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It is common practice when building PCBAs to substitute common passive and discrete components, such as caps and resistors, with alternate parts that have the same or better specifications. This is usually done for part availability or cost reasons.

Should these substitutions be tracked as a manufacturing deviation? It is clear to me that a substitution which modifies the functional parameters of the product should be tracked. But if the substitution is done using a direct equivalent replacement (i.e. same resistor from a different manufacturer), I'm uncertain that there is value in recording the deviation.

Our drawings do not currently have a note stating that substitutions are allowed, or under what conditions they allowed. All substitutions go through engineering approval. I am considering adding that note, and perhaps even specifying that, for example, electrical components that are deemed equivalent by Digikey can be substituted without approval.

What's the best practice for handling these types of substitutions?

rothloup
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    don't specify manufacturers for common parts. specify tolerances and acceptance criteria- what test would pass or fail the device? I imagine a few million 1-ohm resistors in place of a mega-ohm might cause some weight or volume issues. – Abel Mar 08 '22 at 02:00
  • @Abel: My CM will not accept a test spec on their BOM. They need to know what to buy, not a puzzle that tells them how to find what to buy. – rothloup Mar 08 '22 at 06:28
  • so put a part number and manufacturer for reference. when someone needs an alternate, the only way to tell that it is equivalent is to solve the puzzle. For someone in the supply chain, this usually involves handing said puzzle to a supplier or manufacturer. These puzzles are known as specifications! It'd be a pretty shoddy supplier that needs a p/n instead of resistance, tolerance, wattage(or voltage, or current) to get you some resistors. – Abel Mar 08 '22 at 12:48
  • way easier to solve a puzzle that is documented than to hunt a discontinued part from 40 years ago that has no published spec and where the manufacturer got acquired and gutted by the competitor. – Abel Mar 08 '22 at 13:17
  • @Abel: A 40 year obsolete part situation should be solved by an engineer, not a CM. Ultimately, tho, my question isn't about how to spec a part. I'm quite clear on the distinction between specs and BOMs. My question is in regard to documentation of "like for like" substitutions. There is no puzzle there. – rothloup Mar 08 '22 at 21:02
  • Also, the puzzle is documented in the design files. That's what the design documentation is for. Leaking too much of that into the production documentation runs the risk of having engineering lose control of what is being produced. – rothloup Mar 08 '22 at 21:19

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Best practices will depend on what the document is for and what is necessary to meet all foreseeable requirements. In other words, best practice is to develop and stick to best practices. Since there are things that are considered unacceptable by those you work with, there are likely already best practices in place and you should ask them. All I can do is provide some examples that others use, which may only be useful to you if you are newly establishing procedure.

Ultimately one tends to track what was used in every lot (or even serial number!). If something different is used, it gets designated a new lot number, and all component lots and orders get tracked against that. This usually has to do with tracking defects rather than design engineering, and accurately mitigating (recalls) and sharing the risks/costs (sometimes involving litigation) with suppliers should a faulty components make it to the customer.

Some examples of how I've seen things handled:

A design is specified by applicable tolerances and pass/fail criteria. The part is then ordered from suppliers against this, and the supplier handles tracking of individual resistors. The supplier tests what they ship (might be random sampling), but the receiver tests at least a subset.

Manufacturers that need to track individual resistors such as the supplier of the paragraph above, can do so against internal part numbers which have the specification (tolerances) that must be met for such a part to qualify as that part number. Sources are qualified against the specification, and any qualified part is not considered a substitution. (Quality control handles first article inspections, etc so that design engineers are not usually needed unless the tolerances or test criteria are out of the ordinary.)

A small list of qalified suppliers, their equivalent part numbers, etc may be listed in the original spec by the design engineers for initial reference. The full list over time is usually tracked in a database, although design engineers may revise the spec with these for reference.

Selecting potential candidates must be done by someone who understands the specification, otherwise you may end up buying, testing, and failing parts that had no hope of meeting the spec. Usually suppliers know what they are selling and can direct you to what you need. If you are dealing with just a supplier's catalog, you must understand both that and the component specification. If you do not, all you can do is seek someone who can to make the decision and maybe also teach you to do so.

Abel
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