Bill of Materials
09 April 2010
The term ‘BOM’ is frequently used, but what exactly does it mean?
In its simplest form, a Bill of Materials (sometimes also called Build of Materials), or BOM, is a list of all the components, connectors, terminations and wires that make up one printed board assembly.
Normally the design and development team provides this list of parts. It is usually in a form that can be easily read and extracted into other software tools.
Commonly, it is provided to sub contractors in a spreadsheet format, along with other documentation, to allow sales, purchasing and engineering teams to evaluate the product for manufacture and cost the product build and parts. Additionally, it also acts as a technical reference for preferred component suppliers.
The manner in which the list is provided should enable relevant attachments on specific electrical or mechanical requirements to be imbedded, so the Microsoft office program Excel is widely used for the BOM.
Care needs to be taken to ensure that the information provided is complete and detailed so as to make sure that all the parts are correctly defined without being too restrictive. This is because the components need to be purchased at the correct quality levels and at a competitive price.
Specifically the BOM should include:
• The item number
• A design reference number on the PCB (such as C52, IC16, U88)
• A part number from the design department/company for the list of materials
• Where possible, a description with files attached
• The details of a preferred supplier
• The quantity required
• A manufacturing contingency
The example image has been extracted from an Excel file and shows the typical details provided in a BOM from the design department. This example pertains to a product under review during the change from tin/lead to lead-free manufacture.
A simple BOM can make things very difficult for sales teams, purchasing departments and manufacturing engineers.
During a design review of the Bill of Materials, great care needs to be taken, particularly in a subcontract environment when sales staff and purchasing representatives are the first to review a BOM for costing and procurement of parts.
This is because, for example, PoP devices may be listed as one or even multiple parts, so the requirement for PoP assembly may not initially be obvious at the assembly quotation stage.
In fact, it is common to find that each of the devices that make up a PoP structure is listed as separate items with their own supplier part numbers and ID. Having just one reference number may be the case if the part has already been assembled as a pre-assembled PoP stack by a specialist packaging house. Furthermore, when a single component reference designator is used for two, or even three parts (such as U32 or IC13), a BOM may suggest a stacked package, and it is for reasons like this that analysis and diligence is essential.
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