Thermally conductive adhesives are mainly used for disintegrating heat out of power-based electronics. They are mainly for heat sink bonding. So, for example, thermal conductive adhesives are mainly used for reducing strain so as to prevent any kind of loss in performance of preventing any electronic component failure.
Thermal conductive adhesives are primarily used for bonding heat sinks, LEDs as well as other heat-generating electronic components. Most of the thermal conductive adhesives give a strong adhesion to metals as well as plastics and provide a durable physical strength.
Synthetic resins with filler components composed of metallic as well as inorganic materials are used to make adhesives with heat conductivity. Metallic fillers, such as silver or graphite, provide the best thermal conduction coefficients. However, these properties make the glue electrically conductive, which is not desirable in various applications. Adhesives reinforced with mineral-based fillers must be utilized to accomplish thermal conductivity and electrical insulation at the same time.
Now let us understand
Types of Thermally Conductive Adhesives
1. Epoxy Adhesive
Epoxy adhesives are created by using a mixture of two components, primarily the resin as well as the hardener. So, when the resin is appropriately mixed with a proper catalyst, the curing is initiated. Curing is the process through which the molecular chains react with chemically active sites, thereby resulting in an exothermic reaction. Also, it is important to know that the covalent bonds that exist between epoxide groups of the resin, as well as the amine groups of hardeners, arise from the dual combination of the cross-linkage of polymer.
Certain alteration of the mechanical strength properties happens by monitoring curing conditions with the help of temperature and choice of resin as well as hardener compounds. Epoxy adhesives are mainly used as one component or two-component systems. Epoxy adhesives which are one component, can be cured at temperatures of around 250–300-degree F. These conditions result in a higher strength as well as create an excellent adhesion to metals, thereby creating a harsh chemical resistance.
2. Silicone Adhesive
Silicone adhesive is a diverse water-resistant polymer where the primary ingredient is silica and is mainly found in it is mainly a common form of sand that is found in quartz. The term silicone mainly refers to the group of polymers that has a siloxane bond with that of organic compounds. The production of silicone adhesives mainly begins with that of isolation of silicon from silica. The special properties of silicon make it quite resilient. Additionally, the elasticity, as well as other properties, remain quite unchanged of silica at any temperature. Primarily silicone adhesives are used for bonding metals due to their flexibility as well as well as characteristics as well as their ability to bond dissimilar substrates.
The manufacturing of silicone adhesives mainly happens by isolating silica from that of silicon. Silica is generally found in pure form in certain minerals. Silicone adhesives, as well as sealants, are mainly made from polymerized silicone. When silicone is uncured, it creates a highly adhesive gel as well as liquid. It is quite safe to use and is used in a variety of applications that are mainly non-toxic. Silicon adhesives are available in various types like high-temperature silicone adhesive, Room Temperature Vulcanizing Silicone Adhesive (RTV), Silicone Rubber Adhesive, etc.
3. Polyurethane Adhesive
Polyurethane adhesives are mainly thermoplastic glue or a polymer that contains replicas of organic chain units mainly linked to urethane links. The chemical reaction between the polymers results in the creation of an adhesive. Generally, polyurethane adhesives are mainly brown or transparent in color. Pigments like that of green as well as red color can be added mainly for application purposes like that of spraying, thereby making it visible in areas of application. They are used for construction, furniture making, lamination, flooring and roofing, aerospace as well as cold stores. The polyurethane adhesives are mainly available either as a 2-part adhesive or one-part adhesive. The 2-part adhesive generally sets much quicker in comparison to one-part variant taking up around 30 minutes mainly depending on requirements. Polyurethane adhesives are sealants that can be used in challenging environments like that of heat as well as moisture curing. Additionally, polyurethane adhesive materials are also suitable for various materials like that of metal, wood, concrete, rubber, epoxy as well as glass. It is also important to note that PUR adhesives are quite waterproof; however, the water-resistance generally varies as per formulation. Also, polyurethane adhesives are environmentally friendly as they are solvent-free and have a very low VOC. Moreover, they are also food safe.
Polyurethane adhesives or PU adhesives are quite versatile with the best characteristics. They are also modified in order to provide custom formulas that can be designed as per specific applications. These include the ability to modify the physical properties like that of viscosity and application properties like that of pot life. Polyurethane adhesives can be separated into two major classifications, and they are the 1K or ` part and 2Kor 2 part systems with that of the best advantages as well as disadvantages.
The chemical reaction between an isocyanate and a polyol is used as the basis for all polyurethane sealants and adhesives. The isocyanate and the polyol will be made and supplied separately for 2K systems. To establish the chemical reaction and properly cross-link the system, the two components must be mixed just prior to application. To ensure this happens correctly, an exact ratio of components and sufficient mixing must be assured to create the needed polyurethane adhesive qualities.
