one PCB many BOMs how to differentiate assemblies  - Page 1
Forum section: electronics / projects

author: qwaarjet
author_rank: Regular Contributor
timestamp: June 02, 2017, 12:53:42 am
content: I have a PCB design that is going to have several different variants. basically some resistor values will change in each flavor but the PCB will be the same. I don't want to spin a a different silk screen for each variant so I'm looking for a way to mark the board using the pick and place machine since I trust he robot far more than humans. Is there some SMD part that has like numbers or colors that could be used for easy visual identification of the assembly?The only thing I could think of is having a a set of pads that corresponds to the various variants and using a smd resistor as check mark but this seemed inelegant.

author: tooki
author_rank: Super Contributor
timestamp: June 02, 2017, 05:09:07 am
content: Quote from: qwaarjet on June 02, 2017, 12:53:42 amReally? Before I even got to your last sentence, I thought of doing just that, having a few component locations, which you populate purely for ID purposes. I don't think you'll find an alternative that's significantly more elegant. (The only thing I can think of is to use the SMD glue — the kind used to bond SMD components bottom-side for wave soldering — as ink. It's usually bright red.)That said, given that companies routinely use stickers or actual literal marker checkmarks applied by humans, even on super-expensive equipment, I'm not sure why you're worried about humans marking the boards.

author: ajb
author_rank: Super Contributor
timestamp: June 02, 2017, 03:26:08 pm
content: Some assemblers can actually do automatic sticker application as part of their automated assembly process. But with a competent assembler and a clear and accurate assembly package from you, the risk of the wrong sticker or marker mark or whatever being manually applied is no greater than the risk of the wrong component being fitted. That's what QA is for.

author: Benta
author_rank: Super Contributor
timestamp: June 02, 2017, 04:46:22 pm
content: Two suggestions:1: sticker with bar code and serial number placed during assembly, either with a database or with functionality coded into the S/N.2: the hi-tech option: a UHF RFID transponder mounted on the PCB (1 x 1.5 mm) together with the other components. It only needs a slot in the PCB ground plane as antenna. This is a solution with much more possibilities for the future, eg, inventory checking, returns, supply chain tracking etc., but needs a supporting database for storing IDs. Check out NXP UCODE G2XL (no affiliation) or similar from other suppliers.

author: Cerebus
author_rank: Super Contributor
timestamp: June 02, 2017, 04:54:46 pm
content: Quote from: qwaarjet on June 02, 2017, 12:53:42 amMELF resistor? Perhaps pick values that give you three bands the same colour, e.g. 222 = red,red,red, 444=yellow,yellow, yellow.

author: tszaboo
author_rank: Super Contributor
timestamp: June 02, 2017, 05:20:23 pm
content: Just go with the sticker. 90% of the time it works all the time.

author: Jeroen3
author_rank: Super Contributor
timestamp: June 02, 2017, 05:31:50 pm
content: The professional solution is to laser mark the pcb, you can get machines for that. Often a QR code is lasered on.If your assembly house can't do that, maybe you can make multiple electrically identical footprints for the selector resistor that changes. Indicate which board version by which place is populated in silkscreen.Or just add tickboxes that a human must mark during testing.

author: sokoloff
author_rank: Super Contributor
timestamp: June 04, 2017, 12:58:43 pm
content: Quote from: qwaarjet on June 02, 2017, 12:53:42 amIf you have the space on the PCB, this seems like an elegant solution to me. No additional processing steps, little chance for error, very inexpensive. What's not to like?

author: technix
author_rank: Super Contributor
timestamp: June 04, 2017, 05:02:13 pm
content: If your resistors can be replaced with digipots and your board have a microcontroller on it, you can do all the adjusting in software. This allows you to not only differentiate models using just software and thus using only one BOM, you can even sell keys to your customers to unlock additional features on their boards.