Circuit Board Mounts Design Decisions

Owned by Johnson Duong
Last updated: Oct 16, 2020
4 min read

This page will explain all the design decisions that went into creating the circuit board mounts for MSXIV.

What Are Circuit Board Mounts?

The circuit board mounts are exactly what their name suggests - mounts that are meant to secure PCBs to the car.

Here’s an example of a mount designed to hold the Power Selection Board:

And this is what it looks like with the Power Selection Board mounted to it:

We will use this as an example to explain the process of designing all the board mounts for MSXIV which include the: Pedal Board, DC-DC Converter, Precharge (MCI), Power Distribution, Power Selection, Solar Sense, BMS Carrier, Center Console, Charger Interface, and Lights Driver.

Designing the Standoffs

The purpose of the standoffs is to be the contact point that secures the board to the mount. The standoffs are circled in the image below:

Each of the standoffs have holes for heat inserts to be installed. These heat inserts provide threads for fasteners to be screwed into the mount to secure the board. For MSXIV, all the boards use the same heat inserts and fasteners which can be found here:

  1. Brass M2.5 Heat Inserts: https://www.mcmaster.com/94180A323/

  2. Stainless Steel M2.5 Socket Head Screws: https://www.mcmaster.com/91292A010/

All the diameters of the holes in the standoffs were sized to 3.7 mm - the minor diameter of the heat inserts. This allows for the surrounding plastic to flow into the grooves of the heat inserts as they are being heated. Additionally, the holes were cut 2.5 mm deeper than the length of the heat inserts. This was to capture any extra melted plastic that cannot fit into the grooves and prevent it from overflowing out of the hole. Below is a cross-sectional view of a heat insert within a standoff.

Designing the Alignments

The alignments are a feature that helps to position the board so that the holes on the PCBs are perfectly aligned with the standoff holes to make fastening easier. The alignments are circled in the image below:

Each of the alignments have lead-in chamfers that help the board slide into the perfect position. If you take a close look at the board mount assembly, you may notice that the alignments all have a very small gap of 0.35 mm between the PCBs and the mount as shown below.

This is because these alignments are not friction fits! They’re not meant to be another way to secure the boards, but as previously stated, are a device to position the board correctly.

Designing the Supports

When we're installing the connectors and fuses onto the board, the board will have a tendency to bend. If it bends too much, it could crack solder joints and cause bad connections or electrical failures. The supports help to prevent this by opposing any forces that may be causing the board to bend. The supports are circled in the image below:

These supports are placed anywhere along the board that is likely to bend while avoiding contact with any of the pins sticking out from underneath. Below is a bottom view of the board mount assembly with a transparent view of the mount to show the pins underneath.

Fillets, Fillets Everywhere!

As you may have already noticed, the board mounts use A LOT of fillets, pretty much almost every edge or corner has been filleted. You may be wondering why. The answer is, because we can - 3D printing allows us to print complex geometry so why not? It looks nice! That’s one reason, but there are also practical advantages for using fillets as well. Namely, there are fewer stress concentrations on rounded corners and edges as opposed to sharp corners and edges. It also helps to add additional support to any extruded features like the standoffs, alignments, or supports and prevents them from snapping off.

Saving Resources

Circled in the image below are holes made in the base on the board mount. This was simply to save our 3D printing resources by minimizing the amount of plastic that is used.