On the contrary, 1K systems during the manufacture of polyurethane adhesives are mainly manufactured with that of polyol, thereby reacting with that of excess isocyanate so that the polyol chain can be terminated with that of isocyanate group. The ratio of isocyanate, as well as polyol, will help to determine the length of the chain of the terminated polyurethane polymer. Also, it is important to note that this polyurethane prepolymer becomes the major reactive component for 1K adhesive systems.
In order to complete a reaction, a 1K system will need to interact with that of water in order to cross-link fully. It is also important to note that water must be present in a 1K system so as to cross-link, and this water can be moisture from the atmosphere.
Now let us understand some of the
Applications of Thermally Conductive Adhesives
Thermally conductive adhesives are mainly used for potting, coating, as well as other encapsulation applications. Some of the specific applications include
a. Heat Bonding
Certain components of thermally conductive epoxy, as well as silicone compounds, are mainly employed mainly regarding heat sink bonding to electronic components as well as circuit boards for dissipating heat. They are mainly designed in such a way so that they can prevent overheating as well as premature component failure. Heat bonding is used in computers, LEDs, lasers, electric vehicles, refrigerators, gaming systems as well as mobile phones.
b. Potting/ encapsulating sensors
Thermally conductive adhesives are also used for potting as well as encapsulating sensors. They are mainly used as they can be adhered strongly to different kinds of materials and additionally also provide protection from various chemicals. By using thermally conductive adhesive potting as well as encapsulation, formulations are given additional protection from moisture as well as different kinds of corrosive agents. Moreover, by using thermal conductive adhesives in potting, a greater degree of protection is given from any kind of moisture as well as corrosive agents like that of vibrations, shocks, heat build-up, and many more. However, certain epoxies can be a bit rigid; however, they are formulated to make them more flexible, thereby leading to easy retrieval.
c. Chip scale packages
Chip scale package technologies are mainly used for electronic products, and their popularity has been major because of the rise in demand for compact as well as portable electronic systems. Thermally conductive adhesives are greatly used for chip-scale packages due to the increase in demand for portable electronics.
d. Power semiconductors
Conduction, convection, and radiation are the three primary ways by which heat is dissipated from an electronic device. Conduction will carry the bulk of the heat from the heat source in the device’s core through the semiconductor substrate, the lead frame to which the chip is mounted, and the molding material that encapsulates the device to its external surface for a packed semiconductor device. At this stage, heat can be transported further via conduction through any solid substance with which the device comes into contacts, such as a printed circuit board or an external heatsink.
Thermal resistance values for packaged devices are provided by semiconductor makers as design assistance to help assess their power handling capability. This value, usually expressed as a junction-to-ambient thermal resistance, is used to calculate the amount of power that can be safely dissipated inside a device without exceeding its designated maximum junction temperature (Tj).
Hence most of the power semiconductors are equipped with thermally conductive adhesives.
Thermally Conductive Adhesive vs. Thermally Conductive Grease
Before we go ahead to explain the difference between thermally conductive adhesive and thermally conductive let us explain what is thermally conductive grease
Thermal grease, also known as a thermal paste or thermal compound, is a substance used to improve heat transfer between two surfaces, most typically between a microprocessor and a heatsink. Most microprocessors do not have a totally flat top surface. Some contain minuscule grooves, while others have a little curvature, which creates air gaps between the CPU and the heatsink, lowering the heatsink’s cooling capacity. A small layer of thermal grease is applied to the top of the CPU and the base of the heatsink to fill the air spaces.
Now let us understand some of the properties of thermally conductive adhesive
Since thermal grease is electrically insulative and thermally conductive, it can be used in practically any electronic application that requires connectivity between thermal management components. These qualities, however, necessitate certain chemical compositions. Thermal grease and most thermal connection devices are made up of two major compounds:
· A matrix is a polymer basis that is often used.
· Metal filler, liquid or micronized
The thermal and electrical conductivity qualities of the grease are determined by the ratio between these two types of components. Each grease mixture will have its own benefits and implications depending on the percentage.
Now let us understand the relation between thermal conductive adhesives and thermal paste.
Thermal grease can also behave as an adhesive depending on the polymer chemistry of the liquid matrix. Because of its adhesive properties, some producers refer it thermal grease as “thermal glue.” Thermal adhesives come in a variety of shapes and sizes, including a solid-form tape designed to provide a quick fix for adhesion in low thermal conduction situations. These adhesives can also be more typical glues that are applied in liquid form and have sticky characteristics once cured.
Now let us understand some of the benefits of using thermally conductive adhesive.
