making a SMD 500G shock proof - Page 1
Forum section: electronics / projects

author: Simon
author_rank: Unknown
timestamp: April 14, 2014, 07:43:34 am
content: So boss asks me to design a circuit, I do that, Nice little SMD board just a micro and power supply to it. Now he wants it to withstand 500G shocks. Hm I think soic and 1206 packages are ok, what worries me are some though hole connectors and the 4mm diameter electrolytic capacitors that are the only things with any real mass versus their footprint.Any idea's. He is thinking of potting it which will be messy. As I can do wthout the capacitors I am in favor of starting again and using 1206 ceramics maybe a few in parallel.

author: kizzap
author_rank: Unknown
timestamp: April 14, 2014, 07:47:16 am
content: Use through-hole components? Might give a little more support. At a guess though potting will be the most stable.

author: Simon
author_rank: Unknown
timestamp: April 14, 2014, 07:52:22 am
content: yea it's just that we are going front a neat little light board to a monstrosity. next I'll need loads of fixing holes so that the pcb does not rip apart or whatever mounting strategy I use will need loads of mount points.

author: kizzap
author_rank: Unknown
timestamp: April 14, 2014, 08:08:03 am
content: another idea I just had:Some capacitors come with screw down supports. maybe you could make something similar to that?

author: Richard Head
author_rank: Frequent Contributor
timestamp: April 14, 2014, 08:17:34 am
content: Lucky you don't build proximity fuses. They have to withstand 10 000G!Can you imagine a 1g smd component suddenly weighing 10kg?

author: Simon
author_rank: Unknown
timestamp: April 14, 2014, 08:23:51 am
content: ah so is it like for every G/g the weight increases by the amouint of G ? like my 1g part at 500G will weigh 500g ? useful for some basic testing

author: Richard Head
author_rank: Frequent Contributor
timestamp: April 14, 2014, 08:48:14 am
content: SimonYes, that's how it works. Can you imagine 500g for each and every smd resistor?? (Assuming they weigh 1g, but I'm not certain about that)So, if your populated board weighs say 200g it will become 100kg for a brief instant! Can you imagine 10 000g?

author: Kjelt
author_rank: Super Contributor
timestamp: April 14, 2014, 08:53:37 am
content: First thought in my mind, this might require to use special glue on each and every component before soldering. I wonder if SMD components can handle 500G in the first place without potting to distribute those forces.

author: miguelvp
author_rank: Super Contributor
timestamp: April 14, 2014, 08:57:31 am
content: Is the 500G on a predetermined direction?For example in a car for front side collision, you will know the acceleration vector so how the board is mounted in the car will make a difference. I mean it's not like you have to make it 500G proof when someone drops a car flat on it's wheels.Also, will conformal coating help?

author: Simon
author_rank: Unknown
timestamp: April 14, 2014, 08:58:05 am
content: Yes glue around the part is one thing that was suggested. See I'm worried that once posted we go from a 20g board to a 100g lump, so now we have 5 times the strain on our fixing holes that started out as just 2 holes in diagonal corners that now have to take 50Kg so we make the board tougher and heavier. So really designing with 1206 caps will be best and using 90degree connectors instead of connectors sticking up in the air. No 1206 parts will weigh much less than 1g probably more like 1/100 of a gram so I'm quite happy to use those.

author: Simon
author_rank: Unknown
timestamp: April 14, 2014, 08:59:40 am
content: Quote from: miguelvp on April 14, 2014, 08:57:31 amI'm not sure but now that I understand what the effect of the forces are I'll look into finding out as if the board can be appropriately mounted it will make life simpler. But chances are I'll not get much info as I don't think we know much about the vehicle it goes in.

