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When I solder a specific wire, solder won't stick and it leaves my iron tip black.

With many years of hobby electronics, I have never experienced this before. The wire is from a silicone encased LED strip and is dull silver in colour. I stripped back the silicone so that the wires were sticking out in free air. On attempting to solder them, I have the following issues:

  • The solder won't stick. I've tried scraping the wire to remove any coating, rubbing with my flux pen etc, but no luck. The wire has been hot enough to melt solder when touching only the wire, so plenty of heat.
  • The iron tip goes black. It appears as though anywhere that the solder "blob" has touched the iron and wire, the tip ends up black (not happy).

I'm not using any exotic solder. And like I said, I'm not new to this, but this has me stumped.

Has anyone seen this before? Can you offer any suggestions? At this point I don't want to go near it again without an idea of what I'm up against.

Additional info:

LED strip back LED strip front

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    \$\begingroup\$ A few pics might make someone remember they have seen something like that. What temp are you soldering at, what kind of solder, what flux, and what material might the wire be (i.e. is it magnetic, might it be aluminum [melts in flame] etc.) \$\endgroup\$ – PlasmaHH Nov 16 '15 at 23:32
  • \$\begingroup\$ @PlasmaHH, following your comment, I read up about soldering aluminium, and it sounds exactly like what I'm up against. I've added the additional info you requested. If you still think it's aluminium wire, put it in an answer and I'll accept it. \$\endgroup\$ – BlinkyBill Nov 17 '15 at 21:18
  • \$\begingroup\$ You can tell from the photo that even the Manufacturer didn't bother to properly solder the Al wire. The lead going from the bus wire to the first led is just wrapped around, and a poor attempt was made at soldering a single strand of that wire to the "flex-pcb". If you want to make this work I would recommend crimp terminals, but honestly I would get another light strip (5m of flex-pcb LED strip can be had for 10-20 USD) \$\endgroup\$ – crasic Nov 17 '15 at 22:00
  • \$\begingroup\$ Ta, and agreed. I've ordered some screw terminals as a short term fix. Won't bother with this stuff again. \$\endgroup\$ – BlinkyBill Nov 17 '15 at 22:13
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    \$\begingroup\$ Basically you need special flux for Al and perhaps mechanical abrasion beforehand: app.aws.org/wj/2004/02/046 \$\endgroup\$ – Fizz Nov 17 '15 at 22:35
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All of the mentioned symptoms (not accepting solder, silverish gray colour, non magnetic, cheap stuff) make me conclude that it most likely is aluminum.

Soldering properly to aluminum is almost impossible due its tendency to quickly form some oxide layer.

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