How many 0 Ohm, 0402 resistors to conduct 10A??......  - Page 1
Forum section: electronics / projects

author: Smokey
author_rank: Super Contributor
timestamp: June 16, 2024, 11:01:09 am
content: This is sort of a silly question, but I think it would be interesting to hear all the opinions.Say I have a properly sized 10A trace on a board. I want to make the voltage on that trace selectable (5V or 12V) based on component stuffing but I can't add any other line items to the BOM. What I do have is a thick film, 0402, 0 Ohm resistor already on the BOM that I can add more locations to. This part is not specified to act as a high current shunt. It's just a generic 0 Ohm resistor. To make it interesting, lets say that's all the information you have about that particular resistor.So the question is... If it was your only option, how many generic 0 Ohm, thick film, 0402 resistors in parallel would you feel comfortable to conduct 10A.

author: ArdWar
author_rank: Frequent Contributor
timestamp: June 16, 2024, 11:09:45 am
content: Most thick film jumpers are 50mOhm (regardless of size since they all more or less same aspect ratio), and most 0402 are1/16W rated. You can calculate I2R from that. Don't forget to derate.Use metal film jumpers if you're that concerned with current carrying capacity. It's much better than thick films, and not as expensive as current shunt.Example:https://www.digikey.com/en/products/detail/thin-film-technology-corp/D1LPC0402CJUMPF-T10/18158218

author: Monkeh
author_rank: Super Contributor
timestamp: June 16, 2024, 11:10:55 am
content: IME they're usually stated as being up to 50mΩ and 0.5A to 1A. So 20 of them will make me sleep at night. In practice you'd probably get away with 5 or 6.

author: tooki
author_rank: Super Contributor
timestamp: June 16, 2024, 11:13:35 am
content: Quote from: ArdWar on June 16, 2024, 11:09:45 amDid you miss the constraint of “I can’t add any other line items to the BOM”?

author: Psi
author_rank: Super Contributor
timestamp: June 16, 2024, 11:15:32 am
content: I would just solder some onto a random test PCB and then do some testing. Figure out how many works good temp-wise and then add 50% more.Use solder paste and only add a tiny bit when reflowing them to the PCB, you want to replicate how a paste/SMT reflow machine will solder the resistor.If you use an iron you'll probably get excessive solder on the pads and that will give them extra heatsinking.Don't forget you can add extra/wider pcb tracks from a resistor to decrease it's temp for the same power rating. And having them spread out is better than bunched up which makes a hot spot.

author: Smokey
author_rank: Super Contributor
timestamp: June 16, 2024, 11:19:45 am
content: Another interesting aspect is that they would all be in a row bridging a trace that is almost 340mil wide. That's a significant amount of heatsinking on both sides of each resistor into that big trace .

author: langwadt
author_rank: Super Contributor
timestamp: June 16, 2024, 11:28:42 am
content: Quote from: tooki on June 16, 2024, 11:13:35 amchange all of them instead of adding?

author: Smokey
author_rank: Super Contributor
timestamp: June 16, 2024, 11:34:05 am
content: Quote from: langwadt on June 16, 2024, 11:28:42 amYou expect me to pay $0.02333 in 10,000s!! Do I look rich or something?Lets say the company already purchased a lifetime supply of thick film 0 Ohm, so your request was rejected my management. hah.I love that the company that makes that linked part is called "Thin Film Technology Corp" but it's a metal foil resistor. What a bad name for a resistor company.

author: ArdWar
author_rank: Frequent Contributor
timestamp: June 16, 2024, 11:35:39 am
content: Quote from: tooki on June 16, 2024, 11:13:35 amBruh... It's just a remark for when the current carrying capacity ismorecritical...Quote from: Monkeh on June 16, 2024, 11:10:55 amIn reality most of them are actually about 20mΩ (and Vishay do often certify them to 20mΩ). But if it's generic part then nothing wrong with assuming worst case scenario. Who knows what got passed by QC.Quote from: Smokey on June 16, 2024, 11:19:45 amIt helps for sure, but resistor power ratings are often specified with rather unrealistic test footprint in the first place. Usually specified with no other nearby heat source.

