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Loctite on electrical connections

Discussion in 'General Electronics Discussion' started by kilou, Nov 21, 2014.

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  1. kilou

    kilou

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    Feb 22, 2014
    Hi,

    I'm using small PCB boards which are to be connected using eyelet crimped terminals and brass bolts/nuts. The bolts/nuts are M3. I'd like to prevent the nut from unscrewing because the system will be mounted on an electric bike (lots of vibration).

    I'm thinking about using a threadlocking compound such as Loctite 290 or Loctite 270. However I guess these fluids will increase the resistance because zhey aren't supposef to conduct. Do you see any objection to doing that?

    Is it better to use Loctite 3800/3888 which is a conductive epoxy? I cannot find M3 brass nylock nuts since most are made of steel or stainless steel with some coating in zinc (this may trigger galvanic corrosion with brass) so nylock is not an option nor are washers etc which would damage the pcb.

    What would you use on eyelet terminal to make a good contact with pcb yet prevent any nut from getting loose?

    Thanks
     
  2. davenn

    davenn Moderator

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    Sep 5, 2009
    hi there
    welcome :)

    show us a pic of what you are working with

    My initial thought would be ... screw up the eyelet onto the board with the nut, bolt, washer etc
    then just drip a small amount of Loctite to the top of the nut/bolt

    let me try and draw a pic ....
     
  3. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

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    Jan 21, 2010
    If the head of the bolt makes the connection, or the bolt holds the connectors against the board (i.e. you are not relying on conductivity through the bolt to the nut) then loctite will be fine.

    If you're concerned, you could tighten up the bolts then place a small amount of nail polish on the nut/thread. And of course, you're also using shakeproof washers, right?
     
    chopnhack and davenn like this.
  4. davenn

    davenn Moderator

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    gave up drawing a pic haha

    here's a photo of what I made up to demonstrate
    hopefully its along the same lines as what you are doing ? :)

    DSCF5646a.jpg

    OK my standard practice for doing bolted connections to a solder pad are
    1 ... bolt/screw through hole
    2 ... star washer (shake-proof washer)
    3 ... eyelet
    4 ... another star washer
    5 ... nut

    If you dripped a couple of drops of Loctite around the top of the thread where it comes out of the bolt
    it will seep down a couple of turns but wont get to or affect the electrical connection to the board :)

    cheers
    Dave
     
    KrisBlueNZ, chopnhack and Rleo6965 like this.
  5. davenn

    davenn Moderator

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    Sep 5, 2009
    nice Steve
    spot on ! :)

    you responded whilst I was busy taking photo etc
     
  6. chopnhack

    chopnhack

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    Apr 28, 2014
    Spot on, they make one just for that purpose - the liquid is green in color - 200 series - 290 or 220 from Loctite. Another thing you could try is copper based pipe thread sealant - plumbing section.
     
  7. kilou

    kilou

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    Feb 22, 2014
    Thanks for all your answers and suggestions!

    I can't post pics for now as I don't have the parts at hand, it's still on the todo list. But I will build a Y cable with ideal diodes to connect two Li-ion battery packs in parallel (with possibly different voltages). The ideal diodes are pictured here:

    www.re-voltage.eu/electronicsID80V2.html

    These have bolted connections and I want to stack two of them in parallel and connect them to wires using eyelet crimp connectors. This looks very much like your pic, davenn, except that two of the connections will happen through the bolt (connection to parallel the two diodes), the other being exactly like on your pic.

    I didn't think about using washers but I probably should. I'm just unsure about what you mean with star washers? Aren't these digging into the PCB board and damaging it? And is the electrical connection not better if the washer is completely flat (regular washer)? These connections must be able to handle 15-20A at 36V under vibrations (ebike)... I'd say using a star washer just between the nut and the eyelet crimp connector should be enough, no?

    Yes the Loctite 290 is supposed to be used when bolts/nuts are already in place and that included electrical connection (Loctite description). However this stuff is very fluid in order to seep into the thread while the nut is secured and I'm unsure it will increase the resistance overall. Yesterday I put a M3 bolt on a screw and dipped a drop of Loctite 270 (thicker as 290) and it still managed to seep on the whole thread of the nut (but no further). So I guess that 290 would be even worth. Maybe I should stick with the 270.
     
  8. chopnhack

    chopnhack

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    Apr 28, 2014
    Are they available? The bottom of that website you posted says he is out of stock since July!
     
  9. kilou

    kilou

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    Feb 22, 2014
    They are no more available for now but I do have them already as I ordered them last year already. I miss all the other stuff though (too many things to do and had no time to start the project yet).
     
