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Nordic aQFN73 stencil design

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#80189

Nordic aQFN73 stencil design | 9 April, 2018

We are about to do a second iteration of something we struggled mightily with last time and I'm hoping to at least make a step in the right direction.

There are 12 of these parts on the board and virtually all of them required removal and replacement last time. There is a 4.85mm sq. thermal pad under them and we did the typical 4X windowpane layout for the 4 mil stencil that we would normally use for anything like this. They skewed badly.

The manufacturer's recommendation for the paste file is a solid square aperture, just like the customer provided.

That goes against everything I know (and what another engineer specified previously) but before I go with some more sophisticated design (6X, or rounding off the apertures, or....?), I'm thinking maybe I should just go with what the manufacturer recommends.

Has anyone else placed these before, and if you had good success, how did you have your stencil cut for them?

Thanks!

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dwl

#80192

Nordic aQFN73 stencil design | 9 April, 2018

I've never placed one of these before, but it looks like a poorly designed part. It has pads scattered asymmetrically around the body of the part. when it reflows this is going to apply uneven forces to the component, causing it to skew.

I guess having a giant blob of solder for the part to float on might negate the asymmetric forces from the outer pads but I'd be worried about creating voids and outgassing pushing solder under the component body. The sort of things a window pane stencil aperture are are supposed to prevent...

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#80195

Nordic aQFN73 stencil design | 9 April, 2018

Yes. I'm at a loss as to how to best approach this.

I think the giant blob of solder is just going to behave more like a lubricant than anything else.

The thing is, I would like to at least try the manufacturer's recommendation once (and this build is only 6 boards) just to prove that it's a bad design.

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#80205

Nordic aQFN73 stencil design | 10 April, 2018

Hi Steve ...

I found nothing in the manufacturers "paper" to help. Comments are:

* I agree. No blobs. Solder on the thermal pad should me minimized. Too much solder will float the component. Old timers used to talk about 50-80% coverage.

* If you're concerned about missing pads causing imbalance resulting in skewing, epoxy replacement rework pads in the open spots

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#80208

Nordic aQFN73 stencil design | 10 April, 2018

Dave, that's a cool idea and sounds like something that would actually work. I'd be tempted to go with oversized pads to increase the holding force.

Unfortunately as an EMS we would have to work that through the customer approval system and as usual, this one needs to be done yesterday.

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#80209

Nordic aQFN73 stencil design | 10 April, 2018

Oh, I just remembered the other thing that I wanted to mention ... Do the pads match the component leads?

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#80217

Nordic aQFN73 stencil design | 11 April, 2018

Try one aperture 60x60mil

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#80221

Nordic aQFN73 stencil design | 11 April, 2018

Yes - every dimension that I measured on the gerbers checks out against datasheet. Presumably the parts match it as well....?

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#80252

Nordic aQFN73 stencil design | 16 April, 2018

Amazingly enough, every one of these held fast where we placed them with a paste brick sized 1:1 with the thermal pad, no sectioning. Yes, there were voids, but we can live with that in this application.

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dwl

#80254

Nordic aQFN73 stencil design | 17 April, 2018

Thanks for the update!

I guess whoever designed the package knew what they were doing.

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#80255

Nordic aQFN73 stencil design | 17 April, 2018

I attributed it to luck, but whatever keeps you up and running!

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#80267

Nordic aQFN73 stencil design | 18 April, 2018

Further divide the ground block into more than 4 panes. Also think about using, assuming you have good placement of using Loctite chip bonder on two opposing corners. Lots of ways to get good results

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