Thursday, January 31, 2013

Bills of material

Here's a schematic for a simple 5V-to-3V linear regulator than includes input and output filtering (you can click on the figure for a larger version).

The above schematic includes component values, reference designators, part numbers (where appropriate), signal names, and connector pin numbers. However, to actually buy the parts and build the circuit (particularly if someone else is going to buy and build), you need more information. A bill of material (BOM) is a complete list of parts in your project. For example, for the above circuit (again, click on the table for a larger (more readable) version).

As shown here, the BOM must include
  • Line number (for reference)
  • Quantity
  • Manufacturer
  • Manufacturer's part number (complete with all suffixes)
  • Description (standardized, see below)
  • Package (form factor or layout-footprint cell name)
  • Reference designator(s)
  • URL or filename of datasheet (or a copy in the documentation zip file; the PDF version of this BOM includes links to datasheets)
The description line should include the important component parameters, such as value (of course), material composition, power or voltage rating, tolerance, and any other necessary specifications (like temperature range, temperature coefficient, ESR, frequency range, etc.).

Optional, but helpful for small production runs, are columns including retailer, retailer stock number, and cost (for example, item number 4, Digikey, 478-1751-1-ND, $0.77 each).

Getting the manufacturer’s part number correct (complete with all suffixes) is really important.  Here's the part-numbering table out of a Texas Instruments data sheet.

Not only does part number determine the package of the component (like SOIC or TSSOP), but it also determines the number of parts in a shipping package (tube of 25 or reel of 2500). Note the embarrassment that you would suffer if you needed ten tubes of SN74HCT00D, but accidentally ordered ten of SN74HCT00DR (or vice versa).

For volume production, you also may want to include information such as
  • Is this part substitutable?
  • Minimum order quantity
  • Lead time for delivery
  • Internal tracking number
On his blog, Andrew "Bunnie" Huang has a great series of posts on volume production in China called "The Factory Floor". The first installment is the most applicable to this course, but all four parts are a informative read.
Also, don't miss his excellent investigation into some "grey market" MicroSD cards that found their way into his supply stream.

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