author: Jeroen3
author_rank: Super Contributor
timestamp: June 16, 2024, 11:37:03 am
content: What happened to solder jumpers?You can make them selectable with the paste screen.https://learn.sparkfun.com/tutorials/how-to-work-with-jumper-pads-and-pcb-traces/all

author: coromonadalix
author_rank: Super Contributor
timestamp: June 16, 2024, 12:05:15 pm
content: simply put, the design is SHIT if you have to make a tower of 0402 parts, serioulsy,the thermal stress put on many of them while stacking them, physical stress ...even your BOM constraints is stupid, it means the design is flawed from the start

author: langwadt
author_rank: Super Contributor
timestamp: June 16, 2024, 12:11:15 pm
content: Quote from: coromonadalix on June 16, 2024, 12:05:15 pmstacking? obviously multiple parallel footprints ..

author: Smokey
author_rank: Super Contributor
timestamp: June 16, 2024, 12:16:46 pm
content: Quote from: coromonadalix on June 16, 2024, 12:05:15 pmHah. This is obviously not a serious design question. I just wanted to see how many 0402 resistors everyone would feel is enough. I promise this isn't going to actually happen in a 474 control board or something. (at least not by me)

author: tszaboo
author_rank: Super Contributor
timestamp: June 16, 2024, 12:18:10 pm
content: Cut a copper clad PCB, solder your 0402 jumper, run current, see until it's red hot.

author: ksjh
author_rank: Regular Contributor
timestamp: June 16, 2024, 12:34:54 pm
content: YAGEO explicitly states a rated current of 1.0 A and a maximum current of 2.0 A in the datasheet for jumpers of their RC_L series in size 0402.

author: radiolistener
author_rank: Super Contributor
timestamp: June 16, 2024, 01:39:04 pm
content: Just solder a short jumper from a thick copper wire or a flat copper bar.It will be much better than thousands of SMD jumpers in parallel.And it will be much more reliable and more easy to solder.And with smaller voltage drop.

author: ArdWar
author_rank: Frequent Contributor
timestamp: June 16, 2024, 01:49:52 pm
content: I'm not even sure anymore if this is supposed to be production design, hence 0402 jumper in the first place. Here solder jumper can be quite unreliable when done using reflow, and wire jumper obviously need additional manual work.If this is low volume design then anything goes.

author: H.O
author_rank: Frequent Contributor
timestamp: June 16, 2024, 02:57:55 pm
content: For fun and instead of speculating I did some (very) basic tests.I don't have any 0402 but do have 0603 (Yageo RC0603JR-070RL).I soldered 10 of them to a piece of perfboard and run 8A thru the lot, as expected nothing exiting happened so I quickly reduced the number of resistors to 5 and redid the experiment.Code:Initial voltage drop: 43.8mVInitial temperature: 22°CVoltage drop after 20min: 43.8mVTemperature after 20min: 31°C1.6A, 70mW per resistor.These resistors are rated for 100mW with a rated current of 1A or maximum 2A so we're sort of within spec here, no drama.Next I tried with 3 resistors:Code:Initial voltage drop: 71.8mVInitial temperature: 25°CVoltage drop after 10min: 74.4mVTemperature after 10min: 39°CVoltage drop after 20min: 74.8mVTemperature after 20min: 42°CVoltage drop after 30min: 74.8mVTemperature after 30min: 42°C2.7A, 200mW per resistorOK, that's outside spec for the poor resistors and obviously not something one should do in an actual design but it did not seem to get into any thermal issues (under these specific conditions). The voltage drop could be a potential issue.Let's try with 2 resistors....Code:Voltage drop after 10min: 117mVTemperature after 10min: 55°CVoltage drop after 20min: 118mVTemperature after 20min: 57°CVoltage drop after 30min: 118mVTemperature after 30min: 60°CVoltage drop after 40min: 118mVTemperature after 40min: 60°C4A, 472mW per resistor.Of course the resistors get help with heat disipation from the copper strips on the perfboard but so would they on a PCB with a trace designed for 10A.

author: Smokey
author_rank: Super Contributor
timestamp: June 16, 2024, 09:02:25 pm
content: Quote from: H.O on June 16, 2024, 02:57:55 pmHa! 2 resistors survived at almost 5x rated power! Based on this testing from a guy on the internet, the next 747 computer will have exactly 2 more 0 Ohm resistors than before!

author: Smokey
author_rank: Super Contributor
timestamp: June 16, 2024, 09:09:49 pm
content: Quote from: ArdWar on June 16, 2024, 01:49:52 pmNot to get too sidetracked from my silly 0402 thought experiment with actual production engineering, but I have another board with a switch mode power supply that uses an SMT current sense resistor that ends up conducting the full 8A supply current (as recommended by the manufacturer). So far I've had no issues with that design. If surface mount soldering was so inconsistent and unreliable, how is it smt current sense parts aren't blowing themselves off boards left and right? Also, all the boards go through AOI which should catch cold solder joints. There are a ton of things to be afraid of, but bad SMT joints causing current paths to blow up in production probably isn't that high on the list.

