Alloy are relatively similar in size, the atom exchange method usually happens, where some of the atoms composing the metallic crystals are substituted with atoms of the other constituent. This is called a "substitutional alloy". Examples of substitutional alloys include bronze and brass, in which some of the copper atoms are substituted with either tin or zinc atoms respectively. In the case of the interstitial mechanism, one atom is usually much smaller than the other and can not successfully substitute for the other type of atom in the crystals of the base metal. Instead, the smaller atoms become trapped in the
Tin-silver-copper (95%+) tin. For example, the common "SAC305" solder is 3.0% silver and 0.5% copper. Cheaper alternatives with less silver are used in some applications, such as SAC105 and SAC0307 (0.3% silver, 0.7% copper), at the expense of a somewhat higher melting point. SAC alloys are the main choice for lead-free surface-mount technology (SMT) assembly in the electronics industry. SMT is a process where components of circuit assemblies are mounted directly onto the surface of a printed circuit board and soldered in place. SMT has largely replaced “through-hole technology” where components are fitted with wire leads into holes in the circuit