Inverter sintering set to boost range

Improved sintering enables the STPak inverter package to be mounted directly onto heat sinks
(Courtesy of MacDermaid Alpha Electronics Solutions)

MacDermid Alpha Electronics Solutions is working with STMicroelectronics on improved sinter technology for EV inverter assemblies. The process, known as pressure sintering, can significantly increase vehicle efficiency and range as well as lowering production costs (writes Nick Flaherty).

Pressure sintering provides much lower thermal resistance in an inverter, which leads to better power cycling and mechanical reliability over conventional techniques. The biggest contributors to the overall thermal resistance (Rth) from the die to the heat sink are the interconnect materials, including the thermal interface material (TIM), which can account for more than half the total thermal losses. 

By replacing the TIM or solder with a thinner, highly conductive pressure sinter layer, the Rth falls by 10-15%.  This also improves reliability by managing the stress and warpage between the package and heat sink, as the pressure sintering can be achieved at a lower temperature than other approaches.

This allows STMicroelectronics’ STPak package for automotive traction inverters to be mounted directly onto the inverter heat sinks using pressure sintering with Argomax silver sinter paste. The paste has specially designed silver particles to work with low-pressure sinter die and the STPAK package materials.

The Argomax paste creates a strong attachment through silver metallic bonds to direct bonded copper substrates, providing high levels of electrical conductivity and good thermal properties. This improves long-term reliability and reduces the direct conductive losses and inductive losses by placing the STPAK transistors closer together and reducing the number of discrete devices required.

This has led to a 93% efficiency in inverter designs, which adds 10% to the range, said Edoardo Merli, group vice-president of STMicroelectronics’ Automotive and Discrete Group.