When you’re manufacturing or assembling an electronic item such as a PCB, you also need a comprehensive list of materials with estimated costs. This is officially referred to as ‘bill of materials’ (BOM), and every PCB maker needs it to streamline the production process.
The BOM drives positive outcomes from operational processes like manufacturing, parts sourcing and third-party assembly, so it’s important to create a bill of materials that is accurate, organized, and up to date. Let’s look at BOM in a bit more detail and what goes into creating one for a PCB.
Bill of Materials Explained
A bill of materials mentions the assemblies, parts, components and all other items with their desired quantities required to create a product. It also contains the instructions to collect these parts and how to apply them in the production process. Experienced contract electronic manufacturers know where to find these parts and how to assemble them in the required quantity. The BOM is written in a hierarchical manner where the topmost item is the finished product while all the required essentials are mentioned below.
Creating a bill of materials is the standard requirement in the manufacturing industry and less common in creative industries like software development. Some manufacturers also mention the stages of production in their BOM until the final assembly stage.
Bill of Materials for Printed Circuit Board Assembly
Prior to the assembly of a printed circuit board, all stakeholders require a bill of materials. Customers contacting PCB makers for orders first provide a sample BOM containing a list of parts that will go on their PCB. Procurement specialists are the key people who look at BOM and acquire the perfect parts, as mentioned by the customer. They’ll also explore which dealer offers the most cost-effective package and inquire about restocking timelines in case a part is missing.
PCB Bill of Materials vs. Parts List
Both bills of materials and parts lists contain similar information, so it’s not uncommon for people to use these documents interchangeably. However, there are some critical differences between the two, and each of them serves a specific purpose.
By definition, a parts list contains the list of all parts that go into the PCB design. It is a working document created by engineers as the design is coming into shape – because the list is put together during the production, it can be incomplete or even miss out on other essential information. Manufacturer preferences and component model numbers may also be missing from this list.
The bill of materials, on the other hand, includes complete details of how to acquire the parts for a PCB board. It has internal company numbers (if any), descriptions, instructions, quantities and preferred brand names. As a client, it is important to send the right document to your manufacturer to avoid unnecessary complexities in production.
Things to Include in a BOM
You do not need complicated software to create a comprehensive BOM for PCB Assembly. It can be a spreadsheet or a nicely formatted handwritten note. Some clients print their formats on paper for ease of use. It is crucial to add important product data in each relevant category to ensure that the parts are just as you described. Here is a list of things you must add to your BOM document:
- Component Names: Give a name to each part based on its assembly. This is also a trusted method of organizing complex assemblies of printed circuit boards.
- Component Numbers: Assign a number to every component so you can tell them apart with ease. Avoid creating the same number for multiple parts and go with a numbering system that is not 1,2,3,4 but more like ER001, etc.
- Rank: Dedicate a number or rank to each part based on where the item fits within the hierarchy. Anyone looking at this hierarchy can understand the structure and decipher the value of one item versus the others on the BOM.
- Phase Designation: List down the importance of a part according to the phase of the production cycle it will be used in.
- Description: Provide details and descriptions of every component, so it is easier for procurement specialists and vendors to know why a part is required.
- Measuring Unit: Mention a unit of measurement according to which every part will be purchased. You can write ‘each’ or inches, drops, ounces, grams, etc. Information about quantities makes it easy to buy or stock the right number of parts.
- Quantity: mention the quantities of every part to help with purchasing and calculating the bill.
- Designated References: Add notes as to where every component will fit on the board. Mentioning this information on the bill will not only streamline the procurement process but also make the production timeline clear to everyone.
- List of approved vendors: Every business has different preferences for PCB board materials as per their internal guidelines. Companies should make sure that they send a list of approved vendors to the PCBA company. For example, they can request the assembler to procure capacitors from a well-known vendor like Samsung or Panasonic.
- Composition and Specifications: For components where ‘any/open’ is mentioned, the client must add important guidelines so that the parameters are specified. Composition, tolerance, temperature and weight are some guidelines to be mindful of.
It might not look like anything fancy, but the BOM is one of the most critical aspects of PCB manufacturing and assembly. You cannot get any item on your board without a clear BOM in hand. Incomplete or incorrect BOMs will hinder your progress and may shoot up the costs in recalling faulty board pieces. Since no two businesses are alike, there are a lot of variations in every BOM for PCBs.
Some manufacturers ask for separate BOMs for each side of the board. Unless someone asks for something specific, the details mentioned above should be sufficient for a successful order. Make sure you do not build the BOM as you go, because the parts may become short by the time you are done with the entire document. In this case, adding alternate parts is the best idea to keep things on track. Mention the alternate part and company name in the same line as the primary part to keep all relevant information in one place.