Vibration proofing SMD boards - Page 1
Forum section: electronics / beginners

author: stmdude
author_rank: Frequent Contributor
timestamp: September 15, 2015, 06:37:40 pm
content: Hi guys,I'm in the process of finishing off my first properly designed "product".However, there's one thing that I cannot find any information about, and its how I can make the board more resilient against vibrations.Some information about the board:The board is 25x25mm large, all components are SMD, with most of them on the top side (few caps on the bottom).The board is supposed to be used in RC cars and helicopters, so it will experience vibrations in the range of 0 to about 28KHz, as well as some good and proper impacts.The components are 0603 components, a 48 pin LQFP, a 20 pin QFN, one FA-238 crystal (this one worries me), and a SOT-223 LDO. So, nothing with a lot of mass.Now, the crystal worries me, as experience has taught me that they can break inside the component if exposed to too much vibration. Are there crystals that are more resilient against vibration?Pretty much the only thing I've managed to find out online is that leaded solder is better at coping with vibrations than lead-free, but apart from that nothing..So, should the components be glued to the board before soldering?Will conformal coating help?Should I encase it all in a block of epoxy?Where can I read up on this?

author: DaveW
author_rank: Frequent Contributor
timestamp: September 15, 2015, 07:53:33 pm
content: Well, Statek crystals will stand up to high shock. On the downside, they're the reason that my BOM costs for work are almost irrelevant as they make everything else look cheap in comparison.http://www.statek.com/products/pdf/CX3HGSM_10182_Rev%20A.pdfOn a more serious note, at pretty low vibration levels like yours, you are unlikely to have any issues with mounting those components, assuming that the board is well mounted and supported. Glue dot is helpful for heavy components, but in terms of mounting strength, I've had 0603 caps rip the copper tracks off the board (at around 1200g) without the solder joints failing.For general high vibration design, focus on making the board well supported, eg. lots of fixing points, thicker PCBs. However, when you've got components like the crystal which are internally vulnerable, it's good to give it some flexibility in the mounting, which will act as a mechanical low pass filter. Look into anti-vibration mounts if you do run into issues.Edit.Good information about electronic design and analysis for high vibration is the Dave Steinberg book, Preventing Thermal Cycling and Vibration Failures in Electronic Equipment

author: DimitriP
author_rank: Super Contributor
timestamp: September 15, 2015, 11:02:15 pm
content: Conformal or epoxy won't help as much as absorbing the vibration before it reaches the board by wrapping it in a sheet of foam like this one:http://hobbyking.com/hobbyking/store/__26457__Anti_Vibration_foam_Orange_Latex_190mm_x_140mm_x_6mm.html

author: AF6LJ
author_rank: Unknown
timestamp: September 15, 2015, 11:10:40 pm
content: Quote from: DimitriP on September 15, 2015, 11:02:15 pmWe used something similar to that when I worked at Loral.They had pre molded silastic rubber carriers for the crystals in high shock rated units. (1000 Gs and above) The above might be what you are looking for, or you may not need it at all.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: September 16, 2015, 05:37:07 am
content: Make sure the PCB is unable to flex. Heavier thickness as mentioned, or firm mounting to a rigid steel or aluminum backplate (which, in turn, has little flex across it, under all bending moments).The mechanical equivalent of electromagnetic compatibility (shock and vibe) is more or less equivalent, given that there are more mechanical degrees of freedom: x-y-z translational and rotational modes, where a single sprung mass may exhibit the same degenerate frequency of resonance in all modes, or the modes split into coupled peaks. The effects of coupling and multiple resonances manifest as a complex (multiple enveloped) ringing decay waveform following a transient. The nonlinearities are also more significant, as you often have marginally flush surfaces that slap into each other at different frequencies and intensities (and, when they flex apart, the gaps thus formed can exhibit new high frequency resonance peaks).The physics of coupled resonators is all the same as the electrical case, so if you want to dampen something, you can add a spring moment to it; but you must also apply damping to that system, otherwise it will resonate even worse at some frequency than for the fully stiff condition. The damping can be internal (usually lossy rubber or gel or other frictional media) or external (by matching the mechanical impedance to the surrounding system).Shock and vibe is usually tested on a vibrating table, where the "impedance" of the table can be considered to be very low (usually about a hundred kilos of aluminum, quite massive and stiff), so you will need to keep internal impedances low (or else they will resonate against that massive source), or add damping internally.Tim

