Which Mechanical Components Should Be Placed in a PCB BOM?

Every product that includes a circuit board will include some mechanical components, ranging from a simple enclosure to fasteners and heat sinks. When creating a circuit board, it is of course important to specify the mechanical elements that will appear in the final assembly and in the completed product. However, which of these should be specified in the bill of materials, and how will an assembler handle these components when a production order is received?

One strategy for managing the design, procurement, assembly, and packaging process is to use multiple bills of materials for each stage of the manufacturing process. For electronic products, there are generally only two stages: PCBA manufacturing and assembly of the final product. Both of these require a different bill of materials each with different mechanical components.

Mechanicals in a Bill of Materials

There are many possible mechanical components that can appear in the BOM for a PCBA. The mechanical components used in an assembly can span from simple board-level components needed for PCB functionality, or they can include everything needed to assemble the finished product. It all depends on the level of collaboration between the PCB layout engineer, mechanical designer, and manufacturing group.

Some of the most common mechanical elements that could appear in bills of materials for an electronic product include:

• Screws, nuts, bolts, or other fasteners
• Mounting holes (if connected to a ground net in a schematic)
• Mechanical standoffs
• Wire harnesses or flying lead wires
• Fans
• Heat sinks
• Board-level and enclosure-level shielding
• Any adhesives, gaskets, or coatings

Generally, we don’t go so far as to specify every single raw material for a fabrication house or an assembler; this may be something included internally or in fab/assembly drawings rather than in a BOM. However, when producing a product at any appreciable volume, the final assembly and packaging will need to have a set of mechanical components that might not appear in the PCB layout, and they may play no electrical role in the schematic.

So, which of these mechanical elements needs to be included in a bill of materials?

Board, Enclosure, or Assembly?

Typically when designing the PCB for an electronic product, the PCB will have its own bill of materials that is separate from the enclosure and final assembly. This is because the board will go off to a fabrication and assembly company, who will then ship completed PCBAs back to the customer. To begin the production run, the assembler needs the bill of materials only for the PCBA, including any mechanical elements that will appear in the PCBA.

The bill of materials for the PCBA should include any heat sinks, fans that must mount/bond directly to the board, any shielding on the board, any standoffs for board stacks, and any board-to-board cables if part of a multiboard system. If we’re considering heat sinks, these also need to be mounted to target components, so any mechanicals or adhesives (thermal compounds) should also be present if they will be assembled onto the PCBA.

Make sure your PCB assembler has a bill of materials with all the components they need to complete their assembly job.

Beyond the points mentioned above, there will be other mechanical components needed for the final assembly inside an enclosure. These can require a separate bill of materials specifically for the final assembly.

Assembly Bill of Materials

In many cases, even up to moderately high volumes, assembly and packaging of a product will be handled by a different company than the PCB assembler. When this is the case, a different bill of materials is needed that only includes the PCBA and its required mechanical parts.

For product-level assembly, where the PCBA is mounted in its enclosure and the product is packaged, an assembly bill of materials should be used to include the assembly-level mechanicals. Anything that needs to be used to mount to the enclosure, packaged with the PCBA in the enclosure, or plug into the enclosure (like a wire harness) should be included in an assembly bill of materials. In short, all the elements that appear in the final product that have not been mounted or assembled with the PCBA should be included in this bill of materials.

In this case, the assembly bill of materials might appear as follows:

The final assembly will follow this BOM because the product assembler will only receive the parts in the above list; they don’t need the particular details of all other components used to build the PCBA.

If you have a single company handling everything, then that company will most likely need everything in a single bill of materials. Make sure to contact your manufacturer regarding their requirements.

No matter which mechanical components you need to include in your assembly, you can use the CAD tools in OrCAD from Cadence to build your PCB for volume assembly. OrCAD is the industry’s best PCB design and analysis software with utilities covering schematic capture, PCB layout and routing, and manufacturing. OrCAD users can access a complete set of schematic capture features, mixed-signal simulations in PSpice, and powerful CAD features, and much more.

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