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QFN reflow

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gaminn:
Hi,
I have some QFN's from TI, they have pads plated on the bottom of the chip and unplated on the sides (visible copper which will oxidize). During reflow (lead free paste, temperature 255 °C) I can't get sides of the pads covered by solder so one can't visually check the quality of the joint. However, all boards are working.

In you experience, is it possible to solder these QFN also on the non-plated sides during reflow? And is it neccessary? At the moment, I don't think so, the only drawback is that the joint doesn't look well when there is no solder on sides of the pads.

ConKbot:
No need to worry about the side of the QFN leads except in special applications. Some ICs that are automotive rated come in a "wettable flank" QFN package. This gets rid of the stress-riser by letting a nice fillet form, but unless you're in a high vibration application in an un-potted module, it's no problem.
As you said, they are unplated, so they aren't 'solderable'  just a remnant of the lead-frame manufacturing process.

Berni:
Yep this is by design, all the soldering just happens underneath where you can't see it. But if you really want them to be soldered you can likley get them to do so by putting down a lot of flux and running your soldering iron over them a few times. There are also QFN designs that have no metal on the side at all, i don't like those but hey you get what you get.

The ones that are plated are often marketed as a feature since it makes an even more solid connection and makes visual inspection easier.

T3sl4co1l:
Should be PQFN (pullback) actually.  Which is actually an LGA without the grid array.  Follow LGA rules for the pads -- there's no toe fillet, and correct, no way to inspect them visually, you need x-ray for that.

These can be sneaky to identify.  I followed one particularly bad drawing thinking it was a QFN, turns out it was one of these.  And indeed, inspecting the part, there's a 0.1-0.2mm web of plastic around the perimeter, invisible without a loupe.  The pads don't actually come out to the edge, nor do they wrap around to the leadframe exposed on the sides.  The soldering looked terrible -- I left what was supposed to be a generous toe fillet, which just ballooned up instead, what with no face to wick up.  But it connected alright and I fixed the lands in the production revision.

Tim

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