Refers to a soldering process in which the entire solder layer forms an intermetallic phase. Normally, the intermetallic phase in a soldering gap is only in the interface area between the joining partner and the solder material. In diffusion soldering, however, the entire solder alloy forms a phase – for example of CuSn. The melting point of this phase is significantly higher than the melting point of the actual solder alloy. The solder therefore solidifies at a temperature above liquidus. Due to the higher melting temperature after the first soldering process, the joint can be exposed to higher application temperatures. A disadvantage of diffusion soldering, however, is that the intermetallic phases are usually significantly more brittle than the actual solder alloy, which leads to increased cracking under thermomechanical load. In addition, very thin layer thicknesses (10µm) are required in contrast to standard thicknesses (100µm) in order to achieve complete formation of the intermetallic phases. Polished surfaces with very low roughness are also required.