  10. chopnhack

    chopnhack

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    Apr 28, 2014
    Good planning! Got lucky :)
     
  11. davenn

    davenn Moderator

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    that's the whole idea so it stops everything from rotating and working loose ( they don't dig in very far, just grip to the solder)


    these are the type I used in my pic above

    imagesTG3LLZ3A.jpg
    you said .....

    they will stop that, I seriously recommend you use them
    You may get away with just 1 between the eyelet and the nut, I tend to err on the side of caution when vibration is an issue :)

    cheers
    Dave
     
  12. kilou

    kilou

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    Feb 22, 2014
    Great! I'll try to find similar washers in brass for M3 bolts. I'd prefer stay with everything brass as mixing metals may potentially cause galvanic corrosion and increase the resistance I guess.

    Will go with the washers + a drop of Loctite 270 after assembly. Should do the trick.

    Thanks!
     
  13. davenn

    davenn Moderator

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    Sep 5, 2009

    good plan :)
     
  14. kilou

    kilou

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    Feb 22, 2014
    I can't seem to find shakeproof washers made of brass (found just one set for M4 on ebay but I need M3). Do you think that using washers in stainless steel or zinc plated could cause galvanic corrosion with brass? It seems that zinc plating could be a problem when put in contact with brass and thus the washer could corrode faster.

    Or would you know a source of shakeproof washers in brass where ordering small quantities is possible?
     
  15. chopnhack

    chopnhack

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    Apr 28, 2014
    Brass being so soft, the star washer would compress and probably not offer much "bite" - I don't think they make them. That being said, here is one alternative:
    Lock washer
    Or you can just use a double nut ;)

    That being said, I think we should start over and rethink this through:

    You said that the eyelet needs to touch the PCB - therefore the contact for the system is from the eyelet's mating surface to the copper pad on the PCB. If this is a single sided board, the bolt and nut are just a mechanical attaching method and need not enter the electrical equation other than to isolate them from unintended exposure. If the board is double sided, consider tapping the lead to both sides and again the stud/nut will only be of mechanical concern.

    For what its worth the resistivity of 5/95 solder is ~1.959E-07 while that of zinc is ~5.945E-08 ohm/m.
     
    Last edited: Nov 24, 2014
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  16. kilou

    kilou

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    Feb 22, 2014
    I don't exactly get what you mean by tapping the lead to both sides. You mean I should use an eyelet on both sides of the board and connect them together through a piece of wire?

    The board is two sided. For some connections, the eyelet just need to touch the board on one side. However two of the connections will be used to parallel the two diodes using brass standoffs as used on motherboards, such as this:

    [​IMG]

    In that case the eyelet will be placed on one of the two diodes and current must flow from the eyelet through the brass standoff in order to reach the second diode.

    I'm a bit skeptical regarding the use of a shakeproof washer made of zinc plated steel on brass nuts/bolts because as show on the table at http://www.engineersedge.com/galvanic_capatability.htm, the anodic index of zinc plating is far from that of brass and this could trigger galvanic corrosion that would slowly increase the resistance of the electrical connection. So also zinc in itself has a very low resistance, it is probably not a good idea to mix it with brass in the long run.

    Maybe I should just use standoffs, bolts and nuts in stainless steel in order to use stainless steel shakeproof washers. Or just go with your suggestion of using two nuts instead of the shakeproof washer: one of the nut would make the electrical connection and the second nut (on top of the first one) could be secured with strong Loctite (270). Seems not very elegant though....
     
  17. davenn

    davenn Moderator

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    Sep 5, 2009

    Yes definitely agree ... I wouldn't mix metals like that either
     
  18. chopnhack

    chopnhack

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    Apr 28, 2014
    Not through a piece of wire, I was intending to mean that the two leads are from the same wire and can each one side's pad thus eliminating the requirement of conduction through the post..

    I am not quite following what you mean by two of the connections will be parallel. See if this is right? Two eyelets need to be at each diode in parallel:
    If this is correct, the two eyelets can go over each other onto the pad for the diode and be locked down with your choice of hardware. I may be missing how you need your connection to go?
    upload_2014-11-23_9-38-19.png

    Certainly! I used to use a product called No-Alox when combining copper and aluminum connections for that very reason.

    Elegant enough to get the job done, if you can find it in brass, they make acorn nuts that are a bit more of a finished look ;)
     
    Last edited: Nov 24, 2014
  19. kilou

    kilou

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    Feb 22, 2014
    The parallel connection of the two diodes is pictured on page 6 of this PDF (3rd picture):
    http://www.re-voltage.eu/downloads/ID80V2datasheetR1.pdf

    You can see that the two diodes are stacked together and paralleled through spacers (see connection at the right of the diode: the wire is attached to the upper diode only and the spacer transmit current to the lower diode). Therefore the eyelet on these two connections is only attached to one of the two diodes, not both. Basically this means that two of the connections will be used to pass current from one diode to the other.
     
  20. kilou

    kilou

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    0
    Feb 22, 2014
    Is a two nut solution (+Loctite on the upper nut) as "vibration proof" as shakeproof washers? If so I'd go that route indeed! Thanks for the suggestion!
     
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