By Catherine Veilleux on February 05, 2024
Batteries & EVs
Image header courtesy of Appli-Tec
Thermal adhesives are more important than ever to effectively manage heat. Advanced systems like EV batteries and microelectronic devices need them to prevent heat from causing performance problems, damaging electrical components, and creating safety issues.
Thermally conductive adhesives such as tapes, films, and glues are used to bond heat-generating components to heat sinks. Thermal adhesives not only conduct heat, but they can also provide structural integrity, electrical insulation, or electrical conductivity.
Knowing which adhesive is best for your needs is not easy, and you may not know where to start.
This guide is here to help you through your search.
Table of Contents
- Adhesives vs. Non-Adhesives
- Adhesive Application Formats: Liquids, Tapes, and Films
- Adhesive Chemistries: Epoxy, Urethane, Acrylic, and Silicone
- Understanding Properties on Spec Sheets
- List of Adhesive Manufacturers
- Surface Preparation for a Stronger Bond
Adhesives vs. Non-Adhesives
When it comes to heat conductivity, thermal adhesives compete with other types of thermal interface materials called thermal non-adhesives. These include gap pads, gap fillers, phase change materials, and greases. Some potting materials can also provide basic levels of thermal conductivity.
If your thermal material plays a structural role (by replacing other joining techniques such as welding or mechanical fastening), you need adhesives. Otherwise, you need non-adhesives.
This article focuses on adhesives, but if you’re interested in learning more about non-adhesives, here’s a quick overview:
- Gap pads are thin sheets designed to fill gaps and improve thermal conductivity between surfaces.
- Gap fillers are soft materials used to fill gaps and air voids to enhance heat transfer between components.
- Phase-change materials are materials that change phase (e.g., from solid to liquid) during temperature changes, absorbing and releasing heat in the process.
- Greases are viscous compounds with high thermal conductivity. They are used to fill gaps and provide a thermal interface between surfaces.
- Potting materials are compounds used to encapsulate electronic components to provide protection against environmental factors (water, dust, etc.), mechanical stress, and insulation. They are used to seal the housings of electric motors, for example.
Adhesive Application Formats: Liquids, Tapes, and Films
Thermal adhesives can be applied as liquids, tapes, or films. To choose between one format or another, it’s important to consider certain aspects of your application such as strength, electrical insulation/conductivity, surface area, gap size, and so on.
Here are the key characteristics of each application format:
- Liquids (also called glues and pastes) provide strong bonds in addition to thermal conductivity. However, they need to be cured to form an adhesive bond with the substrate. They can be cured with heat or at room temperature. Liquids conform better to the shape of the surfaces and fill air gaps. As such, they provide better contact and coverage than films or tapes, improving heat transfer.
- Double-sided tapes can provide electrical insulation in addition to thermal conductivity. Tapes are also easy to apply, making them convenient.
- Films can provide electrical conductivity in addition to thermal conductivity. They are ideal to bond large or complex areas, where liquids may create voids (air pockets).
Adhesive Chemistries: Epoxy, Urethane, Acrylic, and Silicone
The different adhesive chemistries offer unique properties. To know which one is right for you, start by gathering the following information:
- What substrates do you want to bond?
- Do you need electrical conductivity or electrical insulation?
- What force does the adhesive need to withstand?
- What kind of chemical resistance do you need?
Chemical resistance is the ability of an adhesive to withstand the chemicals to which it is exposed. For example, some adhesives can dissolve in glycol—a common coolant used for electric battery thermal management.
Here’s an overview to help you choose which chemistry fits your application:
| Adhesive Chemistry | General Characteristics |
| Thermally Conductive Epoxy Adhesives |
|
| Thermally Conductive Urethane Adhesives |
|
| Thermally Conductive Acrylic Adhesives |
|
| Thermally Conductive Silicon Adhesives |
|
Understanding Properties on Spec Sheets
To understand an adhesive’s technical data sheet (TDS), compare different products, and identify which products meet your needs, it’s essential to understand the key properties of thermal adhesives. Let’s go over the most important ones.
Thermal Conductivity
Thermal conductivity measures the rate at which heat is transferred through a material. A smaller value indicates that a material is more resistant to heat transfer, whereas a larger value indicates that it is a good thermal conductor for heat sink applications. It is measured in watts per meter kelvin (W/m−1K−1).
Let’s break down the formula with an example.
