Is this a sensible documented BOM? - Page 1
Forum section: electronics / general

author: LukeW
author_rank: Frequent Contributor
timestamp: January 29, 2019, 06:14:55 am
content: If you were the assembly contractor, would you say this BOM looks sensible and clear?Does it include all the sensible information that it should include as a useful BOM?Any information that should be added that is missing? (Obviously excluding the centroid file, Gerbers, placement diagram etc.)See attached pic.

author: mrpackethead
author_rank: Super Contributor
timestamp: January 29, 2019, 06:30:50 am
content: No, that will annoy them. Aggregate all the parts that are the same into one line.Qnty PartNumber manufactuer description5 10k acme 10k resistor 1% 0805 R1, R3, R7, R11, R149 100nF/25V eev 100nF Ceramic Cap C2, C3, C4 .....

author: Berni
author_rank: Super Contributor
timestamp: January 29, 2019, 06:31:33 am
content: That does look pretty sensible. Comments on substitutions can help a lot of the board assembly house is doing parts sourcing for you.But the one thing you really don't want in a BOM is to have multiple lines for the same part. The five 10K resistors should all be in the same line, there designators stuffed into one box and a quantity column added to indicate there are 5 of them. Similar for not fitted components, they should normally not be present on the BOM, we have our PCB tools mark the non placed components with a red X on the assembly drawing.Another tip about BOMs is to use your PCB tool to automatically generate them without any manual editing. Not only is manual editing BOMs time consuming but its also error prone. I also like to include extra columns from my PCB tools such as the footprint name because footprints in description are error prone too (You change the resistors footprint but forget to fix the description).In general the assembly house likely won't complain about your BOM being shit, they just might be a bit annoyed by having to clean it up themselves and you might get an extra question or so, but the guys at your assembly place will certainly appreciate clean and concise documentation as it makes there life easier.

author: SMTech
author_rank: Frequent Contributor
timestamp: January 29, 2019, 11:35:34 am
content: Its pretty good, particularly when you take into account the suggestions from the previous two posters.Without seeing your Centroid file .. I would suggest that no fits are not included in it so there is no need to cross reference the BOM against the centroid locations.. although if you likely to have multiple variants of the same thing differentiated by no fits, including all locations can help (some machines understand variants).I would be tempted to suggest you create your own internal part numbers and include those in both your BOM and centroid files, the reason for this is if at some later date you do need to change one of the items you specify, you only have to change the MPN for your internal part number and let your manufacturer know and its instantly applicable to all your designs as well as letting you specify multiple approved alternatives*. It can also help let you keep an eye on how many different parts you are using across your designs, if you have lots of products it can help reduce the number of lines you or your supplier has to kit to do your runs etc etc..*an example here would be your Molex headers, people like Wurth do knock off direct copies of many of these which can be cheaper while still having all the UL approvals etc

author: T3sl4co1l
author_rank: Super Contributor
timestamp: January 29, 2019, 12:47:36 pm
content: Only thing missing seems to be -- is this hand-edited, not autogenerated? BOM maintenance is a lot of work, and very annoying when it gets out of sync with your design.Not that you necessarily have that choice! I've worked in OrCAD before; what a mess.If it is autogenerated, then I can't add much to what's been said above already. It looks nice!The deleted designators are a nice touch, sometimes you'll get questions about that. In the same vein -- which may or may not be missing from this example, it's unclear -- components that show up in the centroids file, but aren't physical components at all: mounting holes, fiducials, net bridges, logos, those sorts of things. I like to include those and specify DNP. Helps answer that question.You can always add more columns for various purposes, of course; common enough to add a recommended supplier as well, or other comments or notes or values or whatever that help with assembly, or with selection of alternates, or whatever. A CM isn't going to need or want more than what's shown here, but you might add these for your own purposes as you see fit.Incidentally, beware about ceramic capacitor substitution -- I would further include a note like "retains >70% of rated capacitance at XX volts", thus excluding capacitors that do not have C(V) characteristics listed at all, and ensuring adequate selection among those that do.Tim

author: dmills
author_rank: Super Contributor
timestamp: January 31, 2019, 12:13:05 am
content: I would specify the LEDs (and a few alternates), IME these vary more then enough to make "0603, green high brightness" a woefully inadequate spec. Apart from anything else, which way around to fit them is not always as obvious as you would expect.Also, for the passives, include a note that lists acceptable manufacturers (and maybe series), TDK, AVX, Vishay, Murata? Yes, but there are some vendors that I would prefer not to have used on my boards....Regards, Dan.

author: ANTALIFE
author_rank: Frequent Contributor
timestamp: January 31, 2019, 05:37:02 am
content: Like other have said make common components/designators have the same line item, much easier for the assembler to set up the pick & place machine this way.Also no need to bog down your assembler with designator changes, BUT like that you have marked your not fitted (DNF) items, the assembler will find this useful when doing a load checkFinally it pays to specify what components you want to use, that way you have full control of your design, so you definitely want to have extra columns for manufacturer and manufacturer part numberHere is a BOM that I would be pretty happy with:EDIT: If you want to state alternative part numbers (for example ceramic caps) I would add an extra column instead of an extra row/line, the latter makes your BOM a bit more annoying to deal with for the assembler

author: mairo
author_rank: Unknown
timestamp: January 31, 2019, 10:21:08 pm
content: I will also make sure if you allow the assembly house to substitute a component, to first come back to you with the datasheet of the new component for you to decide if this part is OK. Even for a resistor, parameters such as ppm, RoHS could mater. With capacitors you also have RoHS, operating temp, operating hours...

