Automotive Engineering - June 2021 - 25

ELECTRIFICATION | ADHESIVES FEATURE
Nano-fillers have and will continue to provide great
innovations in the field of adhesives development.
Thermally-conductive epoxy and silicone
adhesives can aid in protecting
the battery from environmental
contaminants and
mechanical shock.
materials including the printed-circuit-boards, semiconductors, plastics
and other associated metals used in electronic assemblies.
Careful adhesive selection is a powerful tool in thermal management
of electronic assemblies, including the sensors and processors used in
automated-driving systems. Adhesives can be formulated with mineral
fillers to change their thermal and electrical properties. For example,
electrically insulative potting compounds are used to protect electronic
assemblies from high-voltage arcing, moisture ingress and to structurally
protect the sensitive components from vibration and mechanical
shock. As electronic components generate large quantities of heat, a
thermally conductive potting compound with a high thermal conductivity
helps to dissipate heat from the sensitive electronic components.
This allows them to run more optimally, reducing the negative effects
associated with high-temperature operation.
Depending on the application conditions, the modulus of the potting
compound can be changed to increase rigidity and provide optimal
stress redistribution. Filmic preforms for use as conformal coatings
or encapsulants can be designed to have low modulus and can
deform to make intimate contact with the underlying circuitry.
As materials increase or decrease in temperature, their volume expands
or contracts. Different materials possess different coefficients
of thermal expansion (CTE). When joining two or more dissimilar materials,
the engineer must be aware of any differences in their CTE. If
AUTOMOTIVE ENGINEERING
one material expands more than the other, this manifests
as thermal stress. It can cause solder fatigue,
stress cracking, and enable the ingress of environmental
contaminants. As such, careful material selection
and an understanding of the operating temperatures
are critical to designing multi-component assembly.
Electronic assemblies may contain materials with significantly
different CTEs. Common materials include
silicon, FR-4 glass-reinforced epoxy laminate for PCBs,
solder and epoxy with CTEs of 2.6-3.0, 14-17, 21.5-24.6
and 66-72 ppm/°C, respectively.
Organic adhesives composed of epoxy or other polymeric
materials can be formulated with mineral fillers to
lower their CTE to better match the components present
within the joint. Using an adhesive with a CTE that
is intermediate between the different materials helps to
match the thermal expansion properties and alleviate
the stresses from thermal cycling.
Flippin' chips
The flip-chip assembly method is one of the most common
methods employed in microelectronics including
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Automotive Engineering - June 2021

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