As we discussed in a previous post, thermal interface materials (TIMs) serve an important role in heat transfer systems. By filling the voids between device surfaces and heat sink surfaces, TIMs improve the thermal pathway from where the heat is generated to where it’s rejected. However, applying a TIM is not always an easy, straightforward, set-and-forget process. Here we will discuss some of the practical considerations of applying TIMs in heat transfer systems.
To begin, let’s take a look at the rated performance of some standard TIMs in perfect application conditions. There are a variety of different types of TIMs, including solders, low melting point alloys, structured carbons, and various fillers (pastes, epoxies, adhesives). Figure 1 summarizes the rated performance of different standard TIM options from the Parker Chomerics catalog [1]:
Figure 1: Thermal interface material properties for different TIM types [1].
In short, most TIMs have a rated performance in the 0.1-5 K-cm2/W range. Of course, there are other TIMs out there that may have even better thermal impedance values, but Figure 1 is a good sample of what is commercially available at standard prices. To get the full thermal resistance, divide the thermal impedance from the plot by the area of your device in centimeter units. Each TIM type has its pros and cons, which should be evaluated depending on what your application requires.
In practice there is a lot that can happen to cause TIM performance to fall short of what is shown in Figure 1. For example, do you ever get the feeling that, after two or three years, your computer seems to have slowed down? Of course, friends and colleagues may be buying newer models that make your machine feel extinct, but that’s a separate issue. Why does the same processor, that was whirring along when you first purchased your computer, seemingly not work as well as when it first came out of the box?
You’re not crazy! In fact, this is a common symptom of deteriorating TIM performance. The processor has a built-in thermocouple that is tied to a feedback loop, meaning when your CPU isn’t receiving adequate cooling, the CPU is hard-coded to scale back and operate more slowly. Over time, the silver paste TIM dries out, and heat from the CPU becomes trapped and can’t make its way to the heat sink. A hotter CPU means slower performance, and more time waiting for the blue spinning cursor wheel to stop.
Figure 2: Dried out TIM that only covers part of a CPU surface, restricting heat flow from the processor to the heat sink.
This is just one example for a certain TIM application case, but issues like this are not uncommon. Below are some of the most common practical challenges that cause TIMs to fail to achieve their rated performance [2]:
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Outgassing, Oxidation, and Dryout. Many TIMs experience outgassing and oxidation, causing dryout and degradation over time. Further, the TIM may be subject to dust or impurities throughout its lifetime. So, even if the TIM is applied perfectly at the point of application, the performance of the electronic device may start to falter over time, or worse, experience premature failure. There is early research underway to add nanoscale coatings of higher stability compounds to prevent degradation; this may help mitigate the longevity concerns, though it may result in tradeoffs regarding cost and reduced bulk conductivity.
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Poor Interfacial Adhesion. Even if the bulk thermal conductivity is good, the interfacial contact resistance between the TIM molecules and the substrate molecules can be limiting. For example, carbon nanotube (CNT) TIMs have high thermal conductivity and good mechanical compliance, but often suffer from contact resistances at their interface with the adjacent surface. This is conceptually similar to the issue that TIMs are attempting to solve, but now occurring at the molecular level instead of the bulk TIM level.
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Complex, Multi-Step Application Procedures. Some TIMs are challenging to apply. Special tools, multi-step application procedures, high temperature/pressure activation conditions, and long cure times are a few things that may be required to achieve the TIM’s rated performance. Certain TIMs may require equipment in the $10-100k range and skilled staff with many years of experience just to be applied properly.
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Expensive Verification Equipment. Complex verification techniques may be required to ensure the TIM’s proper application. Uniform coverage of the TIM over the entire footprint area, with no gaps, is critical to the TIM’s functionality. If you are going to space on a 6-year mission, you can’t afford to send a thermal system with an improperly applied TIM. Therefore, complex instruments such as scanning acoustic microscopes are often used to inspect the quality of TIM layers in mission critical applications.
In short, great care must be taken when working with TIMs in order to ensure your expected performance without unexpected thermal behavior. In the case of the computer CPU, it may be easy enough to add a new layer of TIM or replace your machine every few years, but many engineering systems do not have the same flexibility and require significant investment in TIM resources.
References
[1] Blazej, Daniel. “Thermal interface materials.” Electronics Cooling 9 (2003): 14-21..
