Guidelines for Thermal Interface Materials

Versal ACAP Packaging and Pinouts Architecture Manual (AM013)

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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 ACAP 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, Xilinx 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.

Figure 1. Pressure Sensor

Recommended: Xilinx recommends that the applied pressure on the package be in the range of 20 to 50 PSI for optimum performance of the TIM between the package and the heat sink. Thermocouples should not be present between the package and the heat sink, as their presence will degrade the thermal contact and result in incorrect thermal measurements. The best practice is to select the appropriate pressure (in the 20 to 50 PSI range) for the optimum thermal contact performance between the package and the thermal system solution, and the mechanical integrity of the package (with the thermal solution to pass all mechanical stress and vibration qualification tests). The use of a smart-torque tool is suggested when applying a heat sink to control the rate of short-term transient pressure as the TIM material relaxes. Specifications from the TIM supplier should be taken into consideration.
Tip: These recommendations and specifications are the same for both lidded and lidless devices.
Recommended: Xilinx recommends using dynamic mounting around the four corners of the device package. On the PCB, use a bracket clip as part of the heat sink attachment to provide mechanical package support. See the following figure.
Figure 2. Dynamic Mounting and Bracket Clips on Heat Sink Attachment