EEVblog Electronics Community Forum
Electronics => Projects, Designs, and Technical Stuff => Topic started by: rob77 on May 06, 2017, 01:53:06 pm
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Hi Guys,
what's the best way to mechanically secure a big SMD inductor ? for small-ish ones i have no problem to rely on solder and adhesion of the pads to the board... but i'm working on a project wehere i need some bigger ones.
the inductor is 22x22mm and it's weight is 26grams - that's a lot if we consider vibrations and occasional drops.
it's a one-off design only 3-4 devices will be assembled manually so exotic solutions wouldn't be a problem, but i would like to avoid spring loaded clips or other mechanical solutions requiring cut-outs in the board. was considering gluing them to the board but the inductors might run a bit hot (40C above ambient) and i would like to have the devices serviceable so epoxy is out of question..
also attached a photo of the actual inductor next to my another board with more "normal" sized inductors which are only soldered (weight only 4grams).
any suggestions are very welcome !
thanks,
Rob.
(https://www.eevblog.com/forum/projects/how-to-secure-a-big-smd-intuctor/?action=dlattach;attach=313418)
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If you already have a mixed design why not switch to TH inductor, they also might have better specs .
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If you already have a mixed design why not switch to TH inductor, they also might have better specs .
the one i'm working on should be SMD only, because i'm considering a metal core PCB - that's why to avoid cut-outs if possible.
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Give it a zip-tie or two. Yes, that means holes in the PCB, but only simple ones without through-plating etc.
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What about the red glue used for securing larger SMD components during reflow? If it can withstand the temperatures during reflow, it can surely take the operating temperature of the inductor.
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those big tabs will hold 26g just fine no need glue. but if you insist, other than hi-temp glue usually used in china smps to hold hundreds of grams of components supplied from who knows where, you may try creating isolated or gnded pads on both sides, strip a strand of copper wire from its sheath and solder it to the pads wrapped on top of the inductor. not a neat hack but should do for an smd harness. making a custom size retainer like this coin cell's could be a bit neater... http://elcodis.com/parts/5896/3002.html (http://elcodis.com/parts/5896/3002.html)
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Same idea but with copper foil tape along the bottom of either side of it. A nice solder fillet the whole length of two sides should be pretty secure.
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the inductor is 22x22mm and it's weight is 26grams - that's a lot if we consider vibrations and occasional drops.
it's a one-off design only 3-4 devices will be assembled manually so exotic solutions wouldn't be a problem, but i would like to avoid spring loaded clips or other mechanical solutions requiring cut-outs in the board. was considering gluing them to the board but the inductors might run a bit hot (40C above ambient) and i would like to have the devices serviceable so epoxy is out of question...
You've pretty much eliminated all of the reasonable ideas so I have to ask what the true risk of being exposed to vibration/dropping is. If this device is not mounted on a moving vehicle or handheld then it probably won't be at much risk of damage from dropping/vibration. That said, the traditional approach to dealing with vibration damping at the component level is electronics-safe silicone (look for the type that says it emits methanol/ammonia during curing, NOT acetic [ethanoic?] acid).
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thanks for all the suggestions !
the device will be portable and very often transported in a car , so both vibrations and minor drops will be quite common.
in the meantime i decided to definitely go with a metalcore pcb with aluminium case (heat sinking solved), so definitely can't do cut-outs to PCB.
@Mechatrommer,
i like the clip idea ! - that would increase the amount of points fixing the inductor to PCB to 4 so it will have a point of fixation on each side (not just the 2 tabs)
@MagicSmoker,
i think the silicone will limit the serviceability somewhat (have to be cut out and replaced each time) but that's not a big deal.
i think i will go for the clip to increase the holding strength to PCB but combined with a high-temp silicone blob between the inductors and the top of the alu case. i just need to find flat enough capacitors, or the top part of the case will have to have a block of aluminum attached in the area of the inductors.
