WWhy can't solder onto metal?

[Harry]

This has been bugging me. whenever I try to solder the grounding on a pot's shell or the grounding on the bridge the solder won't stick. What am I doing wrong? Do you drip the solder onto it or what?

[PenPen]

You have to heat the surface you are soldering to. Solder only 'sticks' to heated surfaces. Use a wide tip and high wattage iron to heat the back of the pot and wire or whatever you are trying to solder, then touch the solder to the pot near the iron. If done right, the solder will flow towards the heat source.

[R.G.]

Just to amplify a bit;

You need to have:
(a) the right metals to be soldered. Metals like aluminum cannot be soft soldered (i.e. with tin-lead electronic solder) because of their surface oxidation habits.
(b) flux; presumably this is from your cored solder
(c) the metals to be soldered hot enough to melt the solder themselves.

If you don't satisfy any one of these, the molten solder will likely not wet (stick to) the metal.

Some paints or coverings on metal prevent solder sticking. Clean a spot on the back of the pot you're going to solder with fine sandpaper or steel wool until it is shiny. Then heat the pot casing with a well tinned iron. The iron needs to be in good condition to be able to transfer enough heat. Finally, touch the solder to the pot case, not the iron. When the pot case itself melts the solder and the solder flows out smoothly, you can get a good joint.

[gez]

Metals like aluminum cannot be soft soldered (i.e. with tin-lead electronic solder) because of their surface oxidation habits.

Out of interest, how do you solder to aluminium?  What is required?

Or is this a dumb question? 

[mutant_guitar]

this is a common problem in the R/C car world when assembling battery packs. do everything else that was mentioned in this thread, and something that wasn't mentioned that helps is to scuff up the surface you want the solder to stick to. i use a screwdriver to scuff things up, other R/C car driver will use a dremmel. hope this helps. 

[markr04]

I do the same thing Mutant said. I take a file or Dremel and rough the surface to be soldered to. Solder sticks to that easily, unless is it's aluminum.

[R.G.]

Aluminum is very chemically reactive. When exposed to oxygen, it oxidizes in a few seconds to a layer of oxides that eventually stop the contact with air. So all aluminum has a conformal coating of oxides. Solder will not wet (stick to) the oxides at all. You can sand, scrape, or cut away the oxide layer and solder to the aluminum if you prevent oxygen from getting to it. I've seen this done by sanding and covering the sanded part with mineral oil. There are also special fluxes that will eat away the oxide layer. The fluxes are acids, and generally nasty stuff.

It can be done, but is not practical for electronics assembly in general.

[Harry]

Alright, thanks. I did everything mentioned except scuffing, must be the problem.

[grapefruit]

For pots I use a bench grinder. Just watch your fingers and wear eye protection
Good quality solder can make a difference too. I only use multicore these days, though I would like to try a lead free solder.

Stew

[Paul Perry (Frostwave)]

There was a book on elephant hunting once, called "Use Enough Gun". I guess for solderign to pots, you need to "Use Enough Iron". That doesn't necessarily mean a physically huge iron, just one tht can pump out enough heat to get the pot shell surface hot enough for the solder to bond. Try filing, then just putting solder on teh shell (you will be able to see if it sticks). Then try soldering the wire on.
Of course, the "cheat" method would be to use a washer with a solder tag on the pot shaft

[mutant_guitar]

i guess it should be noted that when we solder batteries in the r/c hobby we all use 40+ watt irons, and i use the same iron for soldering the wires on back of the pots for ground. i think it would be pretty hard to get the job done with a 15-20 watt pencil tip.