Using PCB copper planes for heatsinks

[iampoor]

Hi

I am currently designing a Power supply board. I have been trying to cut down on assembly steps, and am considering using a PCB plane as a heatsink. The expected power dissipation of the LM317/LM337 is ~1w. Since I have some extra board space, I have decided to use as much as possible for heat dissipation. I have a plane on the top and bottom. How should these be tied together? Is my via stitching absolute overkill? Just looking for some ball park rules/places to start! 

Thanks!

[tszaboo]

It is not about overkill, but it is not efficiently placed.
First, use D2PAK LM3x7, SMD. Place the vias right outside the package, in the soldermasked region. Make 4-5 row of via, closely placed together. More rows have very little effect.  Dont cover the vias with soldermask, actually after an isolation of say 1mm around the package, leave off the soldermask. you can dissipate 1W on around 2000 mm² without trouble.

[Psi]

As above, you want to couple the top and bottom layers together with vias as close as you can get to the heatsource. After that they are separate heatsink fins, so coupling them at that point does nothing.

Lots of small vias will couple more heat than bigger vias in the same space. But if you put 100's of tiny vias you may annoy the PCB fab house an incur an extra charge for excessive holes. They don't like wearing out their small drill bits.

Go for 0.7mm holes, it's a common size and you wont end up with 100's of vias like you would with 0.4mm holes

[Jeroen3]

Make the via's untented so that tin flows in during the hasl process. You can also try to fill the vias during reflow. More mass means more thermal capacity.
But PCB's are very poor heatsinks. The best you can do is place a through hole connector or probe pin nearby, because these actually help a lot. Or maybe even one of those heatsinks for smd packages.

Google around, there are several application notes form ST, TI or NXP, they'll all conclude that more area does mean more cooling performance, but it's an exponential function. The first few extras mm2's have a huge effect, but this effect degrades dramatically when getting larger.

[tan98010]

Please have a look, a free excel calculator, worth to give it a trial.
http://www.circuitcalculator.com/wordpress/2007/02/16/pcb-thermal-copper-area/

[fivefish]

Just install a small U-shaped heatsink, reduce the size of your PCB and you'll come out better.

[T3sl4co1l]

Vias are important near the device.  Once heat is coupled from top to bottom, it doesn't matter.

The lateral heat spreading length is on the order of 2cm for regular PCB, and maybe 3cm for heavy (2oz?), so you have diminishing returns anywhere the shortest path from source to extremity is greater than that.

Tim

[iampoor]

Thanks guys for all the tips. Answered all of my questions. I am planning on reducing the board size and using a small heatsink along with using the planes for some minor heat dissipation. The power requirements of the project were higher than expected so I am building some extra margin in!