PRODUCTS & SERVICES
Silver lining
Taking advantage of the material properties of silver has led to the development
of supercharged power switching designs for electric vehicle inverters
As the electric vehicle is
forecasted to consume an
ever increasing share of the power
electronics market, the use of
sintered silver as an interconnect
material is enabling a new
generation of higher current density
devices that benefit from the
inherent enhanced reliability.
Examining the previous
generation of power switching
modules, we see that there have
been a number of incremental
improvements that seek to enhance
reliability and power density.
Starting with the
baseline of a soldered
die attach with
wirebond output
interconnects,
the use of low
CTE ceramic
substrates, smaller
diameter wirebonds,
copper wirebonds, aluminum
coated copper wirebonds, higher
creep resistant solders, and
double-sided cooled configurations
have emerged. Even with these
improvements, the only way that
reliability can be significantly
enhanced with solder present is to
reduce temperature swing during
power cycles, and this generally
means reducing switching current
per device, and increasing total
device count.
New semiconductor technologies
such as SiC and GaN collectively
o er compelling opportunities for
reduced switching losses, higher
frequency operation, and higher
current densities. Independent
of the semiconductor technology,
the limitations of solder and
wirebonds prevail.
A solution that is less complex,
based on the material properties of
silver is the use of pressure-assisted
sintering. Silver is a near ideal
material for die and discrete
package attach. Its thermal
conductivity is higher than copper
and it o ers electrical resistance
that is an order of magnitude lower
than the best solders.
The sintered silver interconnects
are deployed, not just for die attach
purposes, but also as the
replacement for bondwires used
with clip architectures.
The replacement of solder with
sintered silver material yields a 50%
increase in current density per
device without an increase in die
temperature, and an order of
magnitude increase in reliability as
measured with power cycling,
making it an ideal material for the
new generation of traction inverter
designs. Sintered silver allows the
device to operate with higher
switching current without the need
to reduce current per device to
162 // July 2019 // www.electrichybridvehicletechnology.com
extend reliability. Sintered silver
also reduces package inductance,
which yields less output voltage
ringing, higher frequency operation,
and also enables a higher usable
output voltage.
The relatively low pressure (10
MPa) and high thermal conductivity
that results (200-250W/mK) makes
this material extremely versatile.
Alpha’s pressure-assisted sinter
silver has been implemented in
electric vehicle inverters based on
Silicon dies, also Silicon Carbide
dies in discrete devices and has also
been implemented in GaN devices.
Designs based on discrete IGBTs
or MOSFETS can also benefit from
sintered silver by deploying it to
attach their packaged devices to
the substrate. This can result in
greater than 10% reduction in the
overall system thermal resistance,
as demonstrated by Alpha
Assembly’s testing.
Developing a process for a new
attach material is a challenging task,
which Alpha has addressed using
expertise based on deep sinter
process knowledge, and production
equipment partnerships to develop
its Alpha Argomax process solution.
By focusing on the appropriate
material set and the correct
processing parameters that will yield
a production ready implementation
is an important part of the process.
This approach helps accelerate the
availability of viable test vehicles and
qualification devices, and also
enables rapid introduction of the
customer’s new product.
Alpha’s Argomax product offering includes sinter paste, sinter fi lm and preforms
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