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  • Author Author: kjachim
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What is a wettable QFN?

kjachim

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As part of the recent KW36/KW35 launch, I saw that this new Bluetooth 5 device was offered in a wettable flanks QFN package and is actually the first Kinetis device to be offered such a package.  So, I wanted to investigate a bit more and try to understand why this is an important for customers.

 

KW36/KW35 was built targeting applications in the automotive, industrial and medical markets so reliability is key.  The device is also packaged in a quad-flat no-lead (QFN) package, which means that there is no exposed pins/terminals that enable you to easily determine whether or not the package successfully soldered on to the PCB.

 

Wettable flanks (WF) are modifications to the exposed terminal ends, which “promote solder wetting for the formation of a solder fillet” that is visible.  Using a QFN package with a wettable flank enables optical inspection of the soldering, which can increase reliability, while reducing cost at the same time.

 

Why is this a big deal?

Since there are such high safety standards for automobiles, the automotive industry requires OEMs to perform 100% automatic visual inspection (AVI) post-assembly. The wettable QFN package allows OEMs to view the solder connections and conduct this mandatory visual inspection.

 

But, even non-automotive applications can benefit from this technology since it can reduce manufacturing costs.  How so?

OEMs must incur additional costs due to yield issues from false assembly failures, along with genuine fails where the assembly process has highlighted poor solder joints.  If they can catch these failures sooner (and without the need for expensive x-ray equipment), then yield improves, while keeping manufacturing costs down.

 

Still not convinced wettable QFNs are right for you? KW36/KW35 is also offered in a traditional laminate QFN as well.

Are you involved in manufacturing and want to learn more? Check out these assembly guidelines from NXP:

https://www.nxp.com/docs/en/application-note/AN1902.pdf

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  • Is it the notch along the edge that makes it "wettable"? I'm not a fan of hand soldering QFN but have found that the solder filet is visible - especially if you leave a longer part of the pad exposed without soldermask.

  • in reply to Fred27

    I'm certainly not a fan either - but I still end up having to do it.

    Lots of QFNs have pads going up the side a bit so I agree it must be the corner profile that makes it wetter.

    I always do my own footprints and make the pads extend beyond the package.

     

    I'm about to push my technique further since I've just ordered some MAXIM parts in 0.4mm pitch QFN.

     

    While we have NXP's attention - this is a nice part but kind of samey - there are lots of integrated processor/radio chips with a BT stack.

     

    What would really make it stand out would be an integrated antenna so the nice wettable QFN was the complete BT interface with no additional RF bits, tuning or hassle.

    I reckon the dev costs from chip set to marketable product for BT are up at the £20k+ level which means I have to use horrible modules (for anything with a build volume of less than 10k) - a complete system QFN would be really nice.

     

     

    MK

  • in reply to michaelkellett

    Is it possible to get an integrated antenna that's comparable to a 2.4GHz PCB antenna?

  • in reply to Fred27

    I don't see why not  - but that kind of thing has to be engineered by the guys with £1M RF lab.

     

    I wouldn't care what was inside - so long as it looks and solders like a single IC.

     

    The module things with the mouse-bite pcb are horrible to deal with - and huge.

     

    MK

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