author: DimitriP
author_rank: Super Contributor
timestamp: September 16, 2015, 06:08:29 am
content: QuoteI have the funny feeling you have more than eloquently and prolifically answered correctly ... the wrong question.If it's unable to flex, mounted on anything rigid all the vibrations and the force of impact will be directly transfered to the components.Car manufacturers spend more than a couple of bucks in order to make sure the force of impact is absorbed and doesn't reach the components err.. I mean the passengers of the vehicle , at full force.The scale is different but the principle is the same.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: September 16, 2015, 06:32:29 am
content: Hello Dimitri,I think you misunderstand the problem.For the specific case -- crystals -- acceleration alone can be a problem.For everything else (rigid and solid, like chip capacitors and most ICs), it's strain (physical stretching) that causes the problem. For these, transmitting the acceleration as uniformly as possible is the best possible solution.A good electromagnetic equivalent to this matter might be EFT (electrical fast transients): the amplitudes are so great that it issimply impossibleto filter the transient approaching your device. Rather, the device must surf over the entire wavefront, so that the local ground potential of the circuit becomes elevated over earth ground. When this is done uniformly, for all inputs, outputs and internal nodes, the circuit doesn't know a thing: it acts like a Faraday cage and experiences no voltage difference.The mechanical equivalent isn't perfect, because acceleration is an independent quantity, not relative. But strain is, and strain is what causes components to fracture and leads to fatigue.So you are correct to note that there are exceptions, which need to be treated accordingly.Regarding passengers: note that the total delta-V of the passengers must be precisely equal to that of the rest of the vehicle.It needn't be done in the same time frame, but if you study some crash test footage, you will see that, in fact, they are very much in the same range, and therefore the peak acceleration is only a few times less than that of the rest of the vehicle. Indeed, seat belts and airbagsare designed specifically to transmit more accelerationto the passengers, not less! They are designed in such a way as to do it in a relatively uniform manner (the belt stretches and carries equal tension along its length; the airbag transmits equal pressure across its face), an important feature for handling squishy "bags of mostly water".Tim

author: DimitriP
author_rank: Super Contributor
timestamp: September 16, 2015, 09:41:28 am
content: QuoteWe are obviously not going to get anywhere with this so as someone said before :QuoteProblem was stated, solutions were proposed and in the end someone will end up trying all , any or none and will figure out what worked and what didn't.

author: Cliff Matthews
author_rank: Unknown
timestamp: September 16, 2015, 09:56:38 am
content: Quote from: T3sl4co1l on September 16, 2015, 06:32:29 amDang! Now I'm off Trek, you just applied tangential force to an old Trek brain..That's the Trouble with Tribbling with Trek memories, they're just too hard to get rid of..

author: Gyro
author_rank: Super Contributor
timestamp: September 16, 2015, 10:17:07 am
content: Haha, well everybody seems to be rightAs has been said, board flex is a very good way of damaging SMDs (cracking capacitors and resistors or damage to their metalizations) ICs will typically survive better due to the flex in their leads.What you need is to rigidly mount the board to a bit of metal or whatever AND elastically mount the metal tp the chassis (foam or other suggested methods). The metal will stop the board flexing and the added mass will work better with the mountings to reduce peak accelerations that you're worried about with the crystal.Simples

author: DaveW
author_rank: Frequent Contributor
timestamp: September 16, 2015, 12:19:56 pm
content: Quote from: Gyro on September 16, 2015, 10:17:07 amThere are a couple of, to borrow Dave's phrase, traps for young players with regards to elastomeric mounts. Firstly, your board assembly will, to take an electrical analogy, act like an LRC circuit. There is likely to be peaking at certain points, which has the effect of amplifying the input displacement. It's therefore important that you ensure that the mechanical resonant points of the PCBA do not fall within excitation frequencies or frequencies where your components are vulnerable.Also, if your soft mounts bottom out, ie. they run out of travel then instead of a a vibration then you get a shock. This can cause extreme damage to your board and is extremely hard to ameliorate. Try to ensure that this doesn't happen! From my experience, T2sl4co11's information is spot on.Hope that helps!

