Thermally Conductive Adhesives - everything you need to know about thermally conductive adhesives

Thermally conductive adhesives are primarily used to dissipate heat from power electronics. They are mainly used for bonding heat sinks. For example, thermally conductive adhesives are primarily used to reduce stress to prevent performance degradation and electronic component failure.

Thermally conductive adhesives are mainly used to bond heat sinks, LEDs and other heat-generating electronic components. Most thermal adhesives will bond to both metals and plastics and provide permanent physical strength.

Synthetic resins with filler components made of metallic and inorganic materials are used to produce thermally conductive adhesives. Metallic fillers, such as silver or graphite, offer the best coefficients of thermal conductivity. However, these properties make the adhesive electrically conductive, which is undesirable in various applications. In order to achieve thermal conductivity and electrical insulation at the same time, it is necessary to use adhesives reinforced with mineral fillers.

Types of thermally conductive adhesives

We distinguish 3 types of thermally conductive adhesives:

1. epoxy adhesives
2. silicone adhesives
3. polyurethane adhesives

1. Epoxy Adhesive

Epoxy adhesives are made from a mixture of two components, primarily the resin and the hardener. When the resin is mixed with a suitable catalyst, curing is initiated. Curing is the process in which the molecular chains react with chemically active sites, resulting in an exothermic reaction. It is also important to know that the covalent bonds that exist between the epoxy groups of the resin and the amine groups of the hardeners are created by the double combination of crosslinking of the polymer.

Some variation in mechanical strength properties occurs by controlling curing conditions with temperature and choice of resin and hardener compounds. Epoxy adhesives are mainly used as one-part or two-part systems. One-component epoxy adhesives can be cured at temperatures of approx. 120-150 C. These conditions result in higher strength as well as excellent adhesion to metals, resulting in tough chemical resistance.

2. Silicone Adhesive

Silicone adhesive is a versatile, waterproof polymer whose main component is silicon dioxide, found primarily in quartz. The term silicone primarily refers to the group of polymers that have a siloxane bond with that of organic compounds. The production of silicone adhesives mainly begins with the isolation of silicon from silica. The special properties of silicon make it very resistant. In addition, the elasticity and other properties of silica remain unchanged at any temperature. Silicone adhesives are primarily used to bond metals due to their flexibility and properties, as well as their ability to bond dissimilar substrates.

Silicone adhesives are primarily made by isolating silica from silicon. Silicon dioxide is generally found in its pure form in certain minerals. Silicone adhesives and sealants are primarily made from polymerized silicone. When silicone is uncured, it forms a highly tacky gel as well as a liquid. It is quite safe to use and is used in a variety of applications that are mostly non-toxic. There are different types of silicone adhesives, such as: B. High temperature silicone adhesives, room temperature vulcanizing silicone adhesives (RTV), silicone rubber adhesives, etc.

3. Polyurethane Adhesive

Polyurethane adhesives are primarily thermoplastic adhesives or polymers containing replicas of organic chain units bonded primarily with urethane links. The chemical reaction between the polymers leads to the formation of an adhesive. In general, polyurethane adhesives are mainly brown or transparent. Pigments like the green or red color can be added mainly for application purposes like spraying, making them visible in the application areas. They are used in construction, furniture manufacturing, lamination, flooring and roofing, aerospace and cold storage. The polyurethane adhesives are mainly available as a 2-component adhesive or as a one-component adhesive. The 2-component adhesive usually hardens much faster than the 1-component variant and takes around 30 minutes, depending on the requirement. Polyurethane adhesives are sealants that can cure even under difficult conditions such as heat and humidity. In addition, polyurethane adhesives are also suitable for various materials such as metal, wood, concrete, rubber, epoxy resin and glass. It's also important to know that PUR adhesives are fairly waterproof; however, water resistance generally varies by formulation. In addition, polyurethane adhesives are environmentally friendly as they are solvent-free and have a very low VOC value. In addition, they are also food safe.

Polyurethane adhesives or PU adhesives are very versatile and have the best properties. They are also modified to provide custom formulas that can be developed based on specific applications. This includes the ability to change physical properties such as viscosity and application properties such as pot life. Polyurethane adhesives can be divided into two main categories, 1-part or 'part' and 2-part or 2-part systems with the best advantages and disadvantages of each.

The chemical reaction between an isocyanate and a polyol serves as the basis for all polyurethane sealants and adhesives. For 2K systems, the isocyanate and the polyol are manufactured and supplied separately. In order to initiate the chemical reaction and properly cross-link the system, the two components must be mixed just before use. In order to ensure this, an accurate ratio of the components and sufficient mixing must be ensured to achieve the required polyurethane adhesive properties.

In contrast to this, 1-component systems in the production of polyurethane adhesives are mainly produced with the polyol, which reacts with the excess isocyanate so that the polyol chain can be terminated with the isocyanate group. The ratio of isocyanate and polyol determines the chain length of the terminal polyurethane polymer. It should also be noted that this polyurethane prepolymer is the most important reactive component for 1K adhesive systems.

