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Thermal relief vs direct connection for heat dissipation

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ricko_uk:
Hi,
normally vias and pads are connected to inner planes with thermal reliefs to relief thermal stress (expansion/contraction). Someone at a PCB fab suggested that they are not needed anymore with the materials used in PCB build nowadays. Not sure how reliable information that is...

If you have a SMD package (lets say TO263) that needs to dissipate a lot of power in a cyclic manner - i.e. is turned on and off every 20-40 seconds resulting in the package heating up to 70-80 degreesC and cooling back to 25C - i.e. high cyclical thermal stress, would you need thermal reliefs? Or can you have still full/direct connection to the internal plane?

I am asking because it would be safer to have them but I think that at having them they would decrease the heat dissipation/conduction into the inner planes, especially those further down the stack (i.e. layer 3 and 4 in a 4-layers PCB) which are the ones which would eventually dissipate the heat into a heat-sink in contact with the bottom side of the PCB.

Thank you :)

tooki:
It’s my understanding that thermal reliefs exist to facilitate soldering, because they create choke points that prevent heat being sucked into big copper planes. High currents are exactly when you can’t use thermal reliefs, because you need low (electrical) resistance. Similarly, you don’t want to use them on pads that are the heatsinks for the component.

ricko_uk:
Thank you Tooki,
what you are saying does make sense about the soldering but I always saw them in vias too where there is no soldering. And by default the CAD packages I used always put them in vias too.

That's another reason for it being a bit confusing...

Thank you :)

Someone:

--- Quote from: ricko_uk on May 04, 2022, 11:57:59 pm ---And by default the CAD packages I used always put them in vias too.
--- End quote ---
The default rules are usually simplified/stupid/conservative. IPC guides don't recommend thermal relief on vias so the question is solidly over to anyone suggesting them to justify it very well.

thm_w:
No one really suggested them, its just the default rule in Altium to thermal relief every feature (holes, vias, pads).
The first rule I'll add to a new design ruleset is IsVia -> Direct connect.

https://resources.pcb.cadence.com/blog/2021-pcb-thermal-relief-guidelines-for-effective-layouts
https://www.vse.com/blog/2020/02/18/pcb-thermal-relief-guidelines-for-manufacturing/

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