author: T3sl4co1l
author_rank: Super Contributor
timestamp: September 16, 2015, 12:37:49 pm
content: And to add more refinement to it: you can think in terms of spring-mass, at least for the simple (first) harmonic modes. In general, modes appear due to transmission line effects, of course again, being in 3D through the material rather than, say, 1D cables, as is the most common case in E&M. You can estimate the mechanical impedance from the elastic modulus, density, dimensions, etc.This is important information because it informs you where the most beneficial places are to secure something like a PCB (which has sheet modes); you can position screws to do the most damping (suppressing or shifting the most low-order modes, and reducing their impedance so they resonate less as well), or identify where a particularly bad case exists.Consider the case of grasping a rod or bar about 20% from one end: it's able to resonate freely with a pleasing tone when struck, but not for most other positions.Tim

author: AF6LJ
author_rank: Unknown
timestamp: September 16, 2015, 01:43:25 pm
content: Quote from: DaveW on September 16, 2015, 12:19:56 pmWell put, a bit about vibration testing for those who may not have been exposed to it.Since in nature most of the vibration resembles varying colors of noise random noise vibration is usually done. Sine wave vibration is very destructive and doesn't accurately reflect real life situations.Soft mounting a whole board has benefits and liabilities. Often times in commercial gear a vibration dampening pad will be installed on a bottom cover near to the center of a circuit board to deaden the vibration that is set up in the board.The temptation is to just over engineer the board which will often do nothing but add weight and complexity to the product.I think the OP should see if any additional consideration in terms of mechanical design is necessary first. he has indicated that his product is to be used in an radio control product of some kind and that implies that size and weight are two of the most important parameters of that design.The vibration may only be a couple of Gs which would require almost no additional consideration. At this point nobody knows what amount of vibration the board will encounter.We all are shooting in the dark.

author: Gyro
author_rank: Super Contributor
timestamp: September 16, 2015, 03:23:27 pm
content: Ok guys so maybe I should have left out the "Simples", I have seen some extraordinary and unexpected excursions on the shaker table (and drop test too - who'd have thought that a PSU could deflect THAT far). Yes, vibration modes need to be swept and mountings, both PCB and chassis, need to be optimized. It is a non-trivial problem. Excursions at characteristic resonances need to be heavily damped.

author: helius
author_rank: Super Contributor
timestamp: September 16, 2015, 04:00:11 pm
content: I'm interested in hearing any experiences with FEA software to simulate mechanical resonances. What are the best tools for this today?

author: stmdude
author_rank: Frequent Contributor
timestamp: October 23, 2015, 07:32:26 am
content: Sorry for the the late reply, but better late than never.First, thanks a lot for all the replys, they certainly gave me a lot more insight than before.Some of you seemed to not quite understand the environment we're talking about, so here's an example (it's a spectacular display, so if you have few moments, enjoy the rules of physics being pushed to the limit)We're talking about vibration from the motor ~20K RPM, from the main rotor (~2K RPM), the tail rotor (~6K RPM), and then the resonances from everything. Oh, and the silly bugger trying to beat the snot out of it..Again, thanks for your insights. Much appreciated/J

author: T3sl4co1l
author_rank: Super Contributor
timestamp: October 23, 2015, 03:53:28 pm
content: Dang

author: Cliff Matthews
author_rank: Unknown
timestamp: October 24, 2015, 01:55:24 am
content: Quote from: stmdude on October 23, 2015, 07:32:26 amNice flying!Perhaps you can post a photo of the PCB? Those who know (I don't claim too) may want to see it mounted in situ. These are gas powered with totally variable pitch rotors right? Unless that video was playing fast with fake audio dubbed later, I can't see electrics pulling off those moves..

author: f5r5e5d
author_rank: Frequent Contributor
timestamp: October 24, 2015, 03:19:26 am
content: http://www.statek.com/products/pdf/Product%20Application%20Note%2014.pdfgives general principles - can search manufacturer's lit for high sock and acceleration sensitivity

author: crispy_tofu
author_rank: Super Contributor
timestamp: October 24, 2015, 04:56:43 am
content: Whoa - that's amazing.

author: f5r5e5d
author_rank: Frequent Contributor
timestamp: October 24, 2015, 08:48:08 am
content: http://www.sitime.com/technology/mems-oscillatorsclaims better than typ Quartz shock and vibration tolerance

author: pickle9000
author_rank: Super Contributor
timestamp: October 24, 2015, 09:06:20 am
content: Awesome vid.

