You Can’t Say BOM on an Airplane

This is part two of the earlier “Baby, it’s the BOM” post and covers what content should be in the Bill of Material (BOM) for electronic contract manufacturing (CM). Most contract manufacturers specialize in adapting to a customers way of doing things, including your BOM format. If the CM makes too many demands about BOM format and content it may be a red flag. I for one have never been an advocate of forcing customers to adapt to the contract manufacturer documentation system. Having said that, it doesn’t relieve the customer of responsibility of BOM quality, BOMs written on a cocktail napkin are generally frowned upon, even by me.

The most important thing about the BOM supplied to your contract manufacturer is accuracy; correct and current manufacturer part numbers, quantities, reference designators, etc. Errors might be discovered during quotation, BOM review, prototype, pre-production, or production phases but usually the further into the process the higher the potential for bad consequences. BOM discrepancies can cause inaccuracy with the quoted cost or lead-time, mistakes in material ordering, rework, project delays, and the dreaded cost adjustment. Make sure all BOM errors found are quickly reported and redline changes are incorporated promptly and accurately. A common error seen is reference designator to quantity discrepancies, many times the error is shown as U1, U6 and should be U1-U6 or vice versa, a quantity discrepancy of four.

As mentioned in the earlier “Baby, It’s the BOM” post, most CMs can work with a Microsoft Excel BOM, it’s a pretty universal format to share. So, just what BOM information does the CM need to accurately quote and manufacture electronic assemblies? Here’s a list of BOM items and a good place to start for content, described in the order to appear on your spreadsheet columns:

Item Number
Basically, just a sequential number showing how many different part numbers are on the BOM. Item numbers are not critical to the assembly but are useful for verbal and email communications, verification, comparison, inspections, etc.

Part Number (Customer PN)
I recommend that customers generate their own part number for each component that goes into the making of an assembly. It’s an area that can quickly become overly complicated if identifying all parameters of a component from the part number is required or desired. From the CMs perspective, it really doesn’t matter how much or little pain was endured to create the number. We just need the number to be unique to the specific component, preferably numeric, and a consistent length. I wouldn’t recommend just taking the manufacturer part number and using that because over time the manufacturer is likely to obsolete or change the number and cause confusion. Also, using a manufacturer’s part number for a customer part number creates confusion if there are multiple approved manufacturers.

Component Revision (Rev)
Primarily used for custom parts like the bare PCB, custom cables, metalwork, plastic etc. Revs can also be useful for tracking firmware versions. It is a good idea to put the rev of at least the bare PCB on the BOM to prevent potential ordering mistakes.

A generic description of the components and items needed to complete an assembly. The description should be as consistent as possible in syntax, capitalization, and order of information presentation. Include component type, value, tolerance, and voltage as applicable. It is sometimes a good idea to include the manufacture’s description and component marking information. Include package type for SMT devices if possible; it can be helpful to process engineers and equipment programmers.

Include the name of the approved company that produces the component, not the distributor. It is important to include only manufacturers that have been verified to work on your product thru engineering approval and/or testing. If testing has confirmed more than one approved manufacturer, alternate or additional manufacturers can be listed with the corresponding manufacturer part number. Be sure to let your CM know that a change to manufacturers without written approval is unacceptable. Changing manufacturers without verification can create performance issues with the end product, components sometimes don’t play well together.

Manufacturer Part Number
List the manufacturer’s part number as it would be ordered, the number’s association to a specific manufacturer should be easily identifiable. Be sure to keep the number current since mfg’s may change the number for specification or performance changes, RoHS, merger/aquisition, etc. Keep in mind that a manufacturer may adjust the part number for packaging (e.g. reel, tube) and can significantly affect product pricing if excess material is amortized by the CM.

Reference Designator
The reference designator, sometimes shown as “ref”, “reference” or “designator” on the BOM is critical to correct printed circuit board assembly. The reference designator on the BOM corresponds to the same marking on the PCB silkscreen, assembly drawing, and schematic. All assembly, inspection, and support personnel depend on the accuracy of the designator to know exactly where each component will be placed. As mentioned earlier, reference designator errors are not uncommon. Missing, extra, duplicate, or the comma/dash switch (U1, U6 versus U1-U6) are errors to be on the lookout for, accuracy and syntax is critical.

Indicates the total number of times a component is used on the assembly. Not much to talk about here, just make sure it’s accurate and matches the quantity specified by the reference designators.

Unit of Measure
Most parts will have a UM or UOM of “ea” for each. For adhesives or sealants you might use “oz” for ounce, “g” for gram, or “ml” for milliliter. For cable or wire you might see “in” for inch, “ft” for feet, “mm” for millimeter, “m” for meter, etc. Unit of measure is helpful in determining the quantity of material being purchased or pulled for a kit but it can also be helpful to assembly workers.

Thanks for reading, I encourage and welcome any questions, comments, or input.