Bill of Materials optimisation - Page 1
Forum section: electronics / general

author: RossJ
author_rank: Newbie
timestamp: February 05, 2020, 07:49:36 am
content: Hello,I would like advice on when/where is a good time/place to optimise the BoM (e.g. reduce unique component count). Currently I tend to do this by updating the schematic, which means all documents are aligned. An alternative of course is to leave the schematic alone and simply tweak the BoM file directly. I assume this would work during assembly as the BoM is the only file connecting a part reference to what it actually is.The reason behind my question is that by updating the schematic, information is lost. For example, a 16V cap might be upgraded to 50V in order to avoid two different parts. But the fact that one only really needs to be 16V is lost. If down the track the design changes and the 50V cap is no longer required, I must reason whether other uses of that cap (i.e. the upgraded ones) really need to be 50V.Of course, not all optimisations can avoid a schematic change (e.g. replace one resistor with two, or a substituted part has a different footprint). Perhaps it's best to simply annotate the schematic that a lesser value is permitted.Any thoughts would be appreciated. FWIW, I'm using KiCad and Altium.Thanks, Ross

author: Daixiwen
author_rank: Frequent Contributor
timestamp: February 05, 2020, 09:15:20 am
content: Updating the BOM without updating the schematic will come back and bite you in the ass sooner or later, in my opinion. It makes it difficult to maintain, it is prone to errors, and when testing a prototype, the schematic that you are looking will not be an accurate representation of the circuit.I'd recommend to update on the schematic each time. About your example, you could add a comment on the schematic each time you make a change, or what I prefer is to keep a list on a separate file with all the components that you have optimized in the BOM. That way each time you do a new optimization round you can go through your list and see if the optimization is still valid.Some tools, like Xpedition, have also a configuration manager where you can have several BOMs on the same schematic. That way you can have the "regular" configuration and the "optimized" configuration. Maybe Altium has the same kind of functionality?

author: SMTech
author_rank: Frequent Contributor
timestamp: February 05, 2020, 10:47:07 am
content: When setting up the SMT I often get the data in two different ways.The first is a BOM and a separate XY file that doesn't contain part numbers. I then have to merge the two either using SMT-Maestro or my own merge tool. As its in this merged form Essemtec likes to receive its data.The other is an XY file that does contain part numbers. It often happens that some people send this with a BOM as well and sometimes those two files are not generated at the same time or the BOM has extra information in comments fields. This method is dangerous, it relies on me knowing which is the authoritative document and spotting anything left to commentary or worse- colour coding. (I convert your excel file to CSV and that is GONE).I would say always generate a complete data pack and always sure all the parts in that pack are in sync with each other, both for you and your fab house.

author: Kasper
author_rank: Frequent Contributor
timestamp: February 05, 2020, 07:51:20 pm
content: You could add extra parameters in the components in your schematic. When you change MPN, make a new parameter 'MPN v1' and put the old one in there. If you have a specific requirement like cap must be rated 16V or higher then make a parameter 'min rated V = 16V'.

author: RossJ
author_rank: Newbie
timestamp: February 06, 2020, 07:00:04 am
content: Thanks everyone for your suggestions. Sounds like I was on the right track by updating the schematic. I'll probably just keep a separate document with the changes, and possibly mark the schematic in some unobtrusive way./Ross

author: Siwastaja
author_rank: Super Contributor
timestamp: February 09, 2020, 03:09:24 pm
content: The PCB toolset schematic capture is supposed to bethesource file for both layout design, and the BOM generation.Sometimes, you just need another higher-level schematic, possibly drawn in a different software. If you need to provide the schematic for the customer for basic troubleshooting / maintenance / etc., this would be the case. Your actual "source" files will likely contain information you don't want to share with everyone, only the ones involved in design.If you have time to source alternatives at the original design time, by all means add extra parameters to the components, for an alternative component. This isn't a total solution, though, it doesn't capture full design intent.A good idea is to add comment text fields to the schematic, to capture the intent. You can add a comment, "16V part will suffice", or even show a calculation of the worst-case peak voltage at that node. This makes your - or the next guy's - work a bit easier.When optimizing the BOM, I tend to copy-paste small "BOM reuse" text boxes all over the schematic, to communicate the reason why the particular value - for example, why I'm using a 4.99k 1% resistor as a generic pull-up instead of, say, 4.7k 5%!