Automotive Engineering - May 2022 - BET25

Using Silver Sintering in Traction
Inverter Assembly
T
raction 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.
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 overmolded
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.
A screw-based dispense valve was adapted with a custom flat
nozzle to enable thick deposits (>400um) of sintering paste to be
rapidly applied onto the pads, eliminating significant material
waste and cleaning issues. High-speed implementation is easy
with the use of existing robot technology, and paste dispense for
most inverter assemblies can be done in below 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 130140
°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.
Battery & Electrification Technology, May 2022
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)
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 heatsink
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 http://info.hotims.
com/82321-424.
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http://info.hotims.com/82321-424 http://info.hotims.com/82321-424
Automotive Engineering - May 2022

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