author: Ian.M
author_rank: Super Contributor
timestamp: October 24, 2015, 09:57:49 am
content: Putting caps on the bottom was probably not the greatest design choice.At 1" square, if the components were top side only, conformal coating for hf vibration damping + environmental protection + putting 3M foam VHB tape on the bottom to act as a compliant mount would almost certainly have been sufficient provided it was attached to a stiff flat surface.Now you've got the problem of preventing brittle ceramic parts hammering into the mounting surface so will probably need to add weight and cost in the form of a mounting plate or frame. It may be possible to bond it onto a sheet of hard low density foam plastic, that has cavities cut out for the caps and still use VHB tape on the back of that. Alternatively, if heat dissipation isn't an issue, and it can be pre-wired, covering it in adhesive lined heatshrink with a dob of hotglue or similar where the wires connect so they bond effectively to the heatshrink, + some sort of mounting clip or VHB foam mounting pad may work well.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: October 24, 2015, 05:30:36 pm
content: Comes to mind, if there's much power on the board, it can be worthwhile to clamp it to the (ideally aluminum) backplate with some Gap-Pad(R) or similar material inbetween. The gooey side conforms around bottom side components, and provides good thermal conductivity (certainly better than air, or foam tape) and damping.Tim

author: NoItAint
author_rank: Contributor
timestamp: October 24, 2015, 05:46:41 pm
content: Wow!I threw up in my mouth just watching thatwhat's the G-force?How much is weight a factor? Seems like any solutions would add weight.Something else you can use besides a crystal?What's the board orientation? Wonder if sheer strength would matter?Nothing but questions from me as it's a totally amazing and fun looking challenge

author: stmdude
author_rank: Frequent Contributor
timestamp: November 10, 2015, 09:44:13 pm
content: Again, sorry for a late reply. I'll haveto figure out how to get email notifications from this forum.Pictures:https://goo.gl/photos/JsprsCm8LbY6HLvS6( It's the sub-board to the right in both pictures. The other board is just the debug stuff for it. Ignore the enormity of the wires on the debug-side, please.)Size: 25x25mmLegend:Large chip - Main MCU - STM32F030C8T6SOT-3 chip - LDO - LM317EMP (this thing isn't using that much power, so I decided to go for a KISS principled design here.QFN chip - RF Transceiver - Nordic Semiconductor nRF24L01+P1 8x3 holes - The servos connects here. 8 channels, three pins each.In the planned future, there will need to be an LNA on there as well to get more range.I have no case/mounting for it yet, as I was unsure about what to do about vibrations.I could probably make the board single-sided, as there's not that much on the bottom. Maybe replace the enormous LM317 with something smaller, and squeeze in that LNA as well?Now, for some answers:G-forces - For pro pilots like in the video, it should be around 10G substained, and up to 25G peaks. From first hand experience, it will launch your receiver battery right out _through_ the fibre-glass canopy if you don't fasten it securely.Weight - It's a factor, but less than one might think. Keep it under 100-150 grams, and it won't be a problem at all.Something other than a crystall - I use the internal RC on the STM32, but the RF chip needs some kind of precision time-source. I'll look over the datasheet and see if I can use an oscillator instead as suggested earlier.Board orientation - "It varies", as people all mount them differently to get more efficient cabling to the servos, gyros, sensors, etc. However, it's almost always perpendicular to one of the three axises.Power on the board - Very little. Using shitty servos that pull some current from the signal lines, I've seen ~100mA draw from my board. Good thing I have a 1.5A LDO on there..As for in-situ, I stopped flying myself a few years ago, and don't even have my equipment here. However, this is a typical setup for an electric heli:The thing I'm making would actually not even be visible in this picure. It's probably stuffed between the two CF plates that the battery is mounted beneath.Fuel-heli vs. Electric - Actually, once LiPos came to the market, the electrics are on-par with the fuel ones in performance, and in some cases better (wider power-band).

author: Ian.M
author_rank: Super Contributor
timestamp: November 11, 2015, 04:26:25 am
content: Sorry, cant ignore: *PLEASE* treat yourself to a reel of Kynar wirewrap wire!That looks fairly sane. If you can go down to a 200mA LDO reg and get enough space to get the caps topside, I don't think you'll have any problems with VHB tape mounting.You need to do a survey of good quality RC flight systems boards to see what crystals or osc. modules the competition is using, otherwise you may be solving a non-existent problem.Once the debug PCB has been cut off, it will be a royal PITA to regain programming/debug access. You may want to put pads on the bottom or a second row of PTH holes within the 1"x1" footprint, so it can be put in a pogo pin programming jig.