Robust power electronics through improved mounting technologies

A feat of material technology for power devices

The isothermal solidification of the solder with diffusion soldering signifi-cantly improves its high-temperature properties.

With conventional solder joints, a eutectic zone forms in the center of the joint and to the metal of the isothermally solidified intermetallic zones. The eutectic zone of the conventional solders determines the thermo-mechanical properties. It has a melting point of 221 °C with SnAg solder and 183 °C with SnPb solder.

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In contrast, with the diffusion soldering method the solder (e.g., SnCu, melting point 227 °C) solidifies isothermally and fully transforms through reacting with the substrate material in the intermetallic phase. The intermetallic phases themselves have a significantly higher melting point than the conventional solder: the Cu3Sn-Phase 676 °C and the Cu6Sn5Phase 415 °C. However, special DCB substrates with a specific coarseness need to be used when working with thin layers (about 10 μm). (A standard DCB substrate has a coarseness of about 20 µm.)

To generate thicker layers, the diffusion solder is filled with a base material such as copper powder. The low-melting components in the solder diffuse in the base material until the entire soldering gap is isothermally solidified.

In diffusion soldering processes, the solder needs to be held at the pro-cessing temperature longer than with conventional soldering processes, especially with thicker layers since the time required for the formation of the intermetallic phase increases due to diffusion with the squared layer thickness. Discontinuing the diffusion soldering process prematurely would lead to a eutectic residue zone and thereby negatively impact the mechanical properties. The major challenge with thicker layers is achieving an even, nonporous mounting layer.

When soldering components, the solder melts when heat is applied (230-250 °C for SnAg solder). Diffusion processes occur at the edges of the metal, creating intermetallic phases. Soldering in a vacuum helps minimize the formation of pores.

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