Bill of materials (BOM) - Electronic Components for PCBA

Bill of materials (BOM): Electronic Components for PCBA (PCB Assembly)

Before starting the assembly process, it's crucial to ensure that all necessary components and equipment are prepared. This includes components, stencils, soldering equipment, flux, solder paste (for SMT), component placement tools, magnifying tools, cleaning supplies, anti-static measures, assembly instructions, quality control tools, safety equipment such as safety glasses and proper ventilation, the PCB itself, and a suitable work surface with adequate lighting.

In this subject, we'll discuss the process of preparing components for assembly, starting from the understanding of BOM, ordering process through verification and documentation, including installation polarity. We'll cover the steps involved in ordering the right components, verifying their specifications and compatibility with the PCB design, organizing documentation such as datasheets and assembly instructions to facilitate smooth assembly, and avoid errors. Additionally, we'll distinguish between washable and non-washable components, emphasizing the importance of correctly identifying them to reduce errors during assembly.

With mentioned subject “Electronics Component for PCB Assembly”, we will discuss all of the above aspects in separate posts to streamline the information and make it easily accessible. We'll start by discussing the Understanding BOM, followed by separate posts on other aspects of PCB assembly.

Bill of materials (BOM)
Before we start purchasing components, it's essential to have the bill of material (BOM) ready. The BOM must include the manufacturer's name, MPN (manufacturer part number), designators (component name on PCB – e.g., R1, C1, U1, IC1, etc.), and quantity per board. Additionally, the BOM should contain more information such as footprint, description, installation instructions, etc.

 Refer to the example in Fig.1a below that shows a sample BOM (Bill of materials).

Bill of materials (BOM) of Electronics component for PCB Assembly
Fig.1a : Bill of materials (BOM)

What is supplier part Number:
As shown in the example in Fig.1a, the Bill of Materials (BOM) includes both the MPN (Manufacturer Part Number) and the manufacturer's name along with the supplier number. A single part may be sold by different suppliers (vendors) who keep stock from the manufacturer and sell the product. Therefore, while the MPN is global and consistent across suppliers, the supplier part number is specific to each supplier. You can find the supplier part number on the respective supplier's website only. However, the manufacturer part number can be found on all supplier websites if those part are available in there inventory.

What is DNI/DNP/Install:
Some customers may mention DNI (Do Not Install), DNP (Do Not Populate), or DNF (Do Not Fit) in the BOM to indicate that certain parts are not required to be fitted or installed on the PCB. These terms are used when those components are not needed for the assembly process. While, "Install" is specified, it indicates that the component needs to be installed on the PCB as part of the assembly process.

Component Type:
The component type mentioned in the BOM indicates whether the component is a surface mount device (SMD) or through-hole component. SMD components are placed and soldered on the surface of the SMD pads on the PCB. They are typically smaller and allow for higher component density on the PCB. In contrast, through-hole components have pins that are placed through holes drilled in the PCB and then soldered on the other side. Through-hole components are often larger and provide greater mechanical strength and reliability.

Designators & Quantity:
Designators refer to the names of components on the PCB, while the quantity indicates how many times the same MPN (Manufacturer Part Number) is used on the PCB. For example, if designators R1, R2, and R3 are listed with a quantity of 3, it means that there are three instances of the MPN RC0603FR-0710KL on the board.

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