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For the best long term reliability, I would use positive retention like a wire tie through the printed circuit board.
But how well would that thick cyanoacrylate glue available at hobby shops work if applied at the interface between the inductor and printed circuit board?
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Some inductors rated for high vibration have an additional pad to help secure to the board
e.g. http://www.coilcraft.com/ser3018h.cfm (http://www.coilcraft.com/ser3018h.cfm)
All other good methods that I've seen involve holes in the board and securing it to the board or chassis with some form of fastener.
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For the best long term reliability, I would use positive retention like a wire tie through the printed circuit board.
But how well would that thick cyanoacrylate glue available at hobby shops work if applied at the interface between the inductor and printed circuit board?
cyanoacrylate don't like heat, it will break down. even if it would withstand the operating temperature of the coil (might be even 90C) it will definitely breakdown during reflow of the board.
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Some inductors rated for high vibration have an additional pad to help secure to the board
e.g. http://www.coilcraft.com/ser3018h.cfm (http://www.coilcraft.com/ser3018h.cfm)
All other good methods that I've seen involve holes in the board and securing it to the board or chassis with some form of fastener.
good catch :) and actually TME has similar ones http://www.tme.eu/gb/details/hci2717-100/smd-power-inductors/ferrocore/ (http://www.tme.eu/gb/details/hci2717-100/smd-power-inductors/ferrocore/) so going to order them and try it.
i can't do cutouts because the metal core PCB will be in contact with alu case to transfer heat. holes through both PCB and case with bolts holding a retaining clip would be possible but i don't like the idea having such a fastener holding the inductor exposed to outside.
so it remains either the 2 tab inductor with a retaining "clip" made of bent copper strip adding 2 extra soldering tabs, or the 3 tab inductor i'm going to order + the hightemp silicone blob between inductor and case.
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Or copper tape on the sides right down to the bottom corner and a solder fillet the whole length of each side as I suggested earlier. Use a tape with a good heat resistant adhesive. It *may* be possible to wrap the tape right over the top, but that increases the risk of eddy current losses from the fringing fields. I prefer the tape because it would be difficult to get your proposed copper strap in close enough contact to the inductor to properly support it unless you glue it on before soldering.
I wouldn't silicone blob to the case - that's a maintenance nightmare. One of those squishy thick heat transfer pads with adhesive on one side of it stuck to the top of the inductor would be good, as long as it will easily pull free from the case for maintenance.
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Or copper tape on the sides right down to the bottom corner and a solder fillet the whole length of each side as I suggested earlier. Use a tape with a good heat resistant adhesive. It *may* be possible to wrap the tape right over the top, but that increases the risk of eddy current losses from the fringing fields. I prefer the tape because it would be difficult to get your proposed copper strap in close enough contact to the inductor to properly support it unless you glue it on before soldering.
I wouldn't silicone blob to the case - that's a maintenance nightmare. One of those squishy thick heat transfer pads with adhesive on one side of it stuck to the top of the inductor would be good, as long as it will easily pull free from the case for maintenance.
finally i got your point ! that's also a great idea ! and actually less effort because of no need to precisely bent the clip, thanks ! :-+ those thick squishy heat transfer pads are very weak though.. those can't withstand any lateral force without shearing so they will not dampen the vibrations. so i will go for the silicone but probably put a piece of some tape to the top cover where the silicone will contact it (that will give me the weak-point for disassembly).
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For the best long term reliability, I would use positive retention like a wire tie through the printed circuit board.
But how well would that thick cyanoacrylate glue available at hobby shops work if applied at the interface between the inductor and printed circuit board?
cyanoacrylate don't like heat, it will break down. even if it would withstand the operating temperature of the coil (might be even 90C) it will definitely breakdown during reflow of the board.
It would need to be added after soldering, doing so before is just asking for trouble, and 90C is way above what I consider a reliable case temperature (Tc) except in special applications.
