During soldering, by what mechanism is solder drawn into the joint?

• A. Capillary action
• B. The heat of the torch
• C. Flux
• D. Gravity

Answer: (A)

Capillary action pulls molten solder into the joint. When the joint is heated and flux cleans and wets the metal surfaces, the solder becomes liquid and adheres to the interfaces. In a narrow gap, the combination of surface tension and the adhesive forces between solder and metal creates a capillary force that draws the molten solder into the space, filling the joint from the inside out. Flux helps by removing oxides and improving wetting, which makes capillary action effective. The torch’s heat is needed to melt the solder, but it’s the capillary forces in the tiny gap that actually move the liquid into the joint. Gravity is negligible here compared with capillary action, especially in small joints.