author: ArdWar
author_rank: Frequent Contributor
timestamp: June 16, 2024, 10:28:41 pm
content: Quote from: Smokey on June 16, 2024, 09:09:49 pmNo, not the component soldering itself. But solder jumper. The kind that use two closely spaced open pads that can be bridged by a solder blob. You really need to overexpand your paste aperture to make it anywhere near acceptable success rate, and hope it actually blobs into the jumper pads and not beading somewhere else. Even then it might still sometimes fail depends on the exact reflow parameter your fab uses.Paradoxically to soldering qualities you might want to achieve with most components, solder bridge practically need a worse soldering to actually work. Increasing reflow temperature making the solder more fluid: fail to bridge. Better flux or solder alloy allowing more surface tension to "autocenter" SMD components: fail to bridge. Lead free solder (older one)? fail to bridge, or sometimes it will bridge when you don't want to.This is one of the reason why actual component jumper is a thing.

author: Smokey
author_rank: Super Contributor
timestamp: June 17, 2024, 12:30:41 am
content: Gotcha. I misunderstood. Ya I would never rely on a solder jumper like that.

author: Someone
author_rank: Super Contributor
timestamp: June 17, 2024, 05:09:34 am
content: Use the 0402 0R (or some other larger resistor) sideways along the terminations ?

author: radiolistener
author_rank: Super Contributor
timestamp: June 17, 2024, 07:41:18 am
content: Quote from: H.O on June 16, 2024, 02:57:55 pmYou're needs to measure voltage drop on it...

author: The Soulman
author_rank: Super Contributor
timestamp: June 17, 2024, 09:45:17 am
content: Quote from: radiolistener on June 17, 2024, 07:41:18 amHe did, and documented it well.

author: Siwastaja
author_rank: Super Contributor
timestamp: June 17, 2024, 10:25:08 am
content: Quote from: Smokey on June 16, 2024, 12:16:46 pmI could expect stuff like this happening exactly on a 747 control board, not because they wanted to save $0.0001, but because each BOM line produces a van full of certification and takes 10 years to certify for aviation, therefore completely unsuitable part being used instead of an obviously correct and easily available part

author: Doctorandus_P
author_rank: Super Contributor
timestamp: June 18, 2024, 05:56:30 pm
content: The only proper way is to look at the datasheet for the current rating, and don't exceed that. If that is not available (your list of constraints make the whole thing a bit silly), then you should at least do some testing. Push some current though it, measure voltage drop (resistance) and temperature rise. Increase current until "excessive" and then use some derating to get to a "safe" value.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: June 18, 2024, 06:12:07 pm
content: Quote from: Smokey on June 16, 2024, 11:34:05 amDepends. What's the average/effective placement cost per part?At some point, it will be worth adding that cursed BOM row. Without numbers, all we can say is that such a point exists.Tim

author: exe
author_rank: Unknown
timestamp: June 18, 2024, 06:51:17 pm
content: Quote from: Jeroen3 on June 16, 2024, 11:37:03 amIf you can do solder jumper, then you can also just connect two tracks together without any solderingI only see a point in jumpers when there are two (or more) intersecting traces. In this case, if one trace is a signal trace, put jumper on the signal trace, rather than on high-current trace.

author: langwadt
author_rank: Super Contributor
timestamp: June 18, 2024, 07:03:16 pm
content: Quote from: T3sl4co1l on June 18, 2024, 06:12:07 pmin volume does it really cost anything extra unless you run out of space on the machine?

author: T3sl4co1l
author_rank: Super Contributor
timestamp: June 18, 2024, 09:39:17 pm
content: Quote from: langwadt on June 18, 2024, 07:03:16 pmIf I can place 1000 parts for the cost of 1, that'll be news to me.Or do you mean adding a reel assuming there's a free slot?Tim

author: langwadt
author_rank: Super Contributor
timestamp: June 18, 2024, 10:04:30 pm
content: Quote from: T3sl4co1l on June 18, 2024, 09:39:17 pmyes, if there's no free slots sure you are going to try to get by with something you already use, but if there are free slot does it actually make sense to place multiple parts to avoid adding different part?