Benefits of Thermally Conductive Adhesive
· A high strength performance as well as good adhesion regarding various substrate materials
· Resistance to a very high as well as low temperature, thereby being able to cope with different types of expansion as well as contraction in between substrate materials that are dissimilar
· A higher resistance to any kind of chemicals, water level as well, as humidity
· Also, by using thermally conductive adhesives, low outgassing is done so as to minimize the risk of damage so as to create sensitive circuitry.
· Moreover, the thermally conductive adhesive also has a non-corrosive formulation, thus making it environment-friendly.
· Thermally conductive adhesives have resistance to thermal shock, impact as well as vibration.
· Thermally Conductive Adhesives are also able to withstand a greater degree of solder-reflow processes.
· Moreover, thermally conductive adhesives also have a greater degree of compliance with that of RoHS and REACH.
Application Methods of Thermally Conductive Adhesives
Thermally conductive adhesives in tapes or liquids, as well as non-adhesives, are currently used for attachment. Silicones, epoxies, and acrylics make up the former, while thermally conductive pads, or greases, make up the latter. Adhesives or mechanical fasteners can be used in conjunction with the pads. Some pads have pressure-sensitive adhesives for easy mounting, while others are clipped in place. Thermally conductive greases, like some of the pads, must be used with metal clips. The clips, which are normally made of stainless steel, allow for easy attachment and are resistant to even the harshest shock and vibration. Microprocessors are the most typical use. Polyimide or polyester-supported films and pressure-sensitive acrylic adhesives have been used to create thermally conductive tapes.
Epoxy adhesives, like tapes, can provide excellent thermal and electrical insulation, making them perfect for heat sinks to transistors, components, packages, and boards. One example is epoxy-impregnated glass cloth. Epoxy preforms, like tapes, are often applied by hand. So, while their installation is straightforward, they require manual hand positioning by operators. As a result, an operator-friendly process may come at the expense of overall product throughput. Epoxy resin adhesive pastes should be examined since they enable for mass production automation. They are the most used substance for component bonding. While screen printing is often used, the preferred approach for high temperatures is epoxy dielectric coatings with high conductivity.
Thermal Adhesives Requirement in Manufacturing Process
Adhesives can support a wide range of application segments and provide the performance characteristics required for manufacturing compatibility, structural support, and protection. Adhesives, for example, must flow freely during manufacture to avoid void formation while still having short curing times to help speed up the whole process. Adhesives must also cling securely to surfaces, forming a strong link between heat sinks and components on PCBs or a die and package for integrated circuits.
Indeed, the interface between the die and the package faces can be subjected to some of the most severe strains in an electronic assembly. When two dissimilar materials are linked together, variations in their coefficients of thermal expansion (CTE) can cause stress and strain, potentially weakening or even fracturing the connection. Thermal cycling can occur even during regular device operation, putting additional strain on this bonding interface. Engineers can eliminate thermal expansion mismatches between die and package by using adhesives with the proper CTE, minimizing stress while providing sufficient structural support for the assembly.
Challenges in the Thermal Adhesive Sector
Engineers are looking for adhesives that are more effective at transporting heat away from sensitive components and complete assemblies as the demand for better thermal management grows. Thermal characteristics of adhesive materials are often specified in terms of bulk thermal conductivity, which describes the material’s ability to transport heat through itself. Thermal management in electrical design, on the other hand, is still more concerned with dispersing or moving heat from a die through a package or from a component through a heat sink.
Internal heat transmission capabilities described by the bulk thermal conductivity criteria only address part of the main goal for an engineer. A more practical quality in a typical product assembly is the material’s thermal resistance. The ease with which heat can flow over the interface between the die (or component) surface and the bonding material, and lastly, through the contact between the bonding material and the packaging is referred to as thermal resistance (or heat sink).
The thickness of the bond and the type of the surfaces meeting at the bond interface can obstruct heat transfer in any product assembly, whether it’s a die linked to a package or a component attached to a heat sink. The rate of heat flow across a medium is inversely related to the thickness of the medium, according to Fourier’s equation of heat conduction. As a result, producers strive for the smallest feasible bond lines when joining materials. In reality, a thin bond line is preferable to a large one since it not only reduces thermal resistance but also reduces stress at the bond joint’s corners. Furthermore, when compared to a thick bond line, a narrow bond line results in less air cavity concentration.
Conclusion
This was all about thermally conductive adhesives. Thermally conductive adhesives for a long have played a major role in the electronic manufacturing, and as per industry trends, the demand for thermally conductive materials are rising. Engineers require materials that can disperse more heat from modern electrical equipment for fast-growing industries such as high-end electronics, LED lighting, and more. Thermally conductive adhesives meet not only a wide range of thermal management needs in this environment but also equally difficult requirements for manufacturing assembly and extended product lifecycles.