Shock and vibration resistant design help - Page 1
Forum section: electronics / projects

author: atwoz
author_rank: Regular Contributor
timestamp: August 13, 2013, 03:00:09 pm
content: Hello everyone!I'm designing a wireless product that will be powered by 2 AA batteries or a Li-Po battery (2100mAH).The product is for industrial use and will be mounted on a piece of equipment that has some serious shock and vibration. It is dropped from 2 feet every 5 minutes (that would be over 100,000 times a year).The highest mass components in the design are the battery, the sensor (some linear hall effect module) and the RF module. The rest of the components will be Surface Mount.Could you guys give me some recommendations and design tips please?I believe a metal case would be more appropriate for the product, maybe aluminium so that the hall sensor can be placed inside the case.Thanks!

author: ovnr
author_rank: Frequent Contributor
timestamp: August 13, 2013, 03:19:09 pm
content: I don't have any concrete suggestions aside from this: Place a large label on it saying "DO NOT DROP". Consider investing in some workplace training. Job done.

author: nctnico
author_rank: Super Contributor
timestamp: August 13, 2013, 03:34:44 pm
content: I'd first try to get some numbers on the amount of G each drop produces. Perhaps your device can be mounted on cushioning. Potting in PU based resin could help to keep components in place.

author: SArepairman
author_rank: Frequent Contributor
timestamp: August 13, 2013, 05:03:42 pm
content: maybe you could connect the batteries through a springy cable (telephone cable?) in order to simplify the design.You might have to replace the cable once in a while but they are pretty cheap and can be added to the machine maintenance.and maybe use through hole instead of SMD if you are gonna pot it

author: SeanB
author_rank: Super Contributor
timestamp: August 13, 2013, 06:05:56 pm
content: SMD can be used if you do a 2 part encapsulation the first being a flexible conformal coat, then a second stiffer coat then the final hard curing low thermal rise epoxy encapsulant. That way the internal parts are going to be supported, though I would suggest the components be glued to the board as if they were going to be wave soldered, and clean the board thoroughly after assembly, test it, bake it for 48 hours then do the first conformal coat.Connect the battery and encapsulate that as well in a separate part of the housing with a thin hard coat so it can be replaced, and give it a thick coat of the conformal soft coat, possibly a sheet of silicone rubber wrap as well around it so it can swell and change shape slightly with charge cycles.Leave a lot of slack on the cable inside and use a multicore fine stranded wire with a tie down to the board with a lacing twine so there is no stress on the solder joints. That is about standard for missile electronics, and they do at least survive for a few hundred hours of test flights under airframe power and a final 50 seconds of self powered flight before the final immolation.

author: KJDS
author_rank: Super Contributor
timestamp: August 13, 2013, 06:34:31 pm
content: There's no need to encapsulate surface mount components for high vibration environments unless there are very large extremes of temperature and the likelihood of humidity. The space industry generally runs with a small gap underneath large SMDs to allow for some board flex. The automotive industry likes flexiterm capacitors for large decouplers.If you can mount the board on a damped mount inside the case, then with care that can reduce the stress on it, however if it hits resonance then you can get a g multiplication affect. You'll need to know what the input vibration spectrum is to ensure that this doesn't occur.

author: DaveW
author_rank: Frequent Contributor
timestamp: August 13, 2013, 06:45:25 pm
content: 1) Cushion the batteries as much as you can; these can rupture2) Glue dot heavier SMD components3) Shock mounts/Soft mounts for the PCB help; However, make sure shock in use doesn't cause the shock mounts to "bottom out", otherwise this can be worse than not having them at all!4) Coating barely helps with shock, potting does help. To get the best results with potting, put the assembly in a vacuum chamber after pouring the potting to remove bubbles5) Assuming you're mounting this to a reasonably strong box, use lots of mounting posts in a grid pattern or similar6) Use a thick board, going from 1.6 to 2.4mm makes a big difference7) Where high mass modules are mounted to the PCB (like the RF module), try to keep these mounted using PTH if possible. If not, keep the tracks wide and the distance to a PTH via short, as these will help secure the board. I've had SMD caps rip the traces straight off boards...8) If you need high reliability, look at ESS techniques for manufacture. Low ramp rate thermal will show up solder creep errors, high ramp rate thermal will show up poor/cracked joints and vibration to NAVMAT or similar will take out a lot of early life failures

author: fcb
author_rank: Super Contributor
timestamp: August 13, 2013, 07:17:09 pm
content: You need G figures before you do anything, otherwise your just making a guess as to whats appropriate. The size of the drop is irrelevant its the deceleration that maters.AD had a problem with the ceramic packaging on there early ADXL50 series sensors, an unaccetable failure rate when they hit the customer. Turned out that when they were put in tubes, they were slamming into each other and exceeding the do-not-exceed G spec.

author: atwoz
author_rank: Regular Contributor
timestamp: August 13, 2013, 09:15:27 pm
content: Thanks for the advice. I'm not exactly sure how many G's will be present. All I know is that they drop a mechanism (that has the module screwed into it) and it slams into another part of the module. It travels around 2.5 feet and it does have some dampening so its not too bad. The bad part is that it will be happening over 100,000 times a year. The good news is that the lifetime is just 1 year.I did a test using one circuit board I designed for other purposes. I attached the circuit board to a wooden bat and began hitting the floor with it (the PCB was on the other side of course, not the side that was hitting the ground). After around 50 hits, many parts (obviously the bigger ones, most SMD) had been ripped off the board. Of course, there were many more G's involved since I was hitting the ground very hard. I know this is not a real indicator of what would happen, I just wanted to see if I could predict what would happen to the PCB. One RJ45 connector, had its shield all bent, just from the G's.Thanks!

author: Mechatrommer
author_rank: Super Contributor
timestamp: August 14, 2013, 02:34:29 am
content: you have a serious mechanical problem here...

author: e100
author_rank: Frequent Contributor
timestamp: August 14, 2013, 12:05:53 pm
content: Li-po batteries tend to catch fire when physically damaged. See Youtube for the scary videos. LFP batteries are less likely to catch fire. Seehttp://en.wikipedia.org/wiki/Lithium_iron_phosphate_batteryFlexible solar panels with a small backup battery may be safer, lighter and more robust.The electronics in guided artillery shells survive several thousand G during firing, so it is possible to build super robust electronics, but the engineering knowledge is unlikely to be found in the public domain.

author: EEVblog
author_rank: Unknown
timestamp: August 14, 2013, 12:58:11 pm
content: Quote from: nctnico on August 13, 2013, 03:34:44 pmThat can be a double edge sword though. Yes, components won't fly off the board, or joints crack, but any shock/vibration on the case is directly coupled to the components. Things like precision crystal may not like this.This is one of those "it depends" questions.Yes, you'll likely need some G numbers to work from.But generally you want to avoid hard rigid mounts between the box and the unit, and the PCB and the box.

author: Johnny2Bad
author_rank: Newbie
timestamp: October 10, 2013, 10:26:07 pm
content: Check out 3M; they have shock/vibration solutions using thermoelastic foam. I forget the name of the subsidiary that actually sells it but you should be able to hunt it down on the 3M site, assuming you know how to search.They have a number of papers on the subject as well.Thermoelastic foam, sometimes called memory foam, has a few grades that consumers would not be familiar with (mostly because they are not very comfortable to sit on).The footwells of F1 cars are lined with this particular grade (F1 orders one of two grades the morning of the race, depending on ambient temperatures); they are used to mitigate spinal injury on hard landings with aircraft and helicopters, etc. They can absorb rather high G loads in about a 2" depth.