To complete a reaction, a 1K system must interact with water to fully crosslink. It's also important to realize that there must be water in a 1K system for it to crosslink, and that water can be moisture from the atmosphere.

Applications of thermally conductive adhesives

Thermally conductive adhesives are primarily used for potting, coating and other encapsulation applications. Some of the specific uses are

a. Bonding to dissipate heat

Certain components of thermally conductive epoxy and silicone compounds are primarily used for bonding heat sinks to electronic components and circuit boards for heat dissipation. They are primarily designed to prevent overheating and premature component failure. Thermal paste is used in computers, LEDs, lasers, electric vehicles, refrigerators, gaming systems, and cell phones.

b. Potting/encapsulation of sensors

Thermally conductive adhesives are also used for potting and encapsulating sensors. They are used primarily because they can be firmly bonded to various materials and also provide protection from various chemicals. The use of thermally conductive adhesives for potting and encapsulation gives the formulations additional protection from moisture and various types of corrosive agents. The use of thermally conductive adhesives in the potting also provides a higher level of protection from any type of moisture, as well as from corrosive agents such as vibration, shock, heat build-up and more. However, certain epoxies can be somewhat rigid; however, they are formulated to be more flexible, resulting in easy recovery.

c. chip scale package

Chip-scale package technologies are mainly used for electronic products, and their popularity is high due to the increasing demand for compact and portable electronic systems. Thermally conductive adhesives are being used extensively for chip-scale packages as the demand for portable electronics increases.

i.e. power semiconductors

Conduction, convection, and radiation are the three main ways that heat is removed from an electronic device. Conduction carries most of the heat from the heat source in the core of the device through the semiconductor substrate, the lead frame to which the chip is attached, and the molding material that encapsulates the device to the outer surface of a packaged semiconductor device. At this stage the heat can be further transported by conduction through any solid substance with which the device comes into contact, e.g. B. by a circuit board or an external heat sink.

Thermal resistance values ​​for packaged devices are provided by semiconductor manufacturers as a design aid to assess their power handling capability. This value, usually expressed as the junction-to-ambient thermal resistance, is used to calculate the power that can be safely dissipated inside a device without exceeding the specified maximum junction temperature (Tj).

For this reason, most power semiconductors are equipped with thermally conductive adhesives.

Thermally Conductive Adhesive vs. Thermally Conductive Grease

Before explaining the difference between a thermally conductive adhesive and a thermally conductive grease, let's explain what a thermally conductive grease is

Thermal grease, also known as thermal grease or thermal paste, is a substance used to improve heat transfer between two surfaces, typically between a microprocessor and a heat sink. Most microprocessors do not have a completely flat top. Some contain tiny grooves, while others have a slight bulge, creating air gaps between the CPU and the heatsink that reduce the cooling performance of the heatsink. A small layer of thermal paste is applied to the top of the CPU and the bottom of the heatsink to fill the air spaces.

Now let's understand some of the properties of thermal adhesive.

Because thermal paste is electrically insulating and thermally conductive, it can be used in virtually any electronic application that requires a bond between thermal management components. However, these properties require a specific chemical composition. Thermal paste and most thermal connectors consist of two main components:

– A matrix is ​​a polymer base that is commonly used.

– Metal filler, liquid or micronized

The thermal and electrical conductivity of the grease is determined by the ratio between these two components. Each fat blend has its own benefits and effects that depend on the proportion.

Now let's understand the relationship between thermal adhesives and thermal paste.

Thermal paste can also behave like an adhesive depending on the polymer chemistry of the liquid matrix. Because of its adhesive properties, some manufacturers refer to thermal paste as “thermal adhesive.” Thermal adhesives come in a variety of shapes and sizes, including a rigid tape that provides a quick fix for adhesion in low heat conduction situations. These adhesives can also be typical adhesives that are applied in liquid form and have tacky properties after curing.

Now let's understand some of the advantages of using thermally conductive adhesive.

Advantages of thermally conductive adhesive

• High strength and good adhesion with respect to various substrate materials
• Resistance to very high and low temperatures, allowing different types of expansion and contraction between different substrates to be accommodated
• A higher resistance to any kind of chemicals, water and moisture
• In addition, low outgassing is achieved through the use of thermally conductive adhesives, minimizing the risk of damage to sensitive circuitry.
• In addition, the thermally conductive adhesive is non-corrosive and therefore environmentally friendly.
• Thermally conductive adhesives are resistant to thermal shock, impact and vibration.
• Thermally conductive adhesives are also able to survive higher levels of solder reflow processes.
• In addition, thermally conductive adhesives meet the requirements of RoHS and REACH to a greater extent.

