Thermo-mechanical behavior and reliability issues for high temperature interconnections

Transient liquid phase (TLP) soldering is one option for high temperature interconnects with the advantage of processing conditions being close to those for conventional soldering. In the Cu-Sn system addressed in the paper, a high post-processed melting point of the solder interconnects is achieved due to the formation of Cu₆Sn₅ and Cu₃Sn intermetallic compounds (IMC). A specific low melting solder paste under development can be used if applications for both power and logic electronics are addressed. Metallic particles are embedded in the solder paste. New challenges concerning the thermo-mechanical reliability of these devices arise as the materials properties of the TLP interconnect differ substantially from those known for soft solders. Based on material characterization of pure IMC effective material characteristics of the TLP joint, consisting of a mixture of different constituents, have been derived based on a micromechanical cell model. A cell model was additionally embedded in a resistor joint with effective TLP properties and the local stresses were studied. It is shown that the higher material stiffness and strongly decreasing ductility of the joining material change the potential failure modes of an assembly made by TLP soldering. The new thermo-mechanical failure risks are evaluated for a mounted chip resistor and a power module, an IGBT on DCB substrate, by both conventional FEA and cohesive zone modeling.