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For the best long term reliability, I would use positive retention like a wire tie through the printed circuit board.
But how well would that thick cyanoacrylate glue available at hobby shops work if applied at the interface between the inductor and printed circuit board?
cyanoacrylate don't like heat, it will break down. even if it would withstand the operating temperature of the coil (might be even 90C) it will definitely breakdown during reflow of the board.
It would need to be added after soldering, doing so before is just asking for trouble, and 90C is way above what I consider a reliable case temperature (Tc) except in special applications.
so solder the inductor and then inject cyanoacrylate into the almost nonexistent gap ? that doesn't work that way i think. you could try to use the thinnest cyanoacrylate and hope it will penetrate that tiny tiny gap, but that's definitely not how you should glue things.
and what's wrong with 90C inductor ? 50C worst case ambient + 40C above ambient gives you 90C worst case while the inductor is specced to operating temp of 125C.. i see nothing wrong there.
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Thin cyanoacrylate wicks into tiny gaps very well by capillary action. As long as you have effectively defluxed the board the result will be effective - maybe scuff up the PCB surface with a fiberglass pencil and the bottom of the inductor with wet&dry paper, then wipe both with a lint free cloth and IPA, before placement and soldering so the surfaces are keyed for better adhesion.
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yes thin cyanoacrylate wicks into gaps, that's why i said you might try a thin one.. but also thin CA is bad at "filling up" those gaps while providing mechanical strength. thin CA is great for contact gluing smooth surfaces while thick CA is great for contact gluing more rough surfaces because thick CA has relatively good "gap filling" properties.
in general thin CA is good for "closing" porous surfaces but i wouldn't rely on the mechanical properties of such a joint because you never know how deep the penetration of the CA under the part was.
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in general thin CA is good for "closing" porous surfaces but i wouldn't rely on the mechanical properties of such a joint because you never know how deep the penetration of the CA under the part was.
That's why you apply it along one edge with the board tilted 45 degrees, till you see the other edge is fully wetted, then wipe off the excess. If the gap is thin enough, it will be reasonably strong but as thin CA tends to be brittle, nowhere near as good as initially gluing it down with Epoxy.
Many epoxies soften at high temperatures, so why not simply pick an epoxy that has reasonable strength at 100 deg C, but softens by 150 deg C, apply before hand soldering the inductor, then for rework, put the board on a preheater to 100deg C and heat the inductor with hot air till its solder joints melt, and the epoxy joint should be hot enough for it to easily twist free.
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Use a thermoset single component epoxy, the same stuff used on the rest of the components. You will need to preheat the board to cure it slightly before reflow, and as you likely will have a very high thermal mass to solder in your inductor it is likely that you will have to spot reflow the inductor after the rest of the board has been done, unless you use a Galden bath to do all at once with minimal thermal shock. Bonus of the Galden is it is also an incredibly effective flux remover.
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http://www.newark.com/dow-corning/3145-90ml/adhesive-sealant-tube-90ml-clear/dp/00Z1764 (http://www.newark.com/dow-corning/3145-90ml/adhesive-sealant-tube-90ml-clear/dp/00Z1764)
This is what it is used on the oil industry to secure components on pcb boards..
Operating temperature range of -45°C to 200°C
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I doubt a silicone adhesive/sealant will significantly help a SMD inductor stay on the board as its modulus of elasticity is so much lower than that of the copper pads and solder, that it will do little to prevent the pads and solder joint being stressed past their yield point. OTOH for cores with leads direct to PCB (no pins or base) with enough slack in the leads to allow them to flex, if applied to a properly cleaned and prepared surface for maximum adhesion, it would be ideal.
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This is harder also used on oil and gas
http://www.emiuv.com/products/ (http://www.emiuv.com/products/)
http://www.emiuv.com/media/files/EMCAST_Selector_Guide.pdf (http://www.emiuv.com/media/files/EMCAST_Selector_Guide.pdf)
take a look at 2401