5 Thermal Interface Material

When using external heat sinks, a suitable thermal interface material must be considered to effectively transfer the heat from the component to the heat sink, and eventually to the environment.

For bare-die flip-chip BGAs, the surface of the silicon contacts the heat sink. For lidded flip-chip BGAs, the lid contacts the heat sink. The surface size of the bare-die flip-chip BGAs and lidded flip-chip BGAs are different. Microchip recommends a different type of thermal material for long-term use with each type of flip-chip BGA’s package.

For lidded flip-chip BGAs, the lid contacts the external heat sink while bare-die flip-chip BGAs, the surface of the silicon contacts the external heat sink. The surface areas of lidded flip-chip BGAs and bare-die flip-chip BGAs are different. The system level thermal design engineer must choose the appropriate TIM to be used.

Thermal interface material is required because the surfaces of the PolarFire SoC package and the heat sinks base are not smooth. The surface roughness reduces the effective contact area between the package and the heat sinks base. The insulating air gaps created by voids between contacting surfaces are too large. The thermal interface materials fill these gaps and allow an effective conductive transfer of heat from the package to the external heat sink.

The selection of the appropriate thermal interface material is critical to ensure the lowest thermal contact resistance. One must consider the thermal conductivity of the TIM:

• The flatness of the surface contact areas
• The applied pressure on the thermal interface material
• The total thermal contact area

In addition to thermal performance, TIMs are selected based on the ease of use in assembly and long-term reliability.