author: Simon
author_rank: Unknown
timestamp: April 14, 2014, 09:19:48 am
content: apparently it's all 3 axises. We think the customer is just trying to cover themselves and being anal about it not realizing the hassle it causes. We are going to pot the whole thing. As usual we don't want to spend money as I'd just do the thing again with 90deg through hole connectors and 1206 or 1812 parts only

author: DmitryL
author_rank: Regular Contributor
timestamp: April 14, 2014, 09:22:07 am
content: Quote from: Simon on April 14, 2014, 07:43:34 amNiceDo you have any facilities to test how your stuff deals with the shock ? Otherwise it might be a bit pity to find out that the homebrewed shock-proof design fails miserably in reality

author: hans
author_rank: Super Contributor
timestamp: April 14, 2014, 09:22:31 am
content: It's in essence all about forces. Gravity is 1G force pulling us down to the surface, as in F=m*a. When free falling all objects will accelerate with approximately 9.8m/s^2 (varies a bit across the earth), until terminal velocity (which is just air resistance building up to an equilibrium with gravity).If that object suddenly stops, it experiences a force to take away the momentum. Depending in how short a distance it stops/stopping time will lead to an acceleration applied, which in turn is a particular force in Newton. For example, 100G would be 980m/s^2 of acceleration, on a 1kg object is 980Newton. That is more or less the same as 100kg at gravity (1G). For imagination purposes it is often expressed as "as large as weighing X kg for a brief moment".You could fake the Newtons by using gravity and have something pulling on it, but I think you need to have a very clever mounting system to apply it in a good way. Also, force/speed/acceleration is not scalar but a vector, and can be applied in 3 dimensions. In addition; "shock" implies a very short trauma, not extended periods of time (milliseconds vs seconds).Honestly I wouldn't really know how to "reliably" test it. You do have those vibration tables, but we often only use those to see if screws & board assemblies remain fixed during transport/shaky equipment usecases. Maybe there are similar pieces of test equipment for shock.Or you can just overengineer the crap out of itNot sure, but I think large (especially tall components) are better to be fixed through hole than SMT. I still prefer through hole USB connectors over SMT USB connectors for that reason.I've also had some large SMT aluminium caps "fall" of a PCB once when the board felt on the ground. So I guess those are also a worry point.

author: Simon
author_rank: Unknown
timestamp: April 14, 2014, 09:28:09 am
content: Quote from: DmitryL on April 14, 2014, 09:22:07 amNope no testing at all, we don't usually do shock. I think the whole thing will be going out for testing

author: Simon
author_rank: Unknown
timestamp: April 14, 2014, 09:30:33 am
content: Quote from: hans on April 14, 2014, 09:22:31 amThat is what we usually end up doing hence we will pot the whole thing so that nothing goes anywhere and screw through the potting.

author: ConKbot
author_rank: Super Contributor
timestamp: April 14, 2014, 10:29:43 am
content: Duration matters a lot. 500G for 1us? 1ms? 1s? for 1uS, you could get away with good design, for 1S, obviously you'd need potting. Is this a single impulse shock? Vibration? "500 G" by itself is almost a worthless spec imo. If you can, pin them down for more information.The nasa hand soldering standard (search online for a pdf) gives some insight into some shock/vibration resistant work. Leaded components are your friend (SMD and TH) as the leads provide some stress relief to keep from stressing the solder joint.Also remember the cube sqare law. Your component volume (and mass) will go down with the cube of the size if you scale a component down, and the cross section of the solder joint (pad size) will only drop with the square, so your mass per mm^2 on the joints drops as you go smaller.Havent tried it, but I'd suspect its a lot harder to use shock and rip off a board one of the flexible terminated 0402 capacitor than a 1206 on the same pcb.

author: Simon
author_rank: Unknown
timestamp: April 14, 2014, 10:46:37 am
content: yea I agree 500g on 3 axises does not tell us alot but unfortunately I'm dealing with the customer and my company seem to have a policy of just saying it can be done and not asking for details rather than try and get as much information as possible.

