The Bill of Materials (BoM) is a list of electronic components, subassemblies, and other parts required for PCB manufacturing. It includes accurate details of all components, assembly, and sub-assembly, drawings, etc that are very important for PCB manufacturers to build a working product. A BoM is required to plan and control the inventory, estimate cost, and reduce the production delay.
BoM is usually arranged in a hierarchy with the comprehensive product at the top followed by sub-assemblies including the list of components, quantity, etc. The main purpose of creating a BoM is to have a master list of all components required in PCB assembly and to have a detailed cost estimate of the product.
Earlier, circuit board designers used to manually create a BoM by adding the component details but now the Electronic Design Automation (EDA) tool can do this job efficiently. The tool used to create a circuit schematic can itself pull the component details and populate a BoM quickly. A BoM helps the designer to:
- Check the components stock in the inventory
- Verify the component’s lifecycle to be active
- Estimate the total cost of the boards to be assembled
- Change the formats easily while providing the BoM to the PCB assemblers
- Place bulk orders based on the list to reduce the purchase cost
- Track the product failures if any, by evaluating the listed sources in BoM
There can be single or multi-level BoMs for a PCB. It can be an Engineering BoM (EBoM) or a Manufacturing BoM (MBoM) also. An Engineering BoM includes components and subassemblies along with a technical drawing of the product whereas a manufacturing BoM includes the parts and assemblies required to ship a product with packaging and labeling items.
A BoM is a tabular representation with rows and columns indicating the necessary details of all the components and subassemblies. We shall discuss 15 important things to be included while preparing a Bill of Materials for PCB assembly:
- Manufacturer Name
This section provides the manufacturer details of the component which is essential to procure the listed part. Knowing the manufacturer name can help in browsing the corresponding website to fetch datasheets and other information about the component.
- Manufacturer Part Number
The part number is the unique identification of a component used in a PCB design. This information is a mandatory input while creating a BoM. PCB assemblers look for the manufacturer part number of all the components before starting the assembly of the circuit board.
- Description
The description includes the electrical ratings of the component like the voltage, current, resistance, etc. It can also contain the dimension, color, and other specifications of the component which is required while sourcing the right part. This is compulsory information to be included in the BoM.
- Quantity
The same component may be used multiple times in a design and mentioning the total quantity of a specific component is a must while preparing a BoM. This section has to be always updated as it can help in bulk ordering from the vendor at a lesser cost.
- Reference designator
A crucial piece of information in the BoM is to include the reference designators of all the components. Notations like U1, U5, U32 for IC chips, or R3-R10 for resistors are examples of such reference designators in the BoM. This information is required to mount a component at the right location on the PCB.
- Component Phase
The present phase of a component in its lifecycle is vital information to be added in the BoM. It may be already in the production phase or maybe a new component in the evaluation phase. Mentioning “Unreleased”, “In Design” or “EOL” for end-of-life parts is necessary to assist the procurement team with inventory planning in advance.
- Package type
Adding the component package information is of great help for the PCB assemblers. It may be a Dual-in-line (DIP) package for a through-hole component or a Quad-flat-package (QFP) for a surface mount part. This information assists in streamlining the production and can make the process cost-effective.
- Placement method
It is suggested to include the placement method of the component like through-hole assembly or SMT assembly and such details in the BoM.
- BoM level
This information is important in hierarchical BoMs to assist in navigation. A multi-level BoM includes assemblies and subassemblies. Mentioning the BoM level will make it easier to identify the component in the BoM structure.
- Board layer
Indicating the board layer in which the component will be mounted like the top layer or the bottom layer can ease the work of a PCB assembler to locate the component on the PCB.
- Unit of measure
Providing the correct unit of measure for each component in the BoM is a good practice. Most likely all IC components will be counted as ‘each’ while resistors, capacitors may be procured in ‘reels’. Mechanical parts like screws may be obtained in units of boxes or by weight.
- ‘Do not mount’ list
Single PCB may be used for multiple applications by mounting associated parts. In such cases, the BoM needs to indicate the parts to be mounted and not to be mounted exclusively. Also, some discrete components are mounted for initial testing and debugging purposes. Once the board is tested completely, such components may be unmounted. Hence including a list of “Do not mount” components is necessary while preparing a BoM.
- Flexibility indicatory
In a design, some of the components are critical for the functionality of the circuit and only the chosen part by the designer has to be mounted. Certain other components may allow flexibility and can be replaced with similar parts. Indicating the flexibility information for each component is important in a BoM.
- Alternate part
Based on the flexibility indicator, there may be alternate parts that can be procured from the vendor at a lesser cost. Listing alternate parts can be helpful while sourcing components with a long lead time also. Usually discrete and small ICs have multiple vendors and are listed in the Alternate part category of the BoM.
- Notes – supporting files, procurement type
A BoM has to include a ‘Notes’ section to add any supplementary details of components and other parts. It may be a caution message for the assembly house or sourcing details to the procurement team. This section allows the designer to share useful information for other BoM users, all through the product development lifecycle.
A well-prepared BoM is a good start point for any PCB assembly provider. It is up to the designer to use a suitable template, include the right amount of information, cross-verify the component details and keep track of all the changes done to the document. Restricting the ‘edit’ permission in the Bill of materials is also a good idea to avoid any wrong entry or unintended modifications. Use the above tips to prepare an effective BoM for your PCB. Also, an experienced PCB assembly provider can offer you some standard BoM formats used in the PCB manufacturing industry.