You reflow the whole board with a hot air station? I've been using an oven. How big a board can you do that way?
Yep. It's not really by choice, but because I have limited space. That being said, I like being able to watch the process and to correct minor issues like a 'tombstone' passive, so I think I'd prefer a hot plate over an oven.
My SMD boards are small, the largest one by length is 7 cm and the largest one by area is 25 sq cm. You could definitely do larger ones, but you'd probably get significant temperature gradients - bad for manufacturing, probably ok for prototyping and one-offs.
We've actually reflowed proto boards here with a hot air station. The trick is to get the whole board warm first and then target the gun (or pencil) at the joints. You can use a hair dryer to get the whole board warm and then use the hot air gun to quickly reflow each joint. It's pretty fast and works well.
If you bump the pads like you mentioned with a little solder, you can then flatten the joints very carefully with a flat piece of metal (but end of a small screwdriver or something similar) and then apply a little flux, and then warm the whole board and reflow as mentioned before. Works like a charm!
I've now manually soldered two dozen or so 0.5mm pitch QFN chips without a single failure. To be honest, I'm somewhat surprised myself. I'm using the footprint recommended by the manufacturer, NXP, which has small vias on the edges of the center pad. Since the center pad is VSS, at least one via would be required anyway.
My process is quite simple:
1) I use a soldering iron to tin all the pads except the large one in the center.
2) I apply no clean flux to the freshly tinned pads. If I wouldn't be too lazy to make a stencil I could probably skip these two steps.
3) I apply a blob of solder paste to the center pad.
4) I place the chip.
5) I reflow the whole board with a hot air soldering station. After the solder has melted, I tap the center of the chip to make sure all the pads on the chip have made contact with the pads on the board.
I actually prefer QFNs over QFPs because on QFPs it's really easy to get solder bridges - easy to fix, but more finicky than preparing QFNs in the first place.
I realise this doesn't really apply to manufacturing, but automated manufacturing already has the processes to deal with QFNs.