A liquid adhesive like Loctite 315 has a thermal conductivity of 0.81 W/m−1K−1 at room temperature, which means 0.81 joules of thermal energy can go through it every second, given it is 1 meter long and there is a temperature difference of 1 kelvin on both sides of the adhesive.
This formula might seem abstract, but it’s a standard used to compare one product to another.
To know if an adhesive is right for your application, the formula must be converted to your situation. After all, it’s unlikely that you’ll apply 1 meter thick of a liquid adhesive and that the temperature difference will be exactly 1 kelvin.
It’s also important to know that the thermal conductivity of a material varies at different temperatures. Understanding the thermal conductivity of an adhesive at different temperatures can be important if your components experience low and high temperatures, such as in automotive, aerospace, and electronics applications.
Flame Rating (UL 94)
Thermal adhesives can be classified based on how they burn in the event of a fire, more specifically how easily they catch fire when exposed to a flame and how quickly they self-extinguish when the flame is removed. This is useful to evaluate if an adhesive is a good flame retardant.
Specific factors that contribute to the spread of fire are evaluated when classifying thermal adhesives. Here are the most important ones:
- The rate at which the adhesive burns
- How quickly the adhesive self-extinguishes
- Whether molten material drips away (flaming drips)
- Whether burn-through holes are created in the adhesive (holes provide pathways for the fire to spread)
UL 94 is a standard called Tests for Flammability of Plastic Materials for Parts in Devices and Appliances. It is used as follows to classify thermal adhesives:
| UL 94 Certification | Meaning |
| 5VA (highest rating) |
Indicates that the adhesive is self-extinguishing in severe conditions, does not drip molten material during burning, and that no holes are created during burning. To achieve this certification, the adhesive was placed vertically, and a 500W flame was applied to its bottom for 5 seconds. The flame was reapplied as soon as the burning stopped (5 times in total). After the fifth flame, the adhesive self-extinguished within 10 seconds. There were no flaming droplets during the test, and there were no holes. |
| 5VB |
Indicates that the adhesive is self-extinguishing in severe conditions, does not drip molten material during burning, but that holes may be created during burning. To achieve this certification, the adhesive was placed vertically, and a 500W flame was applied to its bottom for 5 seconds. The flame was reapplied as soon as the burning stopped (5 times in total). After the fifth flame, the adhesive self-extinguished within 60 seconds. There were no flaming droplets during the test, but there may have been holes. |
| V-0 |
Indicates that the adhesive is self-extinguishing (quick) and that it does not drip molten material when burning. To achieve this certification, the adhesive was placed vertically, and a 50W flame was applied to its bottom for 10 seconds. The flame was reapplied as soon as the burning stopped (2 times in total). After the second flame, the adhesive self-extinguished within 10 seconds. There were no flaming droplets during the test. |
| V-1 |
Indicates that the adhesive is self-extinguishing and that it does not drip molten material when burning. To achieve this certification, the adhesive was placed vertically, and a 50W flame was applied to its bottom for 10 seconds. The flame was reapplied as soon as the burning stopped (2 times in total). After the second flame, the adhesive self-extinguished within 60 seconds. There were no flaming droplets during the test. |
| V-2 |
Indicates that the adhesive is self-extinguishing but that it may drip molten material when burning. To achieve this certification, the adhesive was placed vertically, and a 50W flame was applied to its bottom for 10 seconds. The flame was reapplied as soon as the burning stopped (2 times in total). After the second flame, the adhesive self-extinguished within 60 seconds. There may have been flaming droplets during the test. |
| HB (lowest rating) |
Indicates that the adhesive will burn slowly or that the flame will eventually self-extinguish. To achieve this certification, the adhesive was placed horizontally, and a Tirrill burner flame was applied to one end for 30 seconds. After the flame was removed, the adhesive self-extinguished before the flame reached 100 mm, or the burning rate was slower than 76 mm per minute (for a thickness lower than 3 mm). |
Viscosity
Viscosity is a parameter that is relevant for liquid adhesives. It indicates an adhesive’s internal resistance to flow or, put more simply, how easily it can spread. Understanding viscosity is important to know how the adhesive will behave during application, spreading, and bonding.
- A low viscosity indicates that an adhesive is thinner and can flow more easily. Lower viscosity adhesives are ideal for applications where they need to spread into small gaps or coat irregular surfaces.
- A high viscosity indicates that an adhesive is thicker and won’t flow so easily. Higher viscosity adhesives are ideal when they need to be applied vertically, as they won’t sag or drip. However, they can also create less uniform and thicker bond lines. This can affect thermal conductivity and the strength of the bonded joint.
