Author Topic: QFN float during flux activation  (Read 2505 times)

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Online wraper

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Re: QFN float during flux activation
« Reply #25 on: July 07, 2021, 07:22:17 am »
The pad on the bottom of the chip is significantly larger than the pcb pad and it seems to align itself by the edges of that. I had a moment of panic that I just grossly screwed up the footprint but double checked and I exactly follow the datasheet. The chip center pad size is 3.6x3.6mm while the PCB pad is only 2.79x2.79mm.  :-//
IME don't outright trust footprints in the datasheets. I have a feeling that sometimes they simply don't care and provide it just because. Not to say that for the same package, different manufacturers often provide radically different footprints.
Quote
I scraped off the corner notch but it still pulls.
If the pad on the chip has such a corner as well, there is nothing bad if the pad on the PCB follows its shape as well.
« Last Edit: July 07, 2021, 10:03:15 am by wraper »
 
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Offline Marsupilami

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Re: QFN float during flux activation
« Reply #26 on: July 14, 2021, 03:33:27 am »
Oookay, I got an additional 3 different kinds of solder paste and the 3rd stencil variant with smaller perimeter pins and no reduction at the pin 1 notch. If I can't solder this stupid chip this time I'll give up and move to the Himalayas to be a goatherder.
 

Offline JohnG

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Re: QFN float during flux activation
« Reply #27 on: July 14, 2021, 08:08:28 pm »
Plugged vias are not a panacea for via in pad. The plugs can expand and outgas during soldering. For anything small that I care about, I either use VIPPO or I don't put vias in the pad. It solved many problems and I have never looked back.

If you are doing a one-off and don't want the expense of VIPPO, putting a weight on may be kludgy, but if it's a one-off (or just a few), who cares?

John
"Those who learn the lessons of history are doomed to know when they are repeating the mistakes of the past." Putt's Law of History
 
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Offline Marsupilami

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Re: QFN float during flux activation
« Reply #28 on: October 02, 2021, 12:24:24 am »
Hi All, just in case anybody finds this thread, I managed to get the chips soldered properly.
I got new boards. I decided to go with the ~40% cost increase of a 4 layer, thicker board so that they can plug and overplate my vias. That was a mistake to assume I can kludge it at home. Another change is the solderpaste. I got Loctite GC 10 and it's really nice, it prints a whole lot better too and is apparently a lot less picky about temperature.

Thanks for everyone for chiming in and giving tips!


 
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