We all know that electronics dissipate heat, and the heat dissipated in electronics can be unacceptably high in certain components and application areas. Systems designers and PCB designers also have to contend with form factor restrictions that create new heat dissipation challenges, and oftentimes these cannot be solved by simply adding a fan to an assembly. More elegant solutions are needed for these assemblies, and these can be found through creative use of thermal interface materials.
Thermal interface materials are more than just a thermal compound for attaching a heatsink to a CPU. These are still commonplace in some products, but there are more advanced materials and some ways to use these materials beyond heatsink attachment. Keep reading this article to learn about the range of thermal interface material options on the market, as well as how to select the thermal interface materials to manage heat dissipation.
Thermal Interface Material Options
All thermal interface materials used in electronics play a single, central role: to provide a high thermal conductivity path between a heat-generating component and a heat-sinking element. The “heat-sinking element” could be an extruded or machined heatsink, or it could be the enclosure for the device. The former is most-often recognized as a standard approach in desktop computers and some servers, as well as some boards with higher-power processors (e.g., FPGAs or some application processors). When paired with a fan, you have a very powerful heat management and removal system.
This smaller fan provides airflow a CPU in laptops. These fans will not work in smaller handheld devices, including in devices as large as tablets.
The problem with the heatsink + fan solution is that it is very bulky and are not appropriate for low-profile systems like mobile devices, or any other device that requires a small form factor. Some devices simply need to be quiet, so a fan will not be used in these systems. Fan designers have developed much newer systems that use a flat form factor to pull airflow past the main processor and thus remove heat. This is used in laptops, but it’s difficult to do the same when devices scale smaller.
When you can only rely on a heat-sinking element and there is no room for a fan, a thermal interface material will be important for ensuring efficient heat transfer away from a set of hot components. To see how to solve this problem, a designer should select the right thermal interface material to use for bonding to a heatsink, enclosure, or both.
These materials are more appropriately known as gap-fillers, in that they fill in the gaps between a component surface and a heatsink or enclosure surface. While they can technically be used to bond an enclosure to a component, they are most often used to bond a heatsink to a component. They can be auto-dispensed on an assembly line, or they can be applied manually during assembly.
Thermal paste on CPU.
Thermal Greases/Phase-change Materials (PCMs)
Thermal greases are sometimes lumped into the same category as gap-fillers, but they are not the same type of material. Thermal greases are comparable to thermal pastes in terms of performance, but they can be screen-printed as needed in automated assembly. Some thermal greases are phase-change materials that can provide maximum heat absorption and release within a specific temperature range by taking advantage of latent heat. These materials will solidify or liquefy as they release or receive heat, respectively.
Thermal grease being applied to a processor
Because these materials will liquefy (in contrast to gap fillers that harden), they should not be used on enclosures. They are best used on a heatsink where there is a mechanical mount to hold the heatsink in place.
Thermal pads are solid materials that are easier to work with in manual assembly. They are available as bulk materials that can be cut to size, or as pre-cut pads that will attach to a component package. Thermal pads are common for use in attaching heatsinks to processors, but they are an excellent solution for attaching a device to its enclosure for conduction cooling. The use of thermal pads in conduction cooling is found in smaller devices where there is no room for a heatsink to attach to heat-generating components.
Thermal ads attached to a group of components, which can collectively share a heatsink or can attach to an enclosure. A common thermal pad material is alumina ceramic due to its very high thermal conductivity.
Thermal pads are sometimes considered suboptimal for heatsinks because they do not perform to the level of thermal pastes and can degrade over time when exposed to high temperatures, such as you might find on a CPU/GPU. If the heatsink ever needs to be replaced with a newer heatsink or a fan, the old thermal pad will leave behind some residue that has to be scraped from the component. Thermal pads are a better option for enclosure bonding because they can conform to odd surfaces, and in enclosure situations they do not need to handle as much heat as in CPU bonding.
Which Thermal Interface Material is Best?
This is one of those design questions that does not have a clear-cut answer. There are multiple points to consider when selecting thermal interface materials for a PCBA, spanning from mechanical design to manufacturing. The matrix below outlines matches up when to use different thermal interface materials and airflow techniques for cooling with heatsinks or conduction cooling on the enclosure.
Note that, it is possible to have both airflow and conduction cooling. Airflow can be convective through a vented enclosure, or it can be forced mechanically with a fan. It’s also possible to see conduction cooling to an enclosure for some components as well as cooling fans on specific components. Don’t be afraid to mix and match cooling methods when designing a new system.
When you’re ready to send your board off for fabrication and assembly, make sure you specify your assembly needs for thermal management in your PCBA. Anytime you need to create a new product, make sure you use OrCAD, the industry’s best PCB design and analysis software from Cadence. OrCAD users can access a complete set of schematic capture features, mixed-signal simulations in PSpice, and powerful CAD features, and much more.