Traction inverter power density (KW/L) and efficiency ($/KW) strongly impact electric vehicle (EV) weight, driving range, and cost of ownership. Unfortunately, traditional soldered power modules are not designed for EV traction and suffer from several limitations, which are directly related to poor design and misspecification of thermal and electrical attach layers in the inverter assembly.

The end inverter had double the current capability and 80 percent higher power density at half of the weight of the original inverter. (Photo: ESI Automotive)

ESI Automotive was approached by a leading tier one automotive supplier who needed to improve the reliability and power levels of its inverter for high-end hybrid and EV vehicles. Its previous generation design suffered from several limitations including voltage overshoots during switching, high thermal resistance, and early failures during power cycling and thermal cycling. After exploring multiple solutions, reliability remained an issue. Fortunately, silver sintering held the key.

By working closely with the customer on their material and application process development, soldering and wire bonding was replaced with a fully sintered module. This new approach delivered significant reliability gains in the form of a 10-fold improvement in power cycling, reduced inductance (lower voltage overshoots), improved thermal transfer, and lower thermal resistance. The end inverter also had double the current capability and 80 percent higher power density at half of the weight of the original inverter. These benefits combined create a compelling case for the use of sintering molded SiC modules.

Module Sintering Process

To compensate for the reversible thermal warpage of over-molded modules, thicker bond lines (75-125μm) are needed to mate surfaces and to mitigate stress, since the difference in thermal expansion of the module and heat sink could be >15ppm for each °C rise in temperature.

In addition, >400um thick deposits of sintering paste are applied by either printing or dispense process. High-speed implementation is easy with the use of existing robot technology, and paste application for most inverter assemblies can be done in below 10-30 seconds.

The paste is then dried prior to the placement of the modules. Alternatively, modules can be placed directly onto the wet pads (via the pick-place machine) before drying, which is done at 130-140 °C (for 15-60 minutes depending on module size) to remove the solvents. Finally, the stack is put through the sintering step in a heated press at 250 °C at 8-12 MPa.

Sinter Joint Characterization

The sintering layer then underwent shear, thermal shock, and power cycling testing.

Larger strong sintered joints (above 200 mm2) required a custom shear tool on a 250KN Instron tester for shearing). All assemblies exceeded a 500kg load before mechanical failure, which consistently occurred as a result of failure in the epoxy molding compound. These tests demonstrate that while shear values cannot be used as a standalone quality specification, they are a reliable indicator of assembly quality.

Thermal Shock Testing

The silver sintered and SAC305 soldered module-heatsink assemblies were subjected to liquid-liquid thermal shock testing for 1000 cycles at between -40 °C and 125 °C. Sintered assemblies easily withstood cycles with <5 percent delamination while identical modules based on SAC305 preforms had almost completely delaminated before 500 cycles.

Thermal Impedance and Power Cycling

The soldered and sintered MOSFET module and heat-sink assemblies underwent power cycle testing at a constant temperature gap of 125 °C between the junction and the coolant.

The initial thermal resistance (measured collectively from die to the cold plate) of the soldered assembly was >10 percent higher than for the heat sink. This difference is substantial as it means sintered assemblies can operate successfully with higher power inputs (and temperatures) for the same thermal dissipation.

Prior to cycling, the sintered part could withstand higher currents (~3A higher) for the same temperature differential. When power-cycled with long ON and OFF cycles (to heat up the module to heat-sink joint), the soldered part gradually heated up at a consistently lower current level to maintain the same differential. After 22,000 cycles, a 15 percent reduction in current, the established failure threshold, was recorded. After 56,000 cycles, the sintered part displayed minimal current degradation and remained operational.

Our data illustrates the benefits of combining module to heat sink sintering with silver sintered die attach. Such advancements represent a new frontier for EV traction power modules and inverters, which are becoming increasingly demanding where reliability and power density are concerned. From a systems level perspective, the much-improved current ratings, weight savings, and reduced cooling requirements will accelerate the uptake of sintering technology in power assembly stacks, especially for high-end EVs.

This article is contributed by Gyan Dutt, Power Electronics Specialist, ESI Automotive (Waterbury, CT). For more info visit here .