A BOM is a comprehensive list of all the items required to build a product, saved in the form of a BOM file. In order to build a product, manufacturers always rely on a bill of materials file. This gives them reference information during all the stages of production. Customers, in turn, use this list to help the fabricator understand the components that they require during assembly.
What to include in your bill of materials for a PCB?
The following are the fields required in any BOM irrespective of the PCB manufacturer:
- Item: Gives the index for all the specifications mentioned in the BOM.
- Quantity per board: Specifies the quantity of a particular part to be used in each board.
- Manufacturer part number (MPN): Helps to identify the part easily throughout the process.
- DNI (do not install)/DNP (do not populate): This field indicates whether the components should be populated or not to be populated on the board.
- Reference designator of components: Eliminates any confusion regarding the position of a particular part on the layout of the board as each part will have a specific reference designator.
- Vendor (optional): Name of the vendor.
- Vendor part number (VPN) (optional): Part number designated by the vendor.
- Value (optional): Value of the particular part.
- Size/footprint: The size/footprint of each part number helps to understand the amount of space it is going to accommodate in the PCB layout.
- Part description/specs (recommended): This is the field where designers can add the specifications of each part.
- Manufacturer (optional): Name of the part manufacturer.
What is the purpose of the bill of materials?
We now have a basic idea about what BOM is for PCBs. Let’s have a look at the main purpose of creating it. See our preferred BOM format.
All the components under a single roof
During the PCB assembly process, a BOM provides information about the components under a single roof such as their quantity, reference designators, footprints, etc. Designers will save lots of time and effort during PCB design by preparing a bill of materials with all the updated parts list. If the BOM is properly updated, then there will not be any chances of redesign during the later stages. Hence, an updated BOM saves time and effort.
Cost estimation
As explained earlier, a bill of materials is a file that has all the components required for assembling a circuit board. Cost estimation is a major decision when it comes to PCB projects. When the designers are in the initial stages of figuring out how many boards they need and the parts required for those boards, a BOM list can be really helpful.
Now, most of the time designers have a tough time meeting the target cost of the whole project. Analyzing their BOMs, they can figure out the availability and cost of different components. Hence they can alter their design in the initial stages as required to fit in the target cost range.
Most of the PCB manufacturers these days offer BOM tools that run this cost estimation check with different vendors. These tools compare the parts mentioned in the list with the real-time stocks available with their specified vendors. BOM tools also let the designer know whether a certain part is available for purchase or not. If not, alternate part numbers with the same specifications will be suggested. The discounted prices are considered when it comes to bulk ordering of components. Hence, these tools are very helpful for cost estimation.
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