author: kizzap
author_rank: Unknown
timestamp: April 14, 2014, 12:18:15 pm
content: testing should be easy...hit it with a hammer

author: DaveW
author_rank: Frequent Contributor
timestamp: April 14, 2014, 12:25:17 pm
content: My day job involves around 1,000g-2000g half sine shocks, so these might be overkill. Having said that,If you must use larger components, glue dot them down to the boardDon't mount any components near to the edge of the board (within around 3mm). Neither should any components be mounted too close to supports, ie. where the board is fixed to the enclosureIf you're using larger ceramic components, they can crack under shock. But with lowish levels of shock, you'll probably be OK with 1206The flexiterm components don't seem to help much with shock-this tends to useful with mismatched TCsAre you going through vibe as well? If so, your resonance points in one plane will be at lambda/4, lambda/2 & 3*lambda/4-avoid mounting any large components across these lines

author: Simon
author_rank: Unknown
timestamp: April 14, 2014, 12:51:14 pm
content: Quote from: kizzap on April 14, 2014, 12:18:15 pmwell I think my boss said something about hitting it with a cricket bat but he may have been joking

author: poorchava
author_rank: Super Contributor
timestamp: April 14, 2014, 01:00:08 pm
content: I'd watch out with surface mount ceramic caps. The laminate is not very stiff, which means it can easily deform with 500G applied to it. And the longer the component is in the direction of bending, the greater stress it experiences. And ceramic caps are not very resistant to stress... They simply crack. I would refrain from using anything which is not on some for of leads. Leaded SMD packages are ok, but chip capacitors and resistors are not necessarily a good thing, that is unless you have the pcb screwed to a stiff backing plate of some sort.

author: Simon
author_rank: Unknown
timestamp: April 14, 2014, 02:14:17 pm
content: hm I didn't know you could get leaded surface mount

author: KedasProbe
author_rank: Frequent Contributor
timestamp: April 14, 2014, 02:58:43 pm
content: The way it is mounted to the 500G object can also seriously reduce the G forces on the components. Like a low pass filter.

author: Simon
author_rank: Unknown
timestamp: April 14, 2014, 03:34:20 pm
content: unfortunately military vehicle manufacturers don't tell you all about the vehicle, All we can guess is a gun but then why 3 axises.

author: ConKbot
author_rank: Super Contributor
timestamp: April 14, 2014, 03:47:00 pm
content: Quote from: Simon on April 14, 2014, 02:14:17 pmSOIC, TSSOP, are all leadedthe leads allow for some flex, vs something like a DFN or QFN where its solder pad on solder pad.

author: Simon
author_rank: Unknown
timestamp: April 14, 2014, 03:54:00 pm
content: oh i see, never seen any 1206 leaded parts

author: KJDS
author_rank: Super Contributor
timestamp: April 14, 2014, 03:55:14 pm
content: Quote from: Simon on April 14, 2014, 03:34:20 pmMIL-STD-810 is the universal source for military vehicle vibrations.

author: wraper
author_rank: Unknown
timestamp: April 14, 2014, 04:23:03 pm
content: Be very careful if you use any inductors. They will easily crack before any other parts are affected.

author: Frank
author_rank: Contributor
timestamp: April 14, 2014, 04:27:12 pm
content: As others already said, you will need to know exactly what the "500g" means for the customer. If this figures came from some mil spec, probably you will have to test for shock and vibration.Do you have any connector on this design? With this kind of shocks, the contacts will chatter, and maybe even fail and break.

author: Simon
author_rank: Unknown
timestamp: April 14, 2014, 04:53:09 pm
content: yea we have connectors, honestly if it's really this sort of shock it's pretty gloomy. I was never told of shock requirements when i originally designed it so I'm sort of pissed off and starting to loose interest

author: KJDS
author_rank: Super Contributor
timestamp: April 14, 2014, 06:00:11 pm
content: Quote from: Simon on April 14, 2014, 04:53:09 pmI'd be surprised if you have any real grief unless you've got anything that is very vulnerable on the board. Connectors should be of a locking type but from memory 500g shock isn't very high for a lot of applications.

