Universal high-temperature suitable joint adapting diffusion soldering

This article outlines the realization of a universal joining technology that provides high temperature compliant solder joints. Major achievement of provided solutions is to overcome the conflict of objectives concerning low thermal budget to joining partners during assembly processes and high melting points of the resultant joints during operation. One cost effective approach to solve this conflict is diffusion soldering. In contrast to thermal solidification this soldering variant uses concentration change of materials to achieve at least locally high-melting intermetallic phases. Comprehensive evaluations of potential material systems led to a simple binary system of Sn and Cu with eutectic composition at SnCu0.7 (melting temperature 227°C) and high melting-temperature phases Cu₆Sn₅ (Tm about 415°C) and Cu₃Sn (Tm about 670°C). Diffusion soldering is used already in wafer to wafer and chip to lead frame soldering technologies. To achieve a universal joining technology with focus on wide range of chip to ceramic and second level assembly in the field of surface mount technology the challenge of short bridgeable distances has to be solved by technical creases. This article outlines technical solutions of dispersed Cu particles and special topographic elements that provide the potential to increase joining zones up to 100 μm. Process flows and equipment for major technological solutions are described. Potential adaption in mass production and results of technical reliability are shown. In addition comprehensive analysis results of metallographic investigations are shown to give an introduction to new challenges of diffusion soldered interconnects.