Viscosity is measured in centipoises (cP). The viscosity of liquid adhesives can be as low as 200 cP and as high 1,000,000 cP. For comparison, the viscosity of water is 1 cP (at 20°C), and the viscosity of glycerol is 1,412 cP (at 20 °C).
Volume Resistivity
Volume resistivity indicates an adhesive’s resistance to the flow of electric current passing through its volume. It is expressed in ohm-centimeters (Ω-cm) and is key to understand an adhesive’s insulating or conductive properties. A lower volume resistivity indicates better electrical conductivity, while a higher volume resistivity indicates better electrical insulation.
For example, Loctite Ablestik 5025E is a thermal adhesive film that is electrically conductive. Its spec sheet mentions that it has a volume resistivity of 2 x 10-4. This means that its resistance to the flow of electrical current is 0.0002 Ω-cm and, that it is a very good electrical conductor.
Another example is Loctite 315, a liquid adhesive with a volume resistivity of 1.3 x 1012. This means that its resistance to the flow of electrical current is 1,300,000,000,000 Ω-cm, making it is a very good electrical insulator.
Breakdown Voltage (Dielectric Strength)
Breakdown voltage, also called dielectric strength, is relevant for thermal adhesive that are also used for electrical insulation. It indicates the voltage (in alternating current) at which the adhesive loses its insulating properties and becomes electrically conductive.
Breakdown voltage is measured in volts alternating current (VAC). When selecting thermal adhesives for electrical insulation, it's essential to choose those with a breakdown voltage higher than the requirements of your application.
For example, Bergquist Bond-Ply TBP 1400LMS-HD is a thermally conductive and electrically isolating tape with a breakdown voltage of 5,000 VAC. This means that 5,000 volts of alternating current can be applied to the tape before it becomes electrically conductive. Beyond this point, there is a risk of electrical failure.
Lap Shear Strength
Lap shear strength is most relevant for adhesives used in structural bonding applications. It indicates the adhesive’s ability to withstand forces in a direction parallel to the bond line. This property helps understand the adhesive's ability to provide a strong and durable bond between two substrates.
Lap shear strength is measured in megapascal (MPa) or in pounds per square inch (PSI).
- 10 MPa means that the adhesive bond can withstand 10 newtons of force for each square millimeter of the bond line.
- 10 PSI means that the adhesive bond can withstand 10 pounds of force for each square inch of the bond line.
For example, the CoolTherm TC-2002 is a thermally conductive structural adhesive with a lap shear strength of 15.86 MPa (2300 psi). This means that the adhesive bond can withstand 15.86 newtons of shear stress (i.e., of force per square millimeter). So, an adhesive bond of 10 square millimetres would be able to withstand 158.6 newtons of force.
List of Adhesive Manufacturers
To choose the right thermal adhesive for your application, it’s important that you consider your options. Here is a list of well-known adhesive manufacturers along with a link to their thermal adhesive products.
Surface Preparation for a Stronger Bond
When choosing a thermal adhesive, it’s essential to consider surface preparation. Surface preparation can improve the strength of the bond in several ways:
- By making sure there are no contaminants that interfere with the bonding process
- By modifying the surface roughness to create more bonding sites
- By modifying the chemical composition of the substrate’s surface to create more bonding sites
Surface preparation can be done using several methods: laser texturing, chemical etching, mechanical abrasion, or plasma treatment. Our studies show that laser texturing provides better results than alternatives.
Here is an example comparing laser texturing and sandblasting.
In this test, we compared the effect of both surface preparation methods on the bond strength of Loctite EA 9460, an epoxy adhesive paste.
Results show that laser texturing provides a stronger bond, both in terms of lap shear strength and pull-off strength.
In the event that the adhesive breaks, the stronger bond leads to cohesive failure instead of adhesive failure. This indicates excellent adhesion, as the adhesive (and not the joint) is responsible for the failure.
We have a complete guide on how to do surface preparation for adhesive bonding if you want to explore the subject deeper.
Finding the Best Adhesive Solution
When searching for an adhesive, it’s important to keep in mind that you will need to perform multiple rounds of tests to find the best solution for your needs. We can help with this.
Our experts can work with you to explore how laser surface preparation improves the bond strength of the adhesives you select. We can even work with your adhesive supplier to research the best solution.
With the right surface preparation, you can access a wider range of adhesives that would otherwise not be strong enough.
