What is a BOM?
Bill of materials, or BOM, is an element of the product's technical documentation. It constitutes a centralized source of information containing a list of all materials necessary in order to manufacture one piece of a finished product. Every product that is a result of the manufacturing process, such as electronics assembly, has its own list of materials.
The BOM is a structured document with a hierarchical structure, usually using a general to specific notation. This means that the finished product is displayed at the highest level, while its components and materials are displayed at the lowest level.
Advantages of a well prepared and completed BOM
A well thought out and correctly completed BOM is an essential part of the product documentation. At the same time, it is data that can be successfully used to analyze the entire business process around a given product on various levels:
• Inventory management – an analysis of the BOM for inventory requirements necessary for production, helps in optimizing the purchasing process (the number of suppliers, orders’ frequency, quantities of ordered components) and maintain a lean supply chain;
• Sustainable production – beneficial both regarding the financial results and the environment, thanks to the optimization of the entire supply chain (in terms of ordering what is needed in adequate quantities), the resources saved can be redirected to other areas within the company;
How to create a Bill of Materials for an EMS
Creating a list of materials is an important and time-consuming activity. Managing the BOM, updating the information included there, and sharing product data can become overwhelming, so it is best to make assumptions that will structure the work and make working with an EMS significantly easier from the very beginning.
1. Document format
The BOM is best created in an Excel file. Spreadsheets are a popular tool, always found within corporate office suites, regardless of the organization's size.
Thanks to the use of this format, the submitted bill of materials will have a clear and legible structure and notation, in which each component is saved in a separate cell, and the whole row serves its characteristics (the more extensive it is, the more effective the management of the product manufacturing process is).
2. BOM update
When changes are made to the product, the EMS should also receive an updated bill of materials. This will ensure that changes in the production process run smoothly and without generating additional costs. Sending the contractor an old BOM document with components that have been discontinued can lead to an inability to purchase them and, consequently, a halt in contract electronics manufacturing. Therefore, instead of creating another version of the document, the best practice is to update the previously valid file.
3. Component specification
Each component that is included in the BOM should be described as accurately as possible:
- Specification – unambiguously identifies the item and sets the conditions for its storage or maximum storage period.
- Quantity – required to produce one piece of the finished product (if an element appears in the BOM at different stages of production – it should be included separately at each level of the process, according to the demand)
- A list of alternative parts – with analogous or similar parameters. It constitutes additional protection in case of problems with the availability of the originally planned elements.
- Approved Vendor List – AVL (approved vendor list) level – A list of manufacturers currently approved by the principal to supply the materials specified within the BOM.
- Reference designator (RefDes) – a position on the plate, uniquely defines where the component is to be mounted on the PCBA. It is an essential element for the manufacturing of a product.
4. The submission of an MPN (Manufacturer Part Number)
The MPN should be provided in a way that uniquely identifies the material without leaving any room for interpretation (by entering an incomplete MPN, the series only, etc.). It should be specified separately for each component (do not provide the integrated circuit series). Information should also be provided on the type of packaging and the specific storage conditions (e.g., temperature and humidity) under which the product is to be kept.
5. List of steps to follow
If the BOM refers to box-build or electro-mechanical assembly manufacturing, bulleted steps to be performed should be included in addition. The document can be created in CAD or attached as a PDF with information such as material, dimensions, color, etc.
6. Instructions for creating a product
A complete and comprehensive appendix describing the creation of the product. It should include the following information:
- What materials to use, e.g., glue types, cables;
- Packaging of the final product, if so, provide a specification or ask the contractor about their solutions;
- Visualization of the finished product – in the form of a photo/graphic, although it is best to send a sample (tester). This allows you to avoid misunderstandings.
7. Information on previously contracted sources
If a product was previously made for the employer by another EMS supplier, this information should also be provided to the current contract electronics supplier.
Technical documentation, including the BOM, is an essential tool for communication between the employer and the contractor. Therefore, when creating a BOM, you should include as much product detail as possible. This will have the same impact as staying in constant contact with the EMS on efficient contract manufacturing of electronics. Especially when changes are to be made, the so-called Engineering Change Order (ECO), including those involving materials.