Application methods for thermally conductive adhesive

Currently, thermally conductive adhesives in the form of tapes or liquids as well as non-conductive adhesives are used for attachment. The former include silicones, epoxies and acrylates, while the latter include thermally conductive pads or greases. Adhesives or mechanical fasteners can be used in conjunction with the pads. Some pads come with pressure-sensitive adhesives for ease of assembly, while others attach with clips. Thermally conductive greases, like some of the pads, must be used with metal clips. The brackets, which are usually made of stainless steel, allow for easy attachment and are resistant to even the harshest shock and vibration. Microprocessors are the most typical application. Polyimide or polyester films and acrylic pressure-sensitive adhesives are used to make thermally conductive adhesive tapes.

Like adhesive tape, epoxy adhesives can provide excellent thermal and electrical insulation, making them excellent heat sinks for transistors, components, cases, and circuit boards. An example is epoxy resin impregnated glass fabric. Like adhesive tapes, epoxy preforms are often applied by hand. While easy to install, they require manual positioning by operators. Therefore, an operator-friendly process can come at the expense of overall product throughput. Epoxy resin adhesive pastes should be examined as they allow automation of mass production. They are the most commonly used substance for bonding components. While screen printing is commonly used, high temperature dielectric epoxy coatings are preferred for use at high temperatures.

Requirements for thermally conductive adhesives in the manufacturing process

Adhesives can support a wide range of application segments and provide the performance characteristics needed for manufacturing compatibility, structural support, and protection. Adhesives, for example, must be able to flow freely during manufacture to avoid void formation, while at the same time having short curing times to speed up the entire process. Adhesives also need to adhere securely to surfaces and create a strong bond between heat sinks and components on a circuit board or a chip and an integrated circuit package.

In fact, the interface between the chip and the package side can be subjected to some of the most severe stresses in an electronic assembly. When two dissimilar materials are bonded together, variations in their coefficients of thermal expansion (CTE) can cause stress and strain that can weaken or even break the bond. Even during regular device operation, there can be temperature fluctuations that put additional stress on this connection interface. Engineers can accommodate thermal expansion mismatches between die and package by using adhesives with the correct CTE that minimize stress while providing adequate structural support for the assembly.

Challenges in the field of thermal adhesives

As the demand for better thermal management increases, engineers are looking for adhesives that more effectively dissipate heat from sensitive components and complete assemblies. The thermal properties of adhesives are often specified in terms of thermal conductivity, which describes the material's ability to transport heat through itself. Thermal management in electrical design, on the other hand, is more about dissipating or transporting heat from a chip through a package or from a component through a heatsink.

Internal heat transfer capabilities, described by the criteria of thermal conductivity, are only part of the main goal for an engineer. A more practical property in a typical product assembly is the thermal resistance of the material. The ease with which heat can flow across the interface between the chip (or device) surface and the interconnect material and ultimately through the contact between the interconnect material and the package is referred to as thermal resistance (or heat sink).

The thickness of the bond and the type of surfaces that meet at the bond interface can impede heat transfer in any product assembly, whether it's a chip that's bonded to a package or a component that's attached to attached to a heat sink. According to the Fourier equation for heat conduction, the rate of heat flow through a medium is inversely related to the thickness of the medium. Therefore, when connecting materials, manufacturers strive for the smallest possible connecting lines. In reality, a thin bond line is preferable to a large one because it not only reduces thermal resistance, but also reduces stresses at the corners of the bond line. Also, a narrow glue line results in a lower concentration of air voids compared to a thick glue line.

Conclusion

This was all about thermally conductive adhesives. Thermally conductive adhesives have long played an important role in electronics manufacturing, and according to industry trends, the demand for thermally conductive materials is increasing. Engineers need materials that can dissipate more heat from modern electrical devices for fast-growing industries like high-end electronics, LED lighting, and more. Thermally conductive adhesives not only meet a variety of thermal management needs in this environment, but also equally challenging manufacturing assembly requirements and extended product lifecycles.

 

Frequently Asked Questions

1. How to mix and measure thermal adhesive? ?

It is important to stir all products before weighing or dosing. First, try carefully weighing both the resins and hardeners or powders and binders into separate containers before mixing. Try to mix both parts of the adhesive systems thoroughly before use.

Improper weighing or mixing can result in materials not curing, surface softening and air voids, and elevated temperature softening.

2. How can dissimilar materials be glued?

By selecting an adhesive with a coefficient of thermal expansion that is tailored to the materials to be bonded. If possible, choose a flexible epoxy. Try to remove dirt, oils and grease and mechanically roughen the surface before gluing. Materials should cure at room temperature for 4-16 hours.

3. How should adhesives be diluted for application?

Most manufacturers recommend a commercially available thinning agent for thinning adhesives because it is relatively easy to use. All adhesive thinner labels indicate how much thinner can be used; therefore, you should follow the instructions and precautions accordingly.

4. How to control ceramic adhesives and castable ceramics?

Excess activator or extra water applied to uncured mixes when using ceramic materials can result in cracked and weak castings, encapsulation or bonded joints. Check the mixing ratio used when mixing the components. Castable ceramics reach their maximum strength after heat treatment.