Why does solder (or braze alloy) flow towards heat?

DTM

You hear it over and over again, that solder or braze alloy will flow towards the heat. Another way of saying this is it flows towards hotter metal and away from colder metal. I've done enough soldering to see that it certainly appears to be true. But I've never heard, nor have I found, a good fundamental explanation of why liquid solder or braze alloy flows towards the heat. What force is causing this motion? Any thoughts?

Crazymechanic

I think mostly because of the property of soldier to attract some metals like copper, gold, I don't think that it will attract any metal like iron or aluminium.That's why it is very complicated and you have to use special chemicals to solder aluminum wire.I think it is because of some chemical properties of different alloys , metals and solder itself.that's why they have gold plated connectors so that they would be easier and better to solder to and make a stronger contact , try flowing the hot solder unto a iron bar I bet it won;'t hold nor flow it;'s way.
But maybe someone else has something more advanced to say about this.

Andy Resnick

AFAIK, it's due to Marangoni/thermocapillary flow: the temperature-dependent surface tension. However, I couldn't find a definitive result, and a few reports say that Marangoni flow *opposes* wetting.

Good question!

256bits

Have you tried soldering without using a flux. Flux dissolves the oxides on the metal surface allowing the solder to wet the metal surface. Have you noticed the flux around the perimeter of the solder - the liquid solder follows the flux.

Naty1

Andy is on the right track...see here, but an actual description is not really provided:
http://en.wikipedia.org/wiki/Marangoni_effect

I suspect one can get more of an insight here:

but I did not look through the equation details.....be sure this applies to solids as well as liquids....

DTM

Thank you for the responses. The Marangoni effect is a good explanation. I also like the term "thermo-capillary convection", very descriptive. I'm surprised the Wikipedia article doesn't list soldering/brazing as applications of this mechanism, as it is applicable to anyone who has ever sweated copper fittings. If anyone's an expert they ought to add to the Wiki article.