author: Kean
author_rank: Unknown
timestamp: January 31, 2019, 10:58:16 pm
content: Quote from: mairo on January 31, 2019, 10:21:08 pmAny parts where specific parameters like ppm, etc matter should specifically state "no substitutions without approval".Or the whole BOM and/or contract should state that if it is something where approvals would need to be sought (e.g. aerospace or automotive)

author: mrpackethead
author_rank: Super Contributor
timestamp: January 31, 2019, 11:25:48 pm
content: There is a lot to be said for supplying your own BOM. You save so much work.

author: coppercone2
author_rank: Super Contributor
timestamp: January 31, 2019, 11:35:28 pm
content: make two versions, one condensed one for manufacturing and one with notes for engineering (you need their approval anyway to make changes or its gonna end up crazy). The people making it don't need to know what an acceptable substitution is.They should call you and ask because there are other downsides and its bad practice to say "every part with a higher voltage rating will work". Just check it out if there is a problem. All those components have parasitics and stuff that vary when you change voltage rating. Might make for a EMC issue or something. They are there to put it together. That's it and that's all.The designer should specify everything, including the manufacturer of the component AND the exact part number AND distributor.What you should do is have a database with the exact part number and notifications about stuff going obsolete. I don't know why the assembly should be choosing anything at all. That's engineering work.Your not selling mystery meat. Everything you listed is a highly engineered component. And the distributor too.Why the fuck is the assembly house going to choose your component distributor???If you want digikey and mouser you tell em digikey and mouser, before someone gets the bright idea to buy fucking 'AVX' off alibaba. If something fails you ask, you buy it from digikey? No? Get fucked. They should keep proof of orders and an account for you to verify as well. Don't trust randos. You can't do root cause analysis with this level of outsourcing. At least you can trace it back to a manufacturer this way with nofunny business. If you specify everything you take full responsibility without playing blame games and passing the buck later on. Maybe the component was shipped wrong, maybe they had ESD issues during packaging.. etc... maybe someone at the warehouse went postal and decided to play racket ball with your reels. Monitor all that stuff. Not everyone is going to be like 'lol its a resistor who gives a shit'.Their supposed to solder shit into place, not figure out compliance, approvals, etc. That is your job. You don't want to be calling people up asking what freaking component they placed in a circuit. I would freak out.IMO what you have is a prototyping guide or a design sale for someone to hone, defiantly not a manufacturing worthy document.It's very useful for engineering but its the wrong layer of abstraction for manufacturing.

author: LukeW
author_rank: Frequent Contributor
timestamp: February 01, 2019, 05:01:19 am
content: I think it’s a matter of choosing the right economic tradeoff for the right job. It depends.In some cases, I may want a particular MPN of Johanson 0201 capacitor manufactured by angels which cost $2 each, with their S-parameters specified up into the mid-infrared and transported from the Johanson factory to the assembler in an argon filled case handcuffed to the courier’s wrist. And I’ll do that where it is needed.But this isn’t appropriate for every project.This particular PCBA is fairly simple and non-critical.Where I want to, I’ll specify a particular Murata NP0 part and ship over a reel from Digi-Key myself.But it’s going to be cheaper, easier and faster to allow them to chose, say, a 10k non-critical pull-up resistor or 0.1uF decoupling capacitor. They’ve presumably already got the reels loaded. For certain projects, I want them to do relatively cheap turnkey PCBA and sourcing where I don’t have to organize it.And yes, this is aimed at the Chinese industry. I don’t think they are useless/unethical/stupid as is often believed. And I can get plenty of stupid/unethical/useless right here locally.You can’t always specify everything to be 0.001% nuclear grade. Specify what you need, in terms of tolerance, but not too tight for no reason - basic engineering economics.If I ask Mr China to source their own 10k 0603, 5% or higher, what’s the worst that you could possibly get?You could go 25%, and in this particular case my design will still be fine.Same goes for non-critical decoupling. Plenty of decoupling, multiple capacitors. Capacitors specified for 25V even though they are only operating at 3.3 or 5V in this case so there is plenty of margin for dielectric bias.If you say 0.1 uF is required, X7R/X5R, 0603, 25V or higher, and you leave Mr. China the flexibility to find whatever is in their stock, what’s the worst that could happen?They give you Y5V instead?I should add that the PCBA is not expected to operate in extreme vibration, heat, aerospace etc environments. I don’t expect -40C or +120C performance from the capacitor.Connectors worry me a bit - because the two halves have to mate well.If the PCBA comes from one place and the cable comes from somewhere else, then a change to a “clone” Molex may not mate well, unless the cable manufacturing was kept on the same cheaper brand which mates well to its mate on the PCB. And then if I buy an expensive crimping tool for in-house lab prototypes, will it match the substitutes? The same goes for pre-crimped wire assemblies from Molex, which are handy in the lab.For a number of reasons, it seems like it’s probably a good idea to keep all the boards, cable assemblies, and R&D lab stock all unified as no-bullshit Molex genuine - or, maybe, to change everything to use *only* Brand X cheaper Molex clones.For LEDs, are they really that variable?We’re talking about a batch of say 50 PCBs ordered - not a million. 100 LEDs - likely be LEDs off the same reel.I would prefer LED1 at 2mA to appear fairly consistent with LED2 at 2mA - but if one board is slightly brighter or dimmer than the next it’s not a huge deal.One day I’ll have to write an Eagle script that generates a nicely formatted BOM. The default one includes a lot of noise I don’t want such as my library filename and my part name in the library, which I don’t want.I manually cleaned up this spreadsheet and checked it, which doesn’t scale well, but is okay for this small, relatively simple project.

