Is there vibration or strong air current from fans or a tilt to the PCBA or vibration from motors / etc.
which could motivate motion beyond the usual levels for such things in a reflow process?
It's a vapor phase oven with no movement at all.
Are you pretty sure the solder paste you are using is genuine from a reputable authorized dealer vs. counterfeit?
Is the paste expired or stored / used with improper refrigeration, humidity, etc.?
That's a good point. I'm not. It looks genuine but it's from Amazon and I don't have any practical knowledge of the counterfeit solder paste market
Markings look legit. Expiration date says November 2021 and I was storing it in the fridge, giving it a few hours to warm up before use.
I'll order something completely different from DigiKey or so.
Maybe you're applying too much paste and need a thinner stencil or smaller apertures.
Is your stencil / paste design & use compatible with the paste manufacturer and the IC vendor land / paste pattern suggestions?
I read through a number of app notes about QNF package assembly and one thing I gathered that the recommended stencil thickness for 0.5mm pitch is either 100 or 150um (4 or 6 mil). I have the thinner one. The coverage on the center pad is crucial but what I have is very small at around 35% of the pad area. Most documents mention ~60%.
Pads follow the recommendation of the manufacturer.
Did you make the pads extra long outside the periphery of the QFN so you could use IPC least dense design or manually solder them or something?
Everything is nominal size.
I'm wondering if the problem is not that I don't push the part enough in the paste but I do it too much so that I close the nice escape channels in the paste on the bottom. When the flux starts to do it's thing it lifts the chip slightly as it has nowhere else to go but because I pushed it down too much I smeared the pasted on the perimeter pins to much so they are too low tack to the chip.