The manual studies the patterns of interaction of materials at various stages of solder joint formation. A significant place is given to the surface interactions of solder and soldered metal, which determine the formation of bonds and the conditions for filling solder gaps. The features of the processes of contact melting of materials, the formation of chemical compounds, the crystallization of the solder seam, which largely determines the composition, structure and properties of solder joints, are discussed in detail. The textbook is intended for students studying under the master's program in the direction of training 15.04.01 "Mechanical Engineering" under the program "Soldering Technology and Equipment"; can also be useful for students of other technical specialties of universities and engineering workers.