Capillary action

17 March 2011

Capillary is an interaction between a liquid and a small diameter channel or opening in a solid object. Because of the physics involved, if the liquid can wet the sides of the solid channel or opening, surface tension will draw the liquid up into the capillary.

One of the actions which must be achieved by a soldering flux in electronics is to break down the surface tension of the liquid solder which we know can be different in the alternative alloys used today. The desired result is wetting or complete flow of the solder over the surfaces to be joined, and this can be a considerable distance for a liquid that is not affected by other actions.

The term capillary space refers to the distance between the two surfaces. A classic example of this action in soldering is demonstrated by dipping a stranded wire into a liquid flux that wets the conductor. The small spacings between the individual strands of the wire act as capillary channels, it’s the way solder wick pulls solder off the surface of a joint, which is desirable, but also how solder runs up a stranded wire under the insulation, which is not. It can be seen that the liquid flux will travel for a considerable distance up stranded wire in front of the solder alloy, which is one of the reasons why highly activated flux should not be used in this application.

Capillary action is also how solder joints form on plated through holes with through hole leaded components. But, unlike liquids, the flow of the solder is also impacted by the slow loss of energy, heat, and the actual solderability of the two adjacent metal surfaces.

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