On a design we have in production, we have an Apalis iMX8 module assembled onto the board using the recommendations found in chapter 4 of the Apalis Carrier Board Design Guide. We use the Toradex heatsink for the Apalis iMX8 and have a thermal/mechanical standoff mounted at the center of the module as recommended.
We’ve found that the thermal standoff does not always make a good thermal connection to the module, resulting in widely varying operating temperatures of our final assemblies.
Do you have a recommendation for a thermal material that could be applied to the top of the standoff to improve the thermal conduction?
I asked our team internally if they have any good recommendations and, if they have, I will post the answer here.
Despite that, are you using an air cooler with the heat sink or only the heat sink?
Yes, we have a PWM fan mounted to the heatsink. We found that the assemblies required quite different fan PWMs in order to maintain a consistent operating temperature (eg, 40% for some modules, 80% for others).
In order to reduce the fan noise, we would like to operate in the 40% region. We tried using a cut-down gap filler on the standoff of one of the assemblies and saw a reduction from around 60% PWM to 40% PWM. We are hoping to find a standard or recommended solution to avoid hand-forming a custom solution for each assembly.
From the design guidelines:
To guarantee good thermal conductivity between SoC and the heat sink required for effective heat dissipation, pressure needs to be applied to the thermal interface material between SoC and spreader. As the Apalis module PCB is only 1.2mm thick, adding pressure in the middle of the board would cause undesired bending and flex, which could damage the module. To prevent such bending, every Apalis module features a pad on the bottom side of the module. This pad allows the addition of a support standoff between the module and carrier board.
Can you share a picture or something describing what you want to achieve? So we can have a better understanding of the issue.
The section of the board that contains the Apalis module has the same mechanical/physical layout as the Ixora evaluation board, including the support standoff beneath the module and we are using a Toradex supplied heatsink, so you should be able to replicate our situation in your lab.
I suspect that someApalis modules are not making good thermal connection to the support standoff. The reason I think this is that placing a thin gap filler on the top of the support standoff reduces the fan PWM.
I would like a recommendation for a thermal material that will increase the thermal conduction between the module and the standoff without causing mechanical stress to the module.
after talking with one of our hardware engineers, he suggested some options:
- use a different length standoff underneath the module; even better if it is filled and not with the hole like the one we use (to increase the contact surface) and place on it a thermal interface material (TIM). I am not sure what kind of spacers are available on the market and if they can find a length that could allow having a thin TIM underneath the module (when it comes to TIMs, normally, the thermal resistance increases with the thickness, and this reduces the performances).
- Use some thermal grease with the current spacer. This is probably the easiest solution that should not add too much stress to the module.
For both the options above, it is important that you don’t cause any short circuits/connections/add any stress on the capacitors that are placed near the big pad underneath the processor.
He didn’t mention any specific brands or products that I could recommend here for you.
Let me know if this is enough or if more information is required, so I can pass the questions to him.