Five factors affect the choice, use, and performance of the thermal interface material (TIM) used between the device and the heat sink. Each factor is discussed in this section.
- Thermal conductivity of the material
- Electrical conductivity of the material
- Spreading characteristics of the material
- Long-term stability and reliability of the material
- Ease of application
- Applied pressure from heat sink to the package through the TIM
Thermal Conductivity of the Material
Thermal conductivity is the quantified ability of any material to transfer heat. The thermal conductivity of the interface material has a significant impact on its thermal performance. The higher the thermal conductivity, the more efficient the material is at transferring heat. Materials that have a lower thermal conductivity are less efficient at transferring heat, causing a higher temperature differential to exist across the interface. To overcome this less efficient heat transfer, a better cooling solution (typically, a more costly solution) must be used to achieve the desired heat dissipation.
Electrical Conductivity of the Material
Some metal-based TIM compounds are electrically conductive. Ceramic-based compounds are typically not electrically conductive. Manufacturers produce metal-based compounds with low-electrical conductivity, but some of these materials are not completely electrically inert. Metal-based thermal compounds are not hazardous to the Versal device die itself, but other elements on the Versal device or motherboard can be at risk if they become contaminated by the compound. For this reason, AMD does not recommend the use of electrically conductive thermal interface material.
Spreading Characteristics of the Material
The spreading characteristics of the thermal interface material determines its ability, under the pressure of the mounted heat sink, to spread and fill in or eliminate the air gaps between the Versal device and the heat sink. Because air is a very poor thermal conductor, the more completely the interface material fills the gaps, the greater the heat transference.
Long-Term Stability and Reliability of the Material
The long-term stability and reliability of the thermal interface material is described as the ability to provide a sufficient thermal conductance even after an extended time or extensive. Low-quality compounds can harden or leak out over time (the pump-out effect), leading to overheating or premature failure of the Versal device. High-quality compounds provide a stable and reliable thermal interface material throughout the lifetime of the device. Thermal greases with higher viscosity are typically more resistant to pump out effects on bare-die devices.
Ease of Application
A spreadable thermal grease requires the surface mount supplier to carefully use the appropriate amount of material. Too much or too little material can cause problems. The thermal pad is a fixed size and is therefore easier to apply in a consistent manner.
Applied Pressure from Heat Sink to the Package via Thermal Interface Materials
Measure applied pressure using a calibrated pressure sensor on multiple locations between the device and the heat sink assembly as shown in the following figure.
| Package Type | Supported Pressure Range |
|---|---|
| 0.8 mm, 0.92 mm, and 1.0 mm pitch | 20 to 50 PSI |