author: Simon
author_rank: Unknown
timestamp: April 14, 2014, 08:16:33 pm
content: I have got military grade pcb connectors, basically rugedized and version of molex that go down to -55C. But they are tall

author: mikeselectricstuff
author_rank: Super Contributor
timestamp: April 14, 2014, 08:36:45 pm
content: My instinct would be to go as small and light as possible - less mass = less force under acceleration.

author: Zero999
author_rank: Super Contributor
timestamp: April 14, 2014, 10:11:14 pm
content: Quote from: Simon on April 14, 2014, 03:54:00 pmSome tantalum capacitors are often in leaded 12069 packages.How about replacing some of those larger capacitors with several smaller leaded SMT tantalums? Of course ceramic capacitors and resistors can be replaced with through hole, if space will allow.

author: georges80
author_rank: Frequent Contributor
timestamp: April 14, 2014, 10:20:39 pm
content: I designed a high power LED driver years back that was going into housings (massive) that were used to light a couple of areas of the shuttle during liftoff. i.e. these poor lights were essentially at ground zero in terms of copping some serious badness.The capacitors (electros) sheared off their solder pads when the driver was run on a shake bed at the contractor testing facility. We ended up potting the entire board to have it survive some VERY serious vibrations, in the 10,000g range.Got to see a video INSIDE a bunker near the shuttle launch pad and all the equipment was on air suspension racks and even then everything was bouncing/swaying around. Mind boggling vibrations...cheers,george.

author: Niklas
author_rank: Frequent Contributor
timestamp: April 14, 2014, 11:23:16 pm
content: Are you going to thermo cycle the PCB? If so, be very careful and check the thermal expansion coefficient of the potting or conformal coating. I recently had an issue with SMD connectors, potting and thermo cycling and spoke to an expert at the Fraunhofer Institute about it. His comments were "If you can avoid potting, avoid it at all costs" and "SMD or through hole, both will crack, but the through hole mounted component will in general hold for a few more cycles together with potting". If the potting compound has a larger thermal expansion coefficient than the PCB and components and it can find its way under the components, then it will act as a small balloon and rip the components of the board.See if you can find some info about space electronics. I think they are using both glue and some kind of conformal coating.For the wiring, use multi-stranded wire, not the one with only 7 strands.

author: Simon
author_rank: Unknown
timestamp: April 15, 2014, 05:57:23 am
content: Quote from: Hero999 on April 14, 2014, 10:11:14 pmWell i told my boss I'd want to take the current electrolytics for ceramics that are much smaller and he asked if the current boards "support that" duh, well i could replace the electrolytic with one but I think i need more but they don't want to redo the boards.

author: mzzj
author_rank: Super Contributor
timestamp: April 15, 2014, 07:40:49 am
content: Quote from: Simon on April 14, 2014, 10:46:37 amHow is the previous hand-dryer project with 20m3/second supersonic airflow and 50$ blower motors going?

author: Simon
author_rank: Unknown
timestamp: April 15, 2014, 08:00:04 am
content: Quote from: mzzj on April 15, 2014, 07:40:49 amtechnically it worked out very well but we got lost in paperwork and the customer came up with new requirements at the last minute because they did not realize they'd have to tell us about something requiring a redesign. But for the hand drier their comment was "can it be turned down"so I think that bit of it worked very well, trick is to use a 1.1KW ametek vacuum cleaner motor which is the same design used by comercial hand driers unless your looking at the electric turbo that the dysons have.

author: TerraHertz
author_rank: Super Contributor
timestamp: April 15, 2014, 10:05:14 am
content: Quote from: Simon on April 14, 2014, 09:19:48 amProblem is, most potting compounds are at least a little flexible. Which means that at 500G the potting will be wanting to deform quite a bit, and will apply that force to the components that are mounted to the more rigid fiberglass PCB. How far do the components need to be pushed, to break PCB adhesion, tracks, through-holes, etc?Tldr; potting may make the problem worse, not better.I'd guess you'd have to somehow find a potting material with very similar modulus of elasticity to the PCB.

