Which of the following joints depends on capillary action?

• A. brazed
• B. soldered
• C. welded
• D. threaded

Answer: (B)

Capillary action is what pulls molten filler metal into the tiny clearances between joined parts. In soldering, you melt a low-melting-point filler metal (solder) and use flux to help surfaces wet. The narrow gap between the pipe and fitting acts like a capillary tube, so the molten solder is drawn into the joint by surface tension and wicks all around the joint. As it cools, it solidifies to form a sealed, conductive bond. Welding, threaded joints, and many brazed setups rely on different mechanisms—fusion of base metals, mechanical engagement, or filler metal flow not driven primarily by capillary action—so soldering is the method most dependent on capillary action.