[sdiy] O sweet, magical flux

[KHeck73 at aol.com]

In a message dated 10/3/2001 10:00:53 AM Eastern Daylight Time, 
jdarby at lplizard.com writes:



> What is it about
> flux that makes it tame solder so well? Does it do something to the metals
> chemically to make solder adhere and flow  better? It also seemed to help
> 


Fluxes are compounds that, when liquid, will dissolve or reduce metal oxides. 
Reduction is the opposite of oxidation. When bare metals are exposed to air, 
oxygen joins with the metal to form an oxide. This is usually a film on the 
surface of the metal, and heat speeds up the reaction. Oxide films inhibit 
heat flow, prevent good wetting action (liquids bead up), have high melting 
points, and in most cases are insulators. Also, forming oxides within molten 
metal (the solder) makes it slushy. It then agglomerates into a slag that, if 
trapped, makes the solidified metal less sound.

When you melt with the proper flux, oxygen atoms prefer to associate with the 
flux molecules rather than the metal atoms, and that's the cleaning action. 
The liquified flux also helps transfer heat to the joint while preventing 
oxidation of the joined pieces and the solder. If the solder oxidizes 
extensively while molten, you get a weak 'cold' joint from the formation of 
fine oxide particles. For example, you can observe the dulling of a joint if 
you keep heating it past the point of burning off rosin flux. The solder gets 
dull and clumpy as it oxidizes. Don't make joints that look like this!  Also, 
the reducing action of fresh flux or flux-cored solder makes it easier to 
melt and wick up old (flux-free or oxidized) solder joints. Less oxidation = 
more fluid.

-Karl.




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