How to choose the right thermal interface for compact electronics

Increasing electronic packaging density leads to a greater need for precise thermal management. Hence, for electrical engineers, selecting a thermal interface material (TIM) is a design parameter affecting the reliability of high-wattage CPUs, GPUs, and power converters.

Therefore, engineers must balance trade-offs involving bond line thickness (BLT), surface wettability, dielectric requirements, and mechanical stress relief when selecting a TIM. This FAQ outlines the decision criteria for specifying thermal greases, pads, phase change materials (PCMs), and adhesives in compact designs.

Q: When should I specify thermal pastes versus thermal pads?
A: When it comes to the choice between pastes and pads, the decision depends on the requirements for thermal transfer versus manufacturing repeatability. Thermal pastes (greases) are normally used for high-wattage applications (such as server CPUs and GPUs) where the heat sink and component are in close contact under pressure. The point is that greases provide high surface wettability.

As shown in Figure 1, microscopic surface roughness on metal housings and heat spreaders creates air voids that delay heat transfer. Thermal paste matrices are flowable, which allows them to wet these surfaces and displace air within the interface.

Figure 1. Schematic comparison of interface contact. Non-flowable solutions like pads may leave microscopic voids, whereas greases fully wet the surface. (Image: Dow Corning)

Technical characteristics of thermal paste:

Thermal paste can achieve a BLT of 7–20 microns. Since thermal resistance is linearly proportional to thickness, as such, a thinner layer reduces impedance and supports the highest heat flux densities.
However, pastes are susceptible to “pump-out”, which refers to the migration of material out of the interface due to thermal expansion cycles, and rework requires messy cleaning processes.

On the other hand, thermal pads are often selected for memory modules, VRMs, and PCBs with variable component heights. They are pre-cured and non-flowable. Though compliant, they do not wet the surface as thoroughly as grease (see Figure 1), often requiring higher mounting pressure to reduce contact resistance. Additionally, pads effectively bridge gaps larger than 0.5 mm and provide electrical isolation in compact assemblies.

Q: My design has no space for mechanical clips. What are my options?
A: In ultra-compact electronics, where Z-height constraints prevent the use of screws, clamps, or spring clips, thermally conductive adhesives (TCAs) are a viable option. TCAs serve as a dual-purpose solution, providing both the thermal interface and acting as a structural substitute for fasteners.

As shown in Figure 2, the polymer matrix is thermally insulating. In other words, heat transfer relies on filler particles creating a continuous conductive path bridging the component and the heat sink.

Figure 2. Heat transfer in TCAs relies on filler particles forming a percolation network within the insulating epoxy matrix. (Image: MDPI)

Filler selection deals with dielectric requirements:

Electrically insulating: If shorting is a risk (e.g., bridging traces), specify ceramic fillers such as boron nitride, aluminum nitride, or alumina.
Electrically conductive: Additionally, for thermal performance where isolation is not required, metallic fillers such as silver are used. Silver nanoparticles enhance both thermal conductivity and shear strength.

Rework considerations: It is worth noting that TCAs polymerize into cross-linked structures. While this creates a permanent bond that resists mechanical shock and vibration, it means rework is difficult. Attempting to separate a heat sink bonded with TCA may delaminate the silicon die or damage the PCB.

Q: What is the solution for high-reliability devices prone to “pump-out”?
A: For applications requiring the low thermal resistance of a grease combined with the stability of a pad, PCMs are often specified. PCMs are waxy solids at room temperature that transition to a liquid phase at a specific softening temperature (typically 45–60°C).

In their liquid state, PCMs wet the interface surfaces just like grease, minimizing contact resistance. Figure 3 illustrates the relationship between compression force and thermal impedance. As pressure is applied during the phase transition, the material flows, which results in reducing the BLT and impedance.

Figure 3. Thermal impedance decreases as compression force increases, reducing the BLT. (Image: Wurth Elektronik)

One of the key benefits is pump-out resistance. Unlike thermal paste, which remains viscous and can migrate during thermal cycling (due to CTE mismatch between die and heat sink), PCMs resolidify when the device cools. This re-solidification anchors the material, reducing voiding and dry-out issues over time. Note that rework requires heating the interface to soften the material before removal.

Q: When should I use gap fillers instead of standard pads?
A: Gap fillers are designed for gaps ranging from 0.25 mm to 5 mm, suitable for assemblies with loose tolerances or varying component heights. For example, covering a GPU, VRAM, and MOSFETs with a single heat sink.

A closer look at Figure 4 shows that gap fillers are conformable. They are designed to undergo 10% to 70% deflection under relatively low compression force. This low-modulus characteristic protects delicate components by absorbing mechanical stress associated with vibration, preventing damage to solder joints or component leads in applications such as automotive ECUs.

Figure 4. Gap fillers accommodate varying component heights through deflection (10-70%), protecting delicate solder joints from stress. (Image: Laird)

Though beneficial for mechanical reliability, the increased thickness generally leads to lower overall heat flux capability compared to thin greases or PCMs. Rework is typically clean, similar to thermal pads.

Summary

Thermal pastes offer the lowest thermal resistance but are susceptible to pump-out, while thermal pads simplify assembly and fill larger gaps. Thermally conductive adhesives serve as structural bonds where mechanical fasteners cannot be used, although they limit rework options.

PCMs combine low thermal resistance with pump-out stability, and gap fillers accommodate significant tolerances to protect sensitive components. Evaluating specific application requirements ensures optimal TIM selection for long-term reliability.