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When soldering both plumbing and wiring I have heard the advice that you shoulf apply solid solder opposite to your heat source, since the solder "will flow towards the heat" once it melts.

Is this a true statement, in general?

I could imagine that perhaps an increasing temperature gradient would decrease the solder's viscosity, and maybe that could promote more flow?

(And obviously a really uneven temperature could leave areas below the melting point, so in that case the advice sort of makes sense but only in a trivial way.)

I can think of other reasons why you might want to apply solder away from the heat source - in particular so the heating device doesn't get in your way. Also because by applying the solder far from the heat source your have a high likelihood of the melting temperature being exceeded throughout the joint once it melts at the solder. I wonder if there are the only genuine explanations.

Related:

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    $\begingroup$ It is well known in thin film deposition that when doing a simple heated boat evaporation, depending on the material of the boat and the material being melted, sometimes the melt flows towards the colder ends of the boat and sometimes it makes a nice molten ball in the hot center of the boat. $\endgroup$ – Jon Custer Sep 27 '18 at 12:20
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If you are sitting close to the melt temperature, the hotter the solder gets, the lower its viscosity becomes. Since viscosity is the retarding force in capillary flow, this means that hot solder flows far more easily into small crevices, which yields the impression that solder flows or is drawn towards the source of heat.

Most soldering irons have fairly limited heat delivery rates (20 to 40 watts) which means it's often a challenge to get enough heat into a joint in advance, which magnifies this effect.

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  • $\begingroup$ Isn't there also a surface energy component to that, which is related to contact angle between the surface of a droplet and the (different material) surface it is on? $\endgroup$ – S. McGrew Nov 11 '19 at 2:22
  • $\begingroup$ @S.McGrew, there certainly is; if the solder does not wet the workpieces, it will not be drawn into a hot joint by capillarity. $\endgroup$ – niels nielsen Nov 11 '19 at 6:12
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According to this, viscosity is not the important factor; surface tension is. This relates to the contact angle between the molten solder and the metal surface it is on. When that contact angle is between 0 and 90 degrees, the solder flows. Between 90 and 180 degrees, the solder does not wet the surface. The contact angle of a solder droplet on a given metal is dependent on temperature, and approaches zero at high temperature. This drawing is from Chemistry Libre:

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