author: Smokey
author_rank: Super Contributor
timestamp: June 18, 2024, 11:00:54 pm
content: Quote from: langwadt on June 18, 2024, 10:04:30 pmSince my self described silly thought experiment was designed to make you use only the given generic 0402 0 Ohm parts, lets say both are true....No free slotsAND...it takes an act of god to get a new part approved for an existing assembly.And there is always a per-placement cost factored into the machine run time per assembly. If you are adding 5 more locations to an assembly with only 10 parts on it that would be very different cost increase wise vs adding 5 locations to an assembly with 300 other parts.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: June 19, 2024, 12:02:04 am
content: Quote from: Smokey on June 18, 2024, 11:00:54 pmNo offense but I at least have moved beyond the OP question; it's been conclusively answered (read the datasheet). If that's not a satisfying or acceptable answer, let us know.Also, your setup still leaves open possible questions about what else is in your inventory. Perhaps a 1mΩ resistor is available that will handle more than enough current. If its cost is also amortized to zero, it would be a perfectly viable alternative. But as you've more or less told us to ignore cost -- a peculiar instruction considering it's a production run -- and we don't know the relative costs, or opportunity costs, we have nothing more to say in that direction than "look at the numbers".The rest of this thread as far as I'm concerned is either memeing, or meaningful discussion around related circumstances.Tim

author: Andree Henkel
author_rank: Regular Contributor
timestamp: June 19, 2024, 11:50:04 am
content: KOA SPEER metal plate chip type jumper resistor:0402: TLRZ1ETTB - 10A so you need one ...https://www.mouser.de/ProductDetail/KOA-Speer/TLRZ1ETTB?qs=u4fy%2FsgLU9OXExoEt1rCFw%3D%3DI use 0805 version to slightly rise one SMD-Pin of a photodiode - Soldered on resistor while other is soldered to pcbto avoid unwanted backreflections and resulting problems

author: victorb
author_rank: Contributor
timestamp: June 19, 2024, 03:46:06 pm
content: From my experience, using 0402 0 Ohm resistors for 10A is pretty risky. They're really not meant to handle that kind of current. I once tried something similar, and the resistors got super hot and failed pretty quickly.If you absolutely have to do it this way, I'd say you should start with at least 10 in parallel. But honestly, I'd be super careful and keep an eye on the temperature and performance. It might be better to find a more suitable solution if you can, but if you give it a go, just be ready for some potential issues.

author: bson
author_rank: Unknown
timestamp: June 19, 2024, 09:03:08 pm
content: What is the circuit fused for - presumably a bit more than 10A? Consider the failures modes, what happens if something is dropped on the board, or the board is dropped and cracks say 7 of the 10 resistors and the remainder now see the full 10A current? Sounds like this particular failure mode is a potential fire hazard that can't be fused against. Perhaps if each of the resistors is fused? But then why not just use a fuse as a jumper?

author: Siwastaja
author_rank: Super Contributor
timestamp: June 21, 2024, 07:21:22 am
content: Quote from: bson on June 19, 2024, 09:03:08 pmThat's a good keyword - a board handling 10A probablyshouldhave a fuse, and if it does, then why not just use said fuse instead of 0-Ohm resistors to make the production time choice. It could be something as simple as choosing between two optional locations for the fuse, such that only one fuse is placed and it performs the duties of the fuse, no extra components needed.As always, we can micro-optimize, but if we saw the big picture there could be more sensible optimizations available.

author: Dr. Frank
author_rank: Super Contributor
timestamp: June 21, 2024, 07:47:06 am
content: Quote from: Smokey on June 16, 2024, 11:01:09 amIs your use-case a regular mass-production (with high reliability requirements), or just amateur use?In first case, it's at first not only a component related topic, but more importantly a question about your soldering process, your layout rules, and especially the pad geometry for 0402 of your E-CAD system.As 0402 also implies very small solder joint and layout cross sections, long term reliability of the joints will be affected, if this is not taken into consideration.You need to define proper pads for high current, as well a layout distribution scheme so that you get sufficient thermal separation on the pads (for the reflow process), but as well a proper, even current distribution for each resistor.For that purpose, you either you would do a thermal and electrical simulation, or you have to make appropriate measurements in situ, like current distribution and temperature rise inside the solder joints..Your pad geometry and your soldering process have to assure, that a very consistent solder joint topology can be achieved in mass production.You might also need to investigate on other environmental conditions, like external temperature range, bending of the PCB, vibration, corrosion under humidity conditions, and so forth.For the BOM position of that 0402 resistor, you need to operate with alternative placement positions, i.e. you need to add several BOM entries, which can be selected by your different application cases.Frank