Grease, Putty, and Paste
The use of unsolidified form of TIM-like grease, putty, or paste is commonly used to thermally couple the heatsink to the device. These often can be easily dispensed and controlled and have good thermal properties (conductivity and low thermal contact resistance). Xilinx has found some phase change material (PCM) forms of paste to be a very effective TIM due to further reducing contact resistance. PCM is a type of TIM that employs a matter phase transition usually to/from a solid and liquid form as the material heats and cools, allowing for easy application while delivering superior contact resistance compared to single-phase materials. Xilinx is currently using Laird 780 SP PCM in its Alveo™ products, as it is a good balance of minimum BLT, applicability, and contact resistance. It also provides high reliability due to resistance to leaching, dry out, and other longer-term issues. This and similar materials have been proven to perform well with Xilinx lidded and lidless products.
Thermal pads are generally used when the thermal solution cannot be in direct contact with the device such as in the event multiple devices are connected to the same heatsink. Thermal pads compensate for greater tolerances in height than greases and putty. Xilinx does not recommend the use of thermal pads in direct contact with lidless devices because large BLT compensation compromises the performance improvements that lidless devices offer. Similarly, for lidded devices in thermally challenging situations, Xilinx recommends optimization of BLT that can allow for either a very thin pad, or move to an unsolidified TIM-like grease or paste. When using a pad, PCM-based materials tend to perform better than single-phase materials further optimizing contact resistance.
Thermal Epoxy, Tape, and Adhesive
Xilinx does not suggest the use of thermal epoxy, thermal tape, or other adhesives for use of a TIM or heatsink attachment in a production environment. The following are possible problems that can occur when using adhesives.
- Increased contact resistance
- Uneven thermal contact
- Reliability issues, including the possibility of lid separation
- Voids in TIM1
- Other damaging effects under shock, drop, and vibration
- Difficulty in debug and rework if deemed necessary
Because of this, Xilinx suggests using other materials for TIM rather than adhesives.