author: Doctorandus_P
author_rank: Super Contributor
timestamp: February 01, 2019, 01:34:57 pm
content: giving a screenshot.png to your manufacturer is likely to annoy them far more than not combining similar resistors on one line. This last difference is probably 2 mouse clicks if they have decent software.If you want to be nice to your manufacturer you can ask in what format they want the BOM, and deliver in that format. Whether it is a spreadsheet or a csv file or whatever.Manufacturers may also have guidelines for such things on their website.Usually those manufacturers don't care about waste parts. They may throw 50 resistors in the bin, just because the lead-in from the tape is too short. If you have expensive or hard to find components, make sure you tell them in advance.

author: LukeW
author_rank: Frequent Contributor
timestamp: February 01, 2019, 04:14:34 pm
content: Of course the real one isn’t a screenshot pic

author: SMTech
author_rank: Frequent Contributor
timestamp: February 01, 2019, 04:45:15 pm
content: You're getting a lot of advice some if conflicting and some of it crazy. I am a sub-contractor so I will add to my previous. DON'T GIVE THE CONTRACT MORE WORK TO DO; at the point of quoting they are doing work in the hope of getting business and that work is not paid for.So yes specify exact parts if that is what is needed and specify the necessary parameters for parts where you don't really care as long as its up to snuff, most of the time that's just value, tolerance and wattage on a resistor and value, tolerance, voltage & Dielectric for a cap. Elecs/Tants might need to be more specific when you need low profile, low ESR, Audio Grade and other features that can vary wildly. Not being too specific where possible( simple/common things in particular) lets them do several things:Find alternatives when your preference is out of stock or not available in a suitable qty (a huge problem with small batches, nobody wants a reel of 4000caps when they need 75)Use their existing stock they use for other clients, because you know we already have a perfectly good 0.1uF 0805 in qty, potentially permanently loaded on the machine.Use the above to optimise their production run by slotting it in next to similar work in the schedule.And no @coppercone2 any subcontractor worth their salt can tell you where their parts come from to job level and require their suppliers do the same (standards like ISO require it after all). If every customer insisted on exactly what came from where for every line, then you as the customer would have a bigger bill (because there's bugger all chance it coincides with anyone else at that point), by all means do that for volume(1000s not 100s) but if every single one of a line for your product had to be sourced the way you say it should, dnd then kitted onto the machine specifically for your job run, frankly you should buy it yourself. Some sub contractor do work that way with a preference for customer supplied kitting, but if you do that you need to understand how they like the parts to arrive, which means more time for you to spend or using a dedicated kitting company @ extra cost.

author: coppercone2
author_rank: Super Contributor
timestamp: February 01, 2019, 05:00:01 pm
content: Yes its good to own the board house once you get big

author: SMTech
author_rank: Frequent Contributor
timestamp: February 01, 2019, 05:19:46 pm
content: Quote from: coppercone2 on February 01, 2019, 05:00:01 pmIts not a remotely common thing to see, some PCB manufacturers own assembly houses but even then its quite often actually a separate or related business they work with. (Chinese examples excluded)

author: coppercone2
author_rank: Super Contributor
timestamp: February 01, 2019, 05:25:59 pm
content: Well by own i mean to aquire as a company asset under your control, not to integrate facilities.So you can override any problems to maintain your line, enforce stricter maintenece policies, ensure good equip is used etc.Ie force them down a stop production to implement new machines when it suits your company so you dont need to coordinate. And you get to see dirty laundry like bearly working processes (i.e. if someone is operating at fifty percent yeild or some funny buisness like that). Might prevent one off problems black swan etc.

author: SMTech
author_rank: Frequent Contributor
timestamp: February 01, 2019, 05:38:20 pm
content: But for a contrary example look at AMD, they sold their foundries and rather than turning out to be selling the family silver it lets them take their design to the foundry house that has the best process available to make it. Meanwhile Intel is stuck with designs it can't implement on the process it would like to as its process isn't good enough.