The TIM application pattern is really important as you want complete coverage between the battery cells and the cooling system.
What is TIM?
This is Thermal Interface Material and is designed to bridge the gap between surfaces and hence enable a better heat transfer.
No surface is perfectly flat and hence the TIM is a Gap Filler that would otherwise leave air. As we know air is a good thermal barrier.
The application pattern is important in that the final coverage of the material between the two surfaces is dependent on it. The final coverage needs to be 100% with no air pockets / voids.
There is a lot of online references to application patterns around heatsinks for CPU’s. Puget Systems [1] used a clear plastic so that they could visualise different patterns and the resultant spread / air pockets.
Application
Compressed Resultant
The best result, measured by lowest CPU operating temperature, was the X Pattern. This X Pattern appears to give a complete coverage with only limited voids. This is probably due to the fact that the “X” can spread from the centre and the arms.
Interestingly the bigger issue in CPU applications is paste tends to age, dry out and crack. This means it needs re-applying every 2 to 3 years [3]. Here though the most interesting learning is just the pattern.
The battery difficult application as this is an HV system and designed to last the lifetime of the vehicle.
In the Webasto application you can see that they are using an elongated “X” or “dog-bone” shape that is robotically applied.
We are using this shape, as it provides a perfect “wetting” between the module and housing floor when placing the module at its destination and “pressing” it down. But also to mention: depending on module and housing surfaces and viscosity of the TIM this shape is not the only option.
Webasto
References
- Thermal Paste Application Techniques, Puget Systems
- Battery Systems, Webasto
- Best Way to Apply Thermal Paste – Does the Pattern Matter?, Technewstoday