Screaming Circuits: Speaking of Common QFN Issues...


Speaking of Common QFN Issues...

Classic via in qfn pad Here is the classic QFN via in pad. It simply isn't possible to solder the center pad properly with that much open via real estate.

The best way to deal with this is to fill the holes with something that will still do a good job of conducting heat away and then plate over the holes.

Barring that, you could put solder mask caps over the holes on the component side shown here. Most manufacturers recommend that the soldermask cap diameter be 100 - 125 um wider than the via to minimize voiding and thermal insulation.

Some people will put soldermask caps on the back side of the board. That may work, but it still tends to be problematic. Solder can still wick down in the vias leading to excess voiding. The via caps can pop open resulting in an open via that sucks solder off of the pad.

Duane Benson
Look closely and you might see Paul Lynde in the center via


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I must admit that as a designer I am totally paralyzed in the face of the QFN. I avoid them despite their superior high frequency and thermal performance vs the QFP. The first person who publishes a canonical how-to for QFN layout will earn my eternal gratitude.

For example, some people say that you can use solder mask to plug a via. Other people say that you can't do that because the solder mask bumps upset the coplanarity of the center pad with the other pads. Yet more people say you can or even should solder mask off both sides, but others say that doing so will cause one end or the other to pop.

Compounding all those issues, my board house won't guarantee that plugged vias are actually plugged.

On top of all that, every manufacturer has put out contradictory application notes on how to layout QFNs.

Basically the whole thing is a mess.

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