Factors to consider when choosing thermal interface materials
01 July 2022
Thermal interface material on electronics
From consumer electronics to aerospace, all electronic devices require active thermal management, to effectively dissipate heat away from components. The bottleneck of thermal management is often the conduction between heat-generating components & cooling components...
This article was originally featured in the July 2022 issue of EPDT magazine [read the digital issue]. And sign up to receive your own copy each month.
Unfortunately, many electronic devices cannot get sufficient cooling from heat sinks or fans due to limited space. In these applications, the performance of thermal interface materials (TIMs) is even more important, explains Dr Yong Joon Lee, industry expert in polymer composite materials & contributor to online materials database for engineers, Matmatch…
The most common methods for extracting heat from high-power CPUs (computer processing units) or SoCs (system-on-chips) are thermal paste or grease. While these solutions offer high performance, they also come with disadvantages: they are messy, time-consuming and can have poor long-term reliability. Additionally, conventional high conductive (>20 W/mK) TIMs are known to be expensive and come with manufacturing difficulties and challenges.
However, TIMs are integral to modern electronic device design. As such, engineers and product designers must consider several factors when specifying and selecting TIMs for their application.
There are many types of TIMs, including grease, gel, pads, paste, tapes, phase change materials (PCMs) and even metal. When selecting TIMs, it is important to be familiar with the product and its properties and performance. For thin gap fillers, grease or PCMs are commonly used, while users typically look for pads, gel or putty paste for thick gap fillers.
TIMs can also be categorised as TIM1, TIM2, or TIM1.5. This categorisation is based on where they are used in reference to a device’s die chip and heat spreader or lid. For example, a TIM1 application uses TIMs between the die chip and heat spreader or lid, and its main purpose is to reduce contact resistance and dissipate extreme heat directly from processors. Traditionally, metal solder is used for this application; however, grease, gel, or PCMs can also be used.
On the other hand, a TIM2 application is applied between the heat sink and heat spreader or package level SoC. This application typically uses thicker TIMs, like pads, since the package-level heat management is not extreme, like at chip-level. A TIM2 application may require rework ability and compressibility, which allows for more impact due to the attached heat sink’s needs.
In a TIM1.5 application, the chip die is in direct contact with the cooling component, without heat spreader. This category of TIM is commonly used in mobile devices. Gel and paste are sometimes preferred to pads because their thinner bond lines can allow them to have higher compressibility and thermal performance in comparison to pads. Thermexit™ pads provide a solution to both issues by providing great conformability and thermal performance.
Another important factor to consider is thermal performance. In an electronic device, there are components generating heat and cooling components that determine how much heat can be dissipated. This means that the environment temperature and the presence of active cooling components can determine the thermal budget. Once you know your thermal budget, then you can decide what kind of thermal interface materials are needed, since TIMs can be easily customised to meet your thermal design.
Physical & mechanical properties
The mechanical properties of thermal interface materials, such as hardness, deflection and compression set, should also be considered. Some users prefer compounds or gels to protect their sensitive components, even though these dispensable products may have disadvantages that will be discussed later. It is important to find the right TIM for your application’s pressure and gap thickness design.
Another important factor is whether a TIM is electrically insulating or not. Some applications are very sensitive when it comes to electrical continuity. Most thermal gap pads provide great electrical insulation due to their relatively thick application, unlike grease, gel or PCM products.
When selecting a TIM, you should consider long-term reliability. Today, there are many electronic devices used in extremely harsh environments that require high power cycles. Additionally, it is important for TIMs in automotive applications to be tested under mechanical vibration.
Each form factor is related to an application factor, like applied pressure, attachment method, bond line thickness, geometry and environment. Simple factors such as ease of handling and environment can be easily neglected, but cannot be ignored.
Think beyond the datasheet
While TIMs’ datasheets provide useful information, they should not be the only resource used when selecting TIMs. Much of the data from TIM suppliers is from industry standards or their own test methods to optimise the performance of the product. However, the same TIM product performs differently depending on the conditions. Typically, we would expect that higher thermal conductivity would result in better performance, but is this always the case?
Also, we should ask whether a higher thermal conductivity TIM is needed when a lower thermal conductivity TIM might provide adequate performance for the application. Sometimes, if a higher thermal conductivity TIM is used in an application that does not require it, the TIM will not provide the same benefits.
Gap pad type
Finally, let’s say we decide to use a gap pad. Before we consider thermal conductivity or compressibility, we need to look at basic properties and requirements. For example, the application temperature of the device will determine the necessary resin chemistry. It is also important to know whether the device requires a high-temperature silicone TIM, or if it can use an alternative resin system. Some gap pads have a fabric glass carrier, which may not be good for certain applications. Sometimes, having the correct thickness alone can solve many stringent thermal issues.
Electronic design engineers and product designers can extensively research the properties, performance and application potential of materials using Matmatch’s online materials search and comparison tool. To compare materials and source suppliers, visit the most comprehensive materials database in the world at: matmatch.com
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