author: Simon
author_rank: Unknown
timestamp: April 15, 2014, 11:19:53 am
content: presumably the amount of deflection across the area of something like a 1206 part won't do much though will it ? the board is only 40x20mm

author: ConnorGames
author_rank: Regular Contributor
timestamp: April 15, 2014, 12:36:59 pm
content: Probably pretty obvious,but most xtals are not rated for 500G. You'll want to look for ones that are, or use MEMS oscillators, which need more power but are generally rated for ~10000G (IIRC). I am using the CTS 403 series for a personal project, it is rated for 1500G shock but says nothing about vibration in the datasheet.

author: Simon
author_rank: Unknown
timestamp: April 15, 2014, 12:40:20 pm
content: well I'm running off the watchdog timer in the MCU, it's just a timer circuit, infact I could have probably done it with a comparator and RC input, i think i used MCU because my boss wants to be able to change the delay in commissioning, he can reprogram it but not re-solder a resistor.

author: Richard Crowley
author_rank: Super Contributor
timestamp: April 15, 2014, 12:44:20 pm
content: Those of us who regularly dissect commercial products (some of us have been doing this since before Dave was born!) regularly see hot-glue "goop" squirted around larger components to hold them in place.Even more secure would be to simply "pot" the circuit, but I would strongly consider thru-hole vs. SMD.

author: Simon
author_rank: Unknown
timestamp: April 15, 2014, 12:46:03 pm
content: Well potting is what we came up with. why through hole ? the parts are much heavier and surely the flexing will break component leads.

author: Richard Crowley
author_rank: Super Contributor
timestamp: April 15, 2014, 12:53:22 pm
content: Quote from: Simon on April 15, 2014, 12:46:03 pmBut there is MUCH MORE MECHANICAL support for a thru-hole component. SMD components are mechanically attached only by a thin layer of solder. OTOH, it is true that they have lower mass, so that offsets the attachment problem.

author: Simon
author_rank: Unknown
timestamp: April 15, 2014, 12:54:38 pm
content: yes, in fact i am pleased that I am using through hole connectors

author: Alphatronique
author_rank: Regular Contributor
timestamp: April 16, 2014, 01:52:06 am
content: Himy PCB assembler experience ,go whit tiny SMD for part(0402) + potting + really good board supportand for connector trough hole was mandatoryif you not believe me ,do a simple test solder 2 wire to a 0402 part then make a pull testyou will find that before break connection it need couple of pond and since part have very low mass itmay handle lot of G and not to forgot that it potted to ..but secret was board support it must not have any flex on the PCB ,SMD not like flex ,but smaller was part less it affectedas for connector normally pin may accept more abuse and flex and you cannot reduce mass of connectorfor active part avoid BGA was preferable like SOIC or TQFP Chip maby QFN if it small part < 8mmthat may help you a bit ..http://en.wikipedia.org/wiki/File:M734_Amplifier_and_Oscillator_.jpgthink that motar fuse handel lot of G ;-)

author: luky315
author_rank: Regular Contributor
timestamp: April 16, 2014, 07:28:43 am
content: Can someone recommend a coating material which improves mechanical reliability and is applicable without special equipment?

author: Alphatronique
author_rank: Regular Contributor
timestamp: April 16, 2014, 02:15:28 pm
content: hithat largely depend on what you need and reason to pottingex: for high G protection you need something like epoxy ,that hard and not deformbut have matched Thermal expansion of the PCB and part for avoid stressor if you need to protect automotive under the hood stuff ,better to use kind of jelly stuffthis will better protect from environment and allow much less stress on extreme thermal clyclingMG. chemical made some potting epoxy stuff and available from mouserbut much betterhttp://www.keypolymer.com/potting-encapsulating/tough-seal/but this one have to much flex for Hight G