3 uses something that’s not 30x30mm (someone plz add in here).

COTS

• Pixhawk is not 30.5x30.5mm

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!

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