Silver Sintering in Power Electronics: The State of the Art in Material Characterization and Reliability Testing

Sintered silver is a very promising option in the field of automotive power electronics for die-attach and substrate-attach material as it offers many benefits compared to conventional solders. One main advantage is the higher and therefore better thermal and electrical conductivity, which is necessary due to the usage of SiC and GaN as a basic material for semiconductors. Another one is the higher melting point of sintered silver in comparison to solder, making it possible to use power electronics at higher temperatures. Especially for higher temperatures, it is assumed that this results in an improved reliability. Whereas a lot of information about the behavior of sinter layers is known, e.g., thermal properties, material characterization of mechanical properties are still not understood in detail as the results are highly scattered. A reliable test method has to be developed, and this has to be done for cyclic testing, too, which is essential for determining reliability. As mechanical properties are strongly dependent on process parameters, their influence has to be analyzed as well. For example, the influence of the porosity distribution of a sintered layer in between the semiconductor and the upper side of the DBC on the mechanical properties and their reliability is hardly investigated by now. It can be observed that the porosity varies, and that it often increases from the middle of the plane towards the edges. The influence on the stresses and strains and on reliability has to be determined and if required, improvements have to be developed. In the field of FEM simulation, too, a lot of work has to be done. Therefore, the right input material data, which has to be characterized as already described, is absolutely essential. It is also necessary to implement a material model which describes all of the occurring phenomena like primary and secondary creep. This paper will give an overview on the current status and state of the art of silver sintering, focusing on the issues of material characterization and reliability testing.