Vibration protection - Page 1
Forum section: electronics / projects

author: TomS_
author_rank: Frequent Contributor
timestamp: September 13, 2015, 08:56:08 pm
content: Hi everyone.Ive just done a repair to a small circuit board that drives a seven segment display which indicates which gear my bike is in. Well, when I say repair I actually mean a total rebuild.I completely removed all of the original components and soldered in new ones because trying to repair what looked like some crusty and dry solder joints just wasnt happening.The original components were all gunked up in some kind of glue/silicon, and I am wondering what I should do about my repair.Im guessing that the components left on their own will suffer quite badly from vibration if they arent held more firmly in place, but Im wondering what the likely outcome will be in terms of the damage they will incur? Is it a case of solder joints breaking thus creating intermittent contact, or ..Anyway, attached is a photo showing how the components are now, and Im wondering how I could best go about making sure it stays reliable for years to come. It was a pain having to rebuild it as it is, so I dont want to have to do it again (TBH I dont think I will be able to do it again as some of the PCB traces came off during the re-soldering process, and some others already look to be loose, so the PCB has limited life left.)As a simple quick fix, can I just use some regular sealing silicon to gunk up my components or should I be using something else? Im guessing I may also need to account for higher temperatures, given the board is in a little connector box directly on top of the engine.Can anyone recommend a method/products?Thanks!Tom

author: ovnr
author_rank: Frequent Contributor
timestamp: September 13, 2015, 09:12:04 pm
content: Do not use any "regular" silicone. It will likely corrode the shit out of your components, as it's an acid-cure silicone.Find an electronics-grade RTV silicone, or consider buying a two-component (ideally platinum-cure) silicone and pot the whole thing in it.

author: TomS_
author_rank: Frequent Contributor
timestamp: September 14, 2015, 11:50:47 am
content: Quote from: ovnr on September 13, 2015, 09:12:04 pmThanks for your response.Ive found this:http://uk.rs-online.com/web/p/potting-compounds/0283460/It says its designed for electronic potting applications, so its non corrosive. Specific to my needs it also has a nice temperature range up to 260 degrees C (though I doubt I'll get that high before my bike stops working) and has some vibration and flame resistance to boot.Per your recommendation, the datasheet says "QSil553 contains the platinum catalyst".Looks like a winner.

author: ovnr
author_rank: Frequent Contributor
timestamp: September 14, 2015, 09:09:11 pm
content: Looks like a nice potting compound. Bit pricey, but should be fine - also, once you remove the clamp and mix it, you have to use it all. I personally just buymoldmaking silicone- it doesn't stick as well (kinda the point), but it's way cheaper and will do just as well for vibration damping. In any event, be aware that some silicone chemistries are a bit finicky, and will not cure in the presence of certain compounds. I imagine the potting compound is fairly reliable, though - the mold-making silicone I linked failed to harden in contact with a random ribbon cable I used, which was incredibly amusing to find out.Be sure to mask off any connectors and such; don't want to get it up in there.

author: TomS_
author_rank: Frequent Contributor
timestamp: September 15, 2015, 09:32:26 am
content: Yeah its by far the most expensive part of this project. The components (40 diodes and 10 resistors) came to only £1.42 with free next day delivery!Oh well. Its got to be done.Thanks for your help!

author: MagicSmoker
author_rank: Super Contributor
timestamp: September 15, 2015, 09:57:22 am
content: Quote from: ovnr on September 13, 2015, 09:12:04 pmThis is correct and good advice. However, here in the US (ie - not sure if this applies elsewhere) there are two types of silicone available at hardware stores: the usual "acid-cure" type, which smells strongly of vinegar because it emits acetic acid during curing), and another type which emits methanol and ammonia during curing which is more or less safe for electronics use (an example of such that is available in the US is "Silicone II" by GE [as in, General Electric]).I would reserve the Pt-cure silicone for truly sensitive applications, and/or for products that have to be potted and sealed at the same time.

author: jitter
author_rank: Frequent Contributor
timestamp: September 15, 2015, 03:37:50 pm
content: Indeed. Acid-free silicone sealant is available over here too (e.g.Griffon S-200or similar). We regularly use this stuff on big parts prone to vibration (e.g. big electrolytics) in the production of industrial electronics.

