Brazing and soldering join solid base parts using a filler material that melts and flows into the joint while the base materials remain solid.

The molten filler can be drawn into a narrow joint by capillary action. Good joint clearance is therefore important: a gap that is too wide weakens capillary filling, while an excessively tight or contaminated interface can prevent complete flow.

The surfaces must also be wettable by the molten filler. Oxides and contamination can prevent wetting, so fluxes, shielding atmospheres or controlled surface preparation are often used.

The distinction between brazing and soldering is based on the filler melting-temperature regime, but their common process logic is more important:

prepare joint surfaces
      ↓
assemble controlled gap
      ↓
heat above filler melting range
      ↓
filler wets and flows
      ↓
cool without melting base parts

Because the base material is not bulk-melted, thermal distortion can be lower than in fusion welding and dissimilar materials can sometimes be joined more readily.

Joint strength still depends strongly on geometry. Lap joints provide area for shear transfer, while a simple butt joint offers much less bonded area.

These processes are therefore especially useful when lower joining temperature, dissimilar-material compatibility or distributed bonded area is more important than creating a fusion weld.