Automotive Engineering - June 2021 - 26

HOW EVs AND THEIR ELECTRONICS
ELECTRIFICATION | ADHESIVES FEATURE
STICK
TOGETHER
in sensors and microcomputers. Compared with wirebonded
assemblies, flip-chip exhibits higher speed and
interconnect density due to the short interconnect
lengths that result from its inverted structure. Rather
than a glob-top encapsulant, flip-chip uses an underfill
encapsulant to seal the chip, solder interconnects and
the board substrate.
Adhesives aimed at Hybrids
Designers of hybrid-electric vehicle batteries will be interested
in the structural and thermal-management qualities of
a new adhesive developed by DELO. Called DELO-DUOPOX
TC8686, the structural adhesive allows battery cells to be
bonded into the battery housing while dissipating the heat
generated during operation, the company claims.
DELO-DUOPOX TC8686 is designed for high-volume
manufacturing and is flame-retardant. According to the
company it is suitable for the low-voltage batteries used
in mild hybrid and conventional hybrid vehicles, as well as
e-bikes and scooters. With a tensile shear-strength rating
of 18 N/mm2 on aluminum, and effective temperature
range of -40°C to 85°C, it offers battery designers a onestep
alternative to mechanically connecting the cells and
then using gap fillers for heat dissipation. DELO-DUOPOX
is currently being used by an automotive supplier in the
ramp-up phase of production for a hybrid-vehicle application,
the company stated.
Lindsay Brooke
In addition to providing structural protection, the CTE and thermal
conductivity of the underfill works to dissipate heat and mitigate
stresses that arise from thermal mismatch. By controlling viscosity of
the underfill material, its flow properties can be controlled. Capillaryflow
underfills utilize a low-viscosity coating and capillary action to
completely fill the void space beneath the chip after the chip has
been installed and solder bonded to the substrate surface. Non-flow
underfills have a higher viscosity and thixotropic character; this assembly
method firsts dispenses the uncured underfill onto the substrate
followed by chip placement.
Protecting the EV battery
In EVs, the performance and lifespan of the battery power source
is a design priority. Encapsulation and sealing of the battery are
critical to protecting it from external moisture, oil, dust and corrosive
chemicals that it will be exposed to when in operation.
Additionally, the vast heat that is generated during the battery's
discharge/charge cycle must also be efficiently removed to enable
efficient operation and to mitigate high-risk failure, excess expansion,
or battery rupture.
Thermally-conductive epoxy and silicone adhesives can be used
to seal and protect the battery from environmental contaminants
and from mechanical shock. The chemical resistance of epoxies and
silicones can be further exploited to protect the battery from acids,
bases, fuels, solvents and corrosive salts that it may be exposed to
during the course of its operating life.
Sophisticated battery management systems (BMS) are used to
calculate the state of charge (SoC) as well as monitoring temperature,
cell voltages, charge/discharge rates and capacity fade. As
increased cell temperature results in higher electrical resistance
and lower battery efficiency, battery cooling and thermal management
are critical. Thermally conductive gap filler materials, often
made of polymeric silicone loaded with alumina fillers, are vital for
assuring a high degree of thermal conductance between the battery
cells and the cooling plate. These gap fillers can be engineered
as liquids or gels that cure in place allowing for more efficient
thermal transfer.
When compared with thermal pads, gap fillers enable more intimate
contact with the substrates enabling heat transfer. As air has
high thermal resistance, displacing air with thermally conductive
polymers enables more rapid thermal transfer through conduction.
Robust electronic component design incorporates flame-retardant
adhesives to ensure a safe and consumer-friendly product.
Several mechanisms exist for providing flame retardancy. Halogenfree
flame retardants have been developed and provide flame retardancy
and smoke suppression by absorbing excess heat or through
the formation of a char layer to seal the substrate from oxygen.
The Underwriters Laboratory (UL) has established UL 94 certifiThermally
conductive
adhesive (magenta-colored
for illustration) for low-voltage
battery systems such as those
used in mild hybrid vehicles.
26 June 2021
cation for flame propagation in horizontal and vertical orientations
that must be met to allow use of an adhesive system. In addition
to enabling thermal management and encapsulation, the
optical properties of adhesives such as epoxies can be further exploited
in applications such as optical sensors and in photoelectrochemical
assemblies.
AUTOMOTIVE ENGINEERING
DELO
Automotive Engineering - June 2021

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