Soldering terminology

When two flat metallic surfaces are fitted together and diffused at the interface by heating , then the interface shifts through a migration of vacancies and interstitials. This effect is observed in solder tabs as well. The diffusion can be detected via vacancies in the lattice, especially in face-centered cubic and body-centered cubic metallic crystals. Such vacancies occur most frequently at the interface and the holes or pores (voids) may connect. These connecting gaps are known as Kirkendall holes and emerge for example in Cu and Sn solder joints very easily as these two elements possess very different diffusion coefficients. If phosphorus and oxygen are present, they may accumulate at the interface and increase the likelihood of hole formation. In this manner, the solder joint is degraded by the creation of Kirkendall holes. The problems with solder coating of phosphorous bronze are attributed to the same cause.