Authored By:
Steffen Käss, Ph.D., Sebastian Fritzsche, Ph.D., Wolfgang Schmitt, Dipl. Ing., Jörg Trodler, Dipl. Ing. Heraeus Deutschland GmbH
Hanau, Germany
Daniel Feil, Timo Herberholz, Ph.D.
Robert Bosch GmbH
Stuttgart, Germany
Summary
An innovative interconnection technology developed in the public funded project ‘HotPowCon’ [7] is capable for resistant joints for SMT components, bigger dies, like MOSFETs or IGBTs and even large area substrates, like baseplate or heat sinks. The presentation showed that the combination of Cu Sn based alloys and Cu powder can increase the melting temperature to over 400 °C by creating intermetallic connections with technologies known from standard SMT reflow soldering. The CuSn intermetallic is grown by a two-paste system wherein the liquid solder paste infiltrates in a dried copper paste. By adjusting the copper particle distribution the presented diffusion soldering process can be used to produce currently large area contact of up to 1000 mm2. First results from power cycling investigations show a significant increase of lifetime compared to a SnAgCu305 solder paste. In addition several mechanical properties of the HotPowCon material were investigated. This allows to design future high temperature stable applications with a significant increase of robustness.
Conclusions
It has been demonstrated that with the HotPowCon process a new innovative joining technology was developed. The die attach is realized by a defined process based on a 2 paste system with state of the art manufacturing equipment. The innovative material is a composite of Cu particles as well as intermetallic compounds especially Cu3Sn and Cu6Sn5, formed by isothermal solidification after a commercial available Sn based solder alloy is infiltrated in a Cu depot.
Not all Cu powders are able to form the composite material. This can be tested with a quick wettability test. The infiltration rate of liquid solder, which is the key parameter to attach areas bigger than 1000 mm2 depends on the particle size distribution or the Cu powders. Smaller particle sizes leads to a decreasing infiltration depth. To get a satisfactory amount of intermetallic compounds (high temperature resistivity) and a good infiltration depth a middle particle size distribution will be the Cu paste system of choice.
The resins also decrease the infiltration rate. Despite a lack of resins in the organic vehicle systems, the Cu pastes needs to fulfill special requirements like tackiness. A polar solvent system with higher boiling point, as in the HotPowCon project used, looks promising. Currently the Cu paste have to be dried to avoid drying channels. This leads to die tilt problems during the process due to the lack of tackiness of the dried Cu paste.
Investigations in the tensile strength show that a middle powder size distribution should be preferred to have a residual amount of ductility overcome mechanical stress peaks and high stiffness to bear overloads. First power cycling tests of the new material show that the HotPowCon joints have comparable life times as silver sintering.
In the HotPowCon project a new high temperature stable material is developed. The high infiltration rates allow to join big areas. To check the full possibilities of this material further investigation have to be done. Interesting is the behavior of this composite material in thermal shock tests. Improvements of the Cu paste especially in terms of tackiness have to be realized. Further intensive work is ongoing to get a better understanding of joining big areas like base plates.
Initially Published in the SMTA Proceedings
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