Thermal Interface Materials
We last talked about the challenges for first level thermal interface material. Now we will discuss the third scenario where no integrated heat spreader is presented. This happens commonly in mobile computing platforms as well in many desktop systems. It is the configuration where the chip directly couples to a heat sink. The exact nomenclature is not yet settled throughout the industry may not have a level designation, and is referred to as just a TIM. Since thermal packaging with the integrated heat spreader became common, there was a need to designate the previously existing configuration, thus chip-to-heat sink thermal interface material became known as TIM 1.5.
This distinction at first seems arbitrary, but targeting maximum performance often comes at the cost of specialization, and this is also true with materials used for thermal management. The silicon surface itself is warped, and to make matters worse, this curvature changes with temperature. This will, in turn vary the volume of enclosure between the chips surface and the heat sink or heat spreader which creates a need for material that is resistant to this pump-out effect.
There are other failure modes, depending on the type of the material and chemical base that are used. These modes include an evacuation of the thermal interface material under constant pressure when above certain values, known as squeeze-out. Another type is known as dry-out, which is the loss of plasticity. Due to these types of failure modes, thermal interface material design is a continuing challenge for the semi-conductor industry.