Electronics > Beginners

QFN, thermal pad and vias

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tbscope:
A project I'm working on requires a QFN-16 device. The attachment shows the thermal solder pad requirements for this device.

I have some questions that I think I have the answer to, but I want some confirmation as I do not have much soldering and pcb design experience:
1. The holes are surrounded by pads. But I assume that the whole thermal pad needs to be conductive?
2. I assume that the bottom side of the pcb contains the exact same thermal pad layout to allow thermal distribution? Are there general design rules for the bottom side?
3. The manufacturer advises to use vias, but I want to etch my own pcb and just dril holes where the vias are located and fill them with solder. Is this a good idea?

krenzo:
1. The whole pad is conductive and transfers heat to the vias which relocate it to the ground plane.  There's no isolation between the pad and the vias.
2. The vias should go to a ground plane whether that's an internal layer or a large plane on the back.  If you only have the same sized plane on the back as the pad on the front, it's not going to do much to dissipate the heat, and it may be more beneficial to use a heatsink on the top.
3. I don't recommend etching your own QFN pads unless you have a very precise mask if using a chemical etchant.  Have you ever made your own vias before?  They're a bit more complex than drilling a hole and filling with solder.  It's a big hassle to get solder to fill a hole and have the surface remain flat which is what you need for a thermal pad.  When using QFNs, I don't even bother trying to solder the thermal pad and just rely on physical contact, and I've never had a problem.  However, I'm just a hobbyist, and my hottest QFNs (clock buffers) get up to 150F using this method while other QFNs I've used remain cool to the touch.  Whenever I do want to ensure thermal conductivity with the thermal pad, I use silver epoxy on the pad.

tbscope:
Thanks krenzo.

Codemonkey:
For what its worth, I would check the datasheet for whatever device you are using to see what is connected to the paddle (the big square pad under the QFN package). Not soldering it down is not an option on some chips (indeed, I work for a company that produces such a chip, and we've had problems in the past where customers didn't realise they had to solder it down properly then wondered why the device didn't function correctly). On our device, the paddle is the analogue ground, without it soldered, the digital side of things works fine but as soon as you try and use any of the analogue functionality it does nothing!

The vias are there for a number of reasons:

1) They provide an escape path for vapours/gasses that occur when the solder melts during reflow.
2) They provide a wicking like action when the solder reflows, and this can help "suck down" the device to the board, stopping it from simply floating on the solder.
3) In the case of RF circuits, reduces impedances.

There are probably others too, but that's the reason we specify them to be there.

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