Paper describes solid state phenomena occurring during the formation of interconnections intended to the electronic industry. Diffusion soldering technology is an undemanding and functional tool to use when the growth of intermetallic phase(s) is desired and controlled in order to obtain the joint consisted solely of an intermetallic phase. Scanning and transmission electron microscopy investigations of the microstructure and sequence of appearance of the intermetallics within the joined area are discussed. The diffusion path describing the chemical composition changes across the interconnection was found to correlate with microstructure transformations. The growth kinetics of the δ′[Cu41(Sn,In)11] phase was determined in the temperature range of 300 – 350 °C.