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Thermal disapation through PCB Inner layer?
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jwhitmore:
Hello all,

I'm using an LM5088 for a DC DC converter in a project. I want to potentially push it a bit so want to keep it cool. there is a ground pad under the IC and the footprint includes the corresponding ground pad on the under side of the board.

That's all good but the board is going to be a 4 layer board and I'm wondering could I put another 2 ground fills in the inner layers to conduct the heat away as well. I can't find information on whether that can be done or whether it'll cause problems in the PCB? I can't think of any reason why it'd cause a problem because it'd be 4 ground layers stacked under the LM5088. Having said that soldering the thermal pad might be an issue given all the thermal conduction away from the pad.

Perhaps a better solution would be to wind a piece of copper wire into a coil and solder that into a hole and have it flapping in the breeze beside the IC ;) That'd look like an Antenna and confuse people.
T3sl4co1l:
I'm more concerned that you weren't intending on using the inner layers for ground fill as it is! :o

Tim
bugrobotics:
Here's a good app note from TI about using multiple layers with DC-DC converters.

http://www.ti.com/lit/pdf/snva424
mr_darker:
MCPCBs or Ceramic PCBs are good for thermal dissipation within a board.  Expensive though and not really standard pcb services.

Hand soldering you just turn the heat up so it melts before the heat dissipates, 1 second and no melting is too long and not hot enough.  Reflow soldering is de way to go if you're not doing that.

I'd say just take a few gallons of mineral oil, fill up yer bath tub, and toss her in there.  Liquid cooling.
ajb:
You can and should use an internal layer as a solid ground plane. 

Internal layers can help with thermal dissipation, but not a lot.  The thermal conductivity of the fiberglass that surrounds the internal layers is high enough that once heat spreads into the internal layer it doesn't really have anywhere else to go.  PCB copper layers are thin enough that the thermal resistance in the lateral direction limits the effectiveness of lateral heat spreading.  The best thing you can do (short of adding an actual heatsink) is run a bunch of vias from the top side thermal pad to a copper area on the bottom layer.  With enough vias you can get reasonably low thermal resistance from top to bottom, and then the bottom copper can radiate into the ambient. 
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