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Die Attach of Power Devices Using Silver Sintering - Bonding Process Optimisation and Characterization
Keywords: Silver sintering , Die attach, power electronics
Die attach of power devices remains one of the challenges for high temperature applications. As the operating temperature of a solder joint approaches its melting temperature, it tends to loose its mechanical performances, thus reducing its reliability. Furthermore, soldering alloys with a high melting point generate a high level of residual stress and require components that can withstand a higher soldering temperature. Alternative die attach processes have been presented. They use a different physical mechanism, such as diffusion or sintering, and do not require the entire joint to melt during the process. Therefore, die attaches that can operate at a higher temperature that their process temperature are possible. Among these alternative processes, silver sintering is one of the most promising, as it complies to RoHS directives, and has excellent thermal and electrical properties. In this paper, we propose to describe the bonding of SiC power dies to a ceramic (DBC) substrate. Some commercial silver paste is used (current tests use NBEtech's nanoparticle-based material, but we expect to start using Heraeus' micro-particle material within a few weeks). Compared to the classical soldering process, sintering requires (very frequently) the application of a pressure during the assembly. A special tool (press) has thus been designed and will be described in the paper. This press can apply a pressure ranging from a few MPa to a few tens of MPa, provide a very fast heating rate (over 50°C/min), and follow complex temperature profiles. Other process-related operation will be described such as cleaning or screen printing, as most papers on silver sintering tend to overlook these steps to focus on the results only. Results corresponding to our process will obviously be presented, including shear test as well as optical and SEM inspection, both for new and high-temperature-aged bonds.
Cyril Buttay, Researcher
University of Nottingham, Laboratoire Ampere
Villeurbanne 69621,
France


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