Ask A Scientist Chemistry Archive

name         Joshua K.
status       student
age          16

Question -   How does soldering flux work? What does it do to the
copper to allow it to bond?
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Joshua,

Most metals will react with oxygen in the air -- particularly so if the metal is heated. 
Soldering flux is formulated to remove a film of oxides from the metal and make the 
solder and metal more able to dissolve in each other. Of course, the flux cannot 
remove significant corrosion. That is why it is important to burnish the copper 
surface with something like clean steel wool before the flux is applied and soldering 
commenced. As you may know, it is wise to heat the metal to be soldered (not the solder 
itself) and then apply the solder and allow it to melt on contact with the pre-heated 
metal.

Regards,
ProfHoff 849
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Soldering flux is just a safe, convenient acid for dissolving the oxide skin off the metal 
you want your solder to wet well.  Also dissolves oxide off the liquid solder, making it 
less crusty and therefore more shiny.

Wetting is like dissolving; remember the old saying "like dissolves like".
Oxide is too different from melted metals like tin and lead, to be wetted by them.

Metals have oxide skins because they are usually sitting in air, which has very reactive oxygen.
But low-melting metals like solder (tin and lead mostly)  don't wet oxides, they only wet other metals,
and only those other metals which they can dissolve a little bit of.
Copper, silver, gold always dissolve to a small extent in melted tin or lead.
Then when the solder cools and solidifies, the solder right next to the copper is an 
alloy of the solder and the copper.
If two metals can "alloy", they can be one continuous body of metal, all strongly 
bonded together, with no sharp seam between them.

"Acid" flux is the stronger class of flux; it has something like hydrochloric acid in 
it. (The paste form has zinc chloride.)
This is good for making difficult oxides dissolve so difficult metals like stainless 
steel can be solder-wetted.
But the acid can hang around later trying to corrode the metal it just cleaned for you.
So for electronic stuff we mostly do not use it.
If we do, we scrub it off with things like toothbrush, water, soap, alcohol, baking soda, 
to minimize acid residues.

The flux built into most solder wire is called rosin flux.
I think it is an organic acid (so is vinegar, and tart-tasting "citric acid"),
stuck onto larger molecules that melt only at soldering temperatures.
That is the clear yellow-brownish plaque that sits on the solder's surface when you are 
done.
It does the same stuff as acid flux, but it is milder two ways.
It is only strong enough to reduce weakly oxidizable metals like copper, tin, lead, 
silver.
So it is just strong enough for electronics use, but not for soldering to stainless 
steel or iron or anything with chrome or aluminum.
And rosin-flux goes back to its plastic-like solid form after use, so it does not act 
very corrosive to the metals later on.
So we do not need to clean it away carefully.
It can be cleaned away if you want to work at it, with brush-scrubbing and the right 
"polar solvents".
"Flux remover" is sold in spray-cans for this.  Rubbing alcohol with a dash of dish-soap 
sort of works for me.
Try it and see what it looks like.

Jim Swenson
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