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Understanding Capillary Action in Silver Soldering and Brazing

Capillary action is the fundamental necessity when silver soldering. It is what makes soldering and brazing different to welding and is essential for producing sound, leak free joints.

Understanding Capillary Action in Silver Soldering and Brazing - CuP Alloys

What is Capillary Action?

One definition of brazing is; ‘a process of joining generally applied to metals in which, during or after heating, molten filler metal is drawn into or retained, in the space between closely adjacent surfaces of the parts by capillary action’.

Importance in Silver Soldering

In the realm of metalworking, capillary attraction plays a pivotal role in the efficiency and quality of soldered joints. When heated, the silver solder melts and is drawn into the joint by capillary attraction, ensuring a strong and seamless bond between the metals.

Mechanism Behind Capillary Action in Silver Soldering

Understanding how capillary action occurs, and the conditions required to promote it is fundamental to the production of sound, leak free joints.

Clean Surfaces Matter.

Both surfaces should be generally clean when brazing, free of oxides, oil, grease and rust. Avoid cleaning with any grit based products, eg emery cloth, or sandblasting as these methods can leave residues behind which may impede the flow of the alloy.

Heat Application

Apply heat evenly across the whole joint is crucial. The heat melts the solder, reducing its surface tension and allowing it to be drawn into the joint via capillary attraction. Proper temperature control ensures optimal flow. Too little heat and the alloy won’t melt, or may ‘freeze’ in the joint leaving a void or weak spot. Too much heat can affect the properties of the parent metals, distorting, annealing or even melting it!

Geometry of the Joint

The geometry of the joint significantly influences capillary attraction. Narrow gaps and well-fitted parts enhance capillary flow, promoting efficient solder penetration and stronger bonds. Optimum joint gap is .003-.005 inches. The narrower the gap the greater the capillary action. But note, no gap = no joint. The gap should also be consistent along it’s length.

Understanding Capillary Action in Silver Soldering and Brazing - CuP Alloys

Factors Affecting Capillary Action

Several factors impact the effectiveness of capillary attraction in silver soldering.

Alloy Composition

The composition of the solder alloy affects its fluidity and capillary attraction. Optimal alloy selection tailored to specific metals enhances the solder flow into joints.

Flux Application

Flux is employed to clean surfaces and facilitate solder flow. Proper flux selection aids in reducing surface tension, allowing the solder to spread uniformly through the joint via capillary attraction.

Understanding Capillary Action in Silver Soldering and Brazing - CuP Alloys


How does capillary attraction work in silver soldering?

Capillary attraction in silver soldering relies on surface tension and heat. When heat is applied to the joint, the surface tension breaks, allowing the molten silver to flow into the joint, creating a strong bond.

What role does flux play in capillary flow?

Flux cleanses metal surfaces, reduces oxidation, and helps molten silver flow smoothly through the joint, ensuring effective capillary attraction.

Can capillary attraction be enhanced?

Yes, by ensuring clean surfaces, controlling heat appropriately, using suitable flux, and understanding material properties, capillary attraction can be enhanced significantly.

Where else is capillary attraction present in everyday life?

Capillary attraction is visible in plants drawing water from the soil, in ink moving through pens, and even in the absorption of liquids by paper towels.

Are there limitations to capillary attraction in silver soldering?

Yes, capillary attraction requires clean surfaces and precise heat control. Contaminants hinder its effectiveness, and improper heat might lead to excessive or insufficient flow.


In the intricate world of silver soldering, capillary attraction emerges as the unsung hero. Its seamless flow enables the creation of sturdy, durable joints in various applications. Embracing and understanding this force magnifies the artistry and precision in soldering techniques.

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