How EVs and Their Electronics Stick Together

New adhesives play an increasingly vital role in vehicle structures, battery packs, and in protecting sensitive electronic components from extreme thermal cycling and contaminants.

Certain grades of epoxy can withstand rigorous thermal cycling and shock. Good flow properties and lower exotherm can be desirable for potting and encapsulation applications.

Thermal management and vehicle weight reduction are among the challenges facing electric vehicles (EV) and automated driving systems, as engineers focus on improving vehicle range, performance, and safety. Adhesives, sealants, and coatings are playing a vital role in these developments. Their lightweight, functional properties are key to the industry’s increased use of mixed-material structures, including battery packs. They also serve as electronic encapsulants to protect sensitive electronic components from extreme thermal cycling and contaminants.

Compared with traditional metal fasteners, structural adhesives offer improved inventory management, an overall reduction in design weight and a more reliable joint. Compared with rivets or welds, they can disperse the load over a larger area, thereby reducing the localized stress on a fastener and also improve joint reliability. Dispersing the stresses over a larger area may also allow for caliper reduction of the components with a resulting weight savings.

Epoxy adhesives, two-part or one-part cure, are the workhorse of the automotive industry. Many advances in the packaging and delivery systems of the adhesive further improve manufacturing efficiency and reduce waste. Developments such as dual-cure, UV + Heat curing allow for higher manufacturing speeds and throughput.

Selecting the right adhesive

Thermally-conductive epoxy and silicone adhesives can aid in protecting the battery from environmental contaminants and mechanical shock.

Whether used in a structural capacity or to reinforce and protect electronic assemblies, adhesives must possess the correct mechanical properties. Depending on the nature of the joint, modulus or stiffness will determine the joint stability and whether the adhesive will readily deform to absorb stress or stay rigid and maintain high dimensional stability. In certain assemblies, lower modulus may also allow an adhesive to mitigate and relieve stresses that accumulate due to differences in thermal expansion, or to dampen vibration and shock. Conversely, a higher modulus is important in flip-chip micro-electronic assemblies where the adhesive is used as an underfill. If the modulus is too low, the underfill will inadequately protect the solder joint. And a modulus that is too high may overly redistribute stresses to the silicon chip and result in die cracking.

Depending on the nature of the joint and the applied stresses, the adhesive’s shear, tensile and compressive strengths are critical in joint design as well as the bonding strengths to the employed substrates. Given proper surface prep, a good adhesive bonds well to a variety of 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.

Specialized epoxies may have a silver filler, providing uniform electrical conductivity.

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 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 in sensors and microcomputers. Compared with wire-bonded 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.

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. Capillary-flow 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. Halogen-free 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 certification 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 photoelec-trochemical assemblies.

EMI shielding, process flexibility

With the proliferation of electronics, high-voltage current, cell-phone signals and Wi-Fi, the importance of electromagnetic interference (EMI) shielding is critical. A device or compartment can be shielded from EMI by surrounding it with an electrically conductive shell that creates a discontinuity in the electromagnetic field. Conductive metal films or weaves of metallic threads work but can be heavy and difficult to process with off-cuts and scrap resulting. Alternatively, adhesive coatings and encapsulants loaded with electrically conductive fillers can instead be used to coat surfaces and provide excellent EMI shielding capability, at frequencies ranging from a few hundred MHz up to 10-20 GHz.

Epoxy and silicone adhesives can be formulated to cure at room temperature or at elevated temperatures. The development of dual-cure systems that can be cured with a combination of UV light and thermal post-cure enable improvements to throughput and manufacturing speed. With the case of dual-curing systems, UV light can be used to initiate cure and increase the efficiency of the manufacturing process by providing a strong, near-instantaneous bond.

For one-part and two-part curing systems, the adhesive industry has extensively innovated its packaging and delivery systems. A well-designed system seeks to minimize adhesive waste providing additional efficiencies. Further, the viscosity of adhesive systems can be optimized depending on the desired bond thickness and dispensing rates allowing for use in automated, robotic manufacturing processes.

Sustainable ingredients to lower the carbon footprint of structural adhesives are being utilized in formulations. This has led to the development and use of 100% solid formulations that have low concentrations of volatile organic components (VOC). The European Commission’s Restriction of Hazardous Substances (RoHS) directive has been essential for minimizing the environmental impact of electronic assemblies. In addition to restricting heavy metals such as lead and cadmium, RoHS seeks to prevent the use of toxic brominated flame retardants and biological toxins, including phthalates. As such, it is essential to use only adhesives that comply with RoHS restrictions when manufacturing electronic assemblies.

Future adhesives and coatings

Nano-fillers have and will continue to provide great innovations in the field of adhesives development. There exists in this composite an interfacial area between the polymer matrix and the filler particles. Many mechanical, thermal, and electrical properties can be modulated with the inclusion of functional fillers, and it is often this interface that is critical in controlling the properties.

Micro alumina particles or even nanoparticles of alumina or silica can be formulated to exhibit excellent thermal properties while being non-electrically conductive for electronic applications.

Speed and cost of manufacturing complex parts and prototypes can be improved with 3-D printing and other additive manufacturing processes. Dual-cure, UV + Heat resin systems allow 3-D printing with UV-induced cure while allowing for thermal post-cure to provide superior structural and thermal stability when compared to thermo-plastic printing resins.

Venkat Nandivada, a chemical engineer, is manager of technical support, and Rohit Ramnath is a senior product engineer, for Master Bond Inc.