Thermal vias in thermal pad of IC?

[eem2am]

Hello,
Is it OK to put thermal vias in the heat pad of this IC?

http://www.ti.com/lit/ds/symlink/lp8556.pdf

..Its a linear LED current regulator.

 

[Harald Kapp]

Normally we don't place vias into pads at all. During soldering, solder will penetrate into the hole and will be lost for soldering the pin to the pad.

A thermal via would be counterproductive: as it is thermally isolated from the pad it will reduce the heat transfer away from the IC to the PCB.

I advise to put the necessary vias outside the thermal pad.

 

[OLIVE2222]

You can place thermal pad under via knowing that solder paste are not placed on the whole thermal pad surface but is spread in 4 (in this case) the room between each solder paste "island" is used to place via's (this avoid the soldering penetration mentionned by Harald Kapp)

check the PCB layout of the TI evaluation board here you will see that:

http://www.ti.com/lit/ug/snvu179/snvu179.pdf

 

[eem2am]

http://www.ti.com/lit/ug/snvu179/snvu179.pdf

thanks....the article (above) doesn't show solder paste islands, but I see what you mean.
There are definitely 4 vias under the LP8556 chip, as you say.
I am stil not sure that they have made a complex solder paste mask for that thermal pad. I agree that it seems to make sense.

 

[OLIVE2222]

Yes that's it the solder paste island are on the 4 corner for sure.
it' s a common (and needed) practice to have such complex solder mask.

see the figure 13 here under

http://blogs.mentor.com/tom-hausher...ection-starts-in-the-cad-library-part-15-qfn/

Olivier

 

[KrisBlueNZ]

I don't see any reason not to use thermal vias with that device; it seems like a good candidate.

I did a quick web search and found this informative article on thermal vias: http://www.edn.com/electronics-blogs/the-workbench/4421218/PCB-layout-tips-for-thermal-vias