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I need to solder a wire (tinned coper) to a massive copper busbar, I tried two approaches but to no avail.

I tried heating up the busbar with a heat plate to 300 °F (150 °C) and then tried to solder the wire to the busbar a) with a soldering iron or b) with a heat fan but it didn't work: The solder (we use SAC) doesn't melt properly (it doens'ntbecome fluid, only some small balls are formed) and the copper in the busbar started to change color, which I gather is oxidation in the copper.

Do you guys have any idea how to accomplish this?

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    \$\begingroup\$ Add flux. Or drill a hole and a ring terminal, bolt, and nut. \$\endgroup\$
    – DKNguyen
    Apr 9 '20 at 21:08
  • \$\begingroup\$ Thanks @DKNguyen for your hint. I already tried preheating and the solder I'm using has already flux in it. The iron tip is a pretty big one (200 W) \$\endgroup\$
    – Ken Grimes
    Apr 9 '20 at 21:09
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    \$\begingroup\$ It still doesn't work? Maybe the oxide layer is too thick for the flux to get through on top of all the extra oxidation from heating. What if you use a scour pad to polish the copper first? \$\endgroup\$
    – DKNguyen
    Apr 9 '20 at 21:12
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    \$\begingroup\$ Also, just a thought: add flux before you start preheating and heating. Maybe that will help seal it off from the air as it heats up. Then add more if required once you start soldering. \$\endgroup\$
    – DKNguyen
    Apr 9 '20 at 21:34
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    \$\begingroup\$ If you have ever sweat copper pipe for water... Do the same thing here. Use a torch, and proper flux. Sand the copper first, heat, then bring wire close and heat both while applying solder to joint. Do not put solder in flame. Solder should flow when it touches the hot metal. \$\endgroup\$
    – MadHatter
    Apr 9 '20 at 22:28
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My idea of massive may be different from what your idea of massive is, but if you use a propane or MAPP torch you should be able to solder to the bus bar. I have this set for small jobs that require a bit of oomph (but not so much that an oxyacetylene torch is called for):

enter image description here

You want to heat the copper away from where the joint is to be, so that the solder melts onto the copper, rather than directing the heat at the solder and having it turn into nasty little balls.

You can even silver solder (hard solder) to get a physically strong joint, though that requires a higher temperature and will tend to destroy any insulation on the wire you're attaching.

Copper has very high thermal conductivity (which typically goes hand-in-glove with electrical conductivity for physics reasons) so it's always going to be difficult to solder to if there's a lot of it. You also need to make the copper clean and bright before attempting (and use a good flux).

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  • \$\begingroup\$ Thanks! I will try with a torch and flux. Any recommendations for a good flux? \$\endgroup\$
    – Ken Grimes
    Apr 10 '20 at 7:15
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    \$\begingroup\$ For soft soldering, the flux in the solder should be adequate if you heat indirectly, have a bright clean surface and don’t overheat it. The plumbers acid core flux is more active but usually we want it nowhere near electronics- it has to be washed off very well because it is conductive. For brazing, there are some standard white paste fluxes that work really well and again should be cleaned off well. \$\endgroup\$ Apr 10 '20 at 17:31
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Crimp a ring lug onto the wire, and use a screw and lockwasher to secure it to the busbar. You can either tap threads into the busbar itself, or use a nut on the other side.

Soldering makes no sense on a busbar, since you need to mechanically secure the joint before soldering anyway. NEVER use solder as a structural element!

And when you do need to solder large items together, use a torch, not an iron!

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  • \$\begingroup\$ The contact resistance depends on torque and contact area of threads and crimp lug then wire but often the 2 stage crimp quality is the limiting factor. The OP must define the ground resistance max in μΩ or mΩ and inductance @ 1uH/cm and make contact a gas-tight seal. \$\endgroup\$ Apr 9 '20 at 21:57

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