I went and got online quotes from two assembly houses for a simple board with 60 surface mount components (all on one side), no fine-pitch, bga, through-hole parts or connectors, and lead-free processing. Medium delivery time, neither rush or extended.
I then requested quotes for 1, 2, 5, 10, 20, 50, 100, 200 and 500 boards. Only the second house would quote over 100 boards online. Then I plopped the #'s into Excel.
Here are the prices (in $) per PCB for various quantities for the first assembly house:
and here are the numbers for the second house:
The actual figures range from $318 for one board down to $11 / board for 100 boards for the first house, and $448 for one board down to $16 / board for 500 boards for the second house. Guess which one is based in China.
What I find interesting, is although the prices are quite different, the shape of the two curves is almost identical. Prices rise from their lowest point until they double at around 20 boards, and in both cases there is a very sharp knee right at 5 boards. I had thought the knee would be much further right, i.e. having to pay a steep premium for even quantities of 10 or 15 boards, but I guess that isn't the case.
Cost is most likely not going to be any different between a DFN/QFN or SOIC. The main reason to generally have SOICs instead of QFNs is to facilitate debugging, hand soldering, number of layers on your PCB board, and more.
Debugging is much easier with SOICs since you have direct access to the pin and you do not need to remove the solder mask. With a QFN, you would generally expect your PCB boards to be fully functioning and mass produced. To debug a board with QFNs, you would generally need to remove the solder mask to rpobe the specific copper trace.
Hand Soldering is done when you have only a few PCB to do and it is much more cost effective than buying a reflow oven for mass production.
PCB Area comes in handy when you have a lot of parts to dispatch through the board and area is limited. A QFN chip is smaller in size and will usually require less area. In the other hand, you will most likely need more layers for traces, which will also require more vias and so on. This is a trade-off that you have to factor in.
PCB Layers is important as I mentioned above. If you have a lot of layers and can afford to route the traces that way, a QFN could be very useful. But for the typical smaller projects which have 2-4 layers, a QFN part is generally not used. Or atleast, a big IC 4x4 and more.
With all that being said, it is critical to follow Design Rule Check (DRC) to make sure your traces on a QFN meet the requirements.
Best Answer
Many assembly houses these days do 0402 with the same machines they do anything else, possibly using a different needle, though I'd suspect they'd be using the 0402 capable needle for 0603 and 0805 as well.
Is it still true you'll have more choices when you don't go below 0603? Sure. Most likely. There's a lot of cheap assembly houses that are cheap because they use the old equipment of the others, probably everywhere around the world. Some assembly houses may do down to 0201, but not be too happy to do the smaller stuff, because it requires extra operator attention.
However, when you go into volume, the cost of the more advanced assembly house will likely not weigh up to the cost difference for smaller circuit boards, more efficient systems and/or lower component cost. And some times using 0402 or even 0201 offers better per-component performance as well, such as lower parasitic effects.
Obviously if a 5000 unit reel of 0603 capacitors costs $15 and the 10000 unit reel of 0402 of the same value cost $20, that'll add up when you're making 10's of thousands with 10 each, but not really do much at all below using a reel per month.
Because boards are now almost always made to 5mil/5mil standard, the board won't likely be much more expensive if you make anything more compact with tiny components, but at high volumes the board space savings will start weighing as well. If a panel costs $100 and with 0603 the panel can fit 20 PCBs, but with 0402 it can fit 25 PCBs, that usually saves much more in volume than any extra cost you have at assembly in high quantities.
In all, if you want to be fully sure you'd need to do a cost estimation, including an RFQ to a few assembly houses that tickle your fancy. All the assembly/full-service houses I use are always ready to pick up the phone or answer an e-mail with questions about comparative costs. And more often than not I find the cost increase of something "unwise" 10 years ago falls into the less than a few percent now. And the same will happen later to stuff we think expensive now, so, really, you need to regularly keep asking them if things have changed if you want to be the best designer you can be.
Summarising: The only reason I don't do at least 0402 in my designs is if it's a hobby thing for me or others, where I want to be as quick as possible with replacing components, or I want others to be able to use my design as well, as I am not even noticing significant cost increase up to 160mmx160mm boards at 10 units. Over average past orders.