author: AF6LJ
author_rank: Unknown
timestamp: September 15, 2015, 03:53:09 pm
content: Well I can tell you the first mistake you made, and this is common so don't feel bad about it.Any part that is going to be in a high vibration enviroment must have some some of its leads exposed to support the part above the board. Your glass diodes should have a couple of mm of clearance between the board and the glass body.Even if you pot it one or more of them will fail so will the resistors.

author: TomS_
author_rank: Frequent Contributor
timestamp: September 16, 2015, 03:48:59 am
content: Quote from: AF6LJ on September 15, 2015, 03:53:09 pmThanks for the tip!Ive actually been wondering if I might try and find some local help to design and produce a PCB, and use SMT components instead. I dont suppose they would be any more likely to survive longer in such an environment?I mean, I could always just get a new gear indicator, but as an interest exercise its appealing - and where would be the fun?

author: AF6LJ
author_rank: Unknown
timestamp: September 16, 2015, 03:51:57 am
content: Quote from: TomS_ on September 16, 2015, 03:48:59 amThey will survive much better, that actually is a good idea.Make the board thicker than normal and conformal coat it when you have it tested and finished.If you can use lead solder, as others here have pointed out in other threads it will hold up better.

author: CatalinaWOW
author_rank: Super Contributor
timestamp: September 16, 2015, 04:30:47 am
content: Another approach if you don't want to get into SMD assembly is to design two circuit cards - one for each end of the components. This packaging technique is known (in the US anyway) as cordwood packaging. It is a pain to design but for a device of this complexity is very doable. It is also somewhat of a pain to assemble, but particularly with the addition of some potting is very robust. It won't be quite as good as SMD (less mass is almost always good for vibration survival). You still need to give some lead spacing to the boards.

author: e100
author_rank: Frequent Contributor
timestamp: September 17, 2015, 06:20:57 am
content: From experience I can tell you that general purpose platinum cured moulding silicone, non acidic and acetic acid silicone rubber (from your local hardware store) all have very poor adhesion to the components typically used on a PCB. The only thing that reliably sticks to cured silicone rubber is more silicone rubber. As a vibration dampener its OK, but it isn't going to keep out moisture, just limit the rate at which it wicks its way down through the exposed wires.If you have a look at the Cypres Parachute AAD Teardown,you'll see they used re-entrant potting compound which really does stick, and therefore will keep the water out.

author: ez24
author_rank: Super Contributor
timestamp: September 17, 2015, 06:30:55 am
content: Sorry if I missed it but how do you do this? IE determine which gear you are in. What kind of sensors do you use?thanks

author: DanielS
author_rank: Frequent Contributor
timestamp: September 17, 2015, 03:19:43 pm
content: Quote from: CatalinaWOW on September 16, 2015, 04:30:47 amIf your PCB design package allows creating parts with overlapping pads on different layers, possibly using blind via-in-pad for holes just to make DRC shut up, you can simply create a 2/4 layer board, then split them between top and bottom PCB.

author: nfmax
author_rank: Super Contributor
timestamp: September 17, 2015, 03:58:32 pm
content: Quote from: MagicSmoker on September 15, 2015, 09:57:22 amBeware! Polycarbonate plastics (e.g. Makrolon) are attacked by ammonia...

author: TomS_
author_rank: Frequent Contributor
timestamp: September 25, 2015, 02:08:22 pm
content: Brilliant information on this forum.Thanks everyone for their input. Im just potting it up now as I type. It will do for now I guess, maybe give me a few more years. If not I'll look at producing an SMD based board, been watching some of Daves videos on this subject and its got me thinking I want to give it a try!Might even be able to sell it as a replacement on ebay or something - Im sure there are others out there that have got similar problems!

author: AF6LJ
author_rank: Unknown
timestamp: September 25, 2015, 02:09:53 pm
content: Good luck, I hope it works out for you.

author: TomS_
author_rank: Frequent Contributor
timestamp: September 25, 2015, 04:16:31 pm
content: Quote from: ez24 on September 17, 2015, 06:30:55 amConveniently on my bike the gearbox has effectively a rotary position switch to indicate which gear it is in, so it s quite easy to use a simple diode matrix to display the actual gear afterwards. I guess you need a special connector to get access to this switch. The gear indicator was on my bike when I bought it, so Im not sure exactly what kind of connector it is, its behind the sprocket cover and I havent gone poking around in there yet...Others interface with the tacho and speedo and make a guess which gear youre in based on revs and speed.