The Importance of Bill of Materials in PCB Design

What is Bill of Materials from the viewpoint of PCB Design?

Bill of Materials is a formal list that includes details of all parts, items and raw materials. For an electronic product, a bill of materials lists all the components that are to be used.

It is a list of all items that is needed to build a product and saved in the form of a BOM file. Manufacturers rely on a bill of materials file for the purpose of building a product. During all stages of production this gives them the reference information. This list is used by customers and others alike –For eg. by the fabricator to understand the main components which are needed during the process of assembly.

The following are the key fields available in any Bill of Materials document:

  • Item: For all specifications that are mentioned in the Bill of Materials, it gives the index.
  • Quantity Per Board: It specifies the quantity of a particular part that has to be used in each board.
  • Reference Designator of Components: It helps to end any confusion in regard to the position of any particular part on the layout of the board as each part has a specific reference designator.
  • Vendor: Name of Vendor
  • Size / Footprint: The amount of space a part is going to use in the PCB layout.
  • Part Description / Specs (Recommended): In this field, designers can add the specification of each part.
  • Manufacturer Part Number (MPN): Throughout the process it helps to find the part.
  • Vendor part number (VPN): Vendor designates part number
  • Manufacturer: It includes the name of the part manufacturer.

Purpose of Bill of Materials:

During the process of PCB assembly, a BOM provides information in regard to the components such as footprints, reference designators, quantity etc. During PCB design or PCB design services, designers will save a lot of effort and time by preparing a bill of materials with an updated parts list. There will be no chance of redesign during later stages if the BOM is not updated properly. BOM saves a lot of effort and time on rework and product engineering.

Cost Estimation

When it comes to projects related to PCB, cost estimation is one of the major decisions. A Bill of Materials list can be helpful, when the designers are in the first stage of figuring out not only how many partsthey need but also those that are required for those boards.

Most designers have a challenging time meeting the target cost of the project. By analyzing their respective Bill of Materials, they can be aware of the availability and cost of various components. Designers mostly alter their designs in the first stages as it is required to be fit in their respective target cost range.

 How can a Bill of Materials be created?

By using a spreadsheet and entering the list of parts that are needed, Bill of Materials can be created. This is a tedious processes that requires a lot of effort and time. In modern times, many designers opt for software tools that will help them to create bill of materials. The BOM is generated from the schematic ECAD tool during the design phase.

Designers initially can draw a PCB schematic by using ECAD tools. These tools have information in regard to the part numbers in the library. The information of the part is pulled from this library when designers add each part to the schematic. For the purpose of creating a bill of materials for that particular schematic, ECAD tools use the same information. Once BOM files are created, they help designers in understanding total cost, availability of parts, bulk ordering price, easy format change, life cycle etc.

After having a first look at the template that is offered by the PCB manufacturer, designers can generate a bill of materials in the specified format and use a BOM tool to examine it and get a circuit board quote.

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