We are looking at starting PCB design in house, up until now we have outsource design and manufacture. This means that we have yet to see the real cost of the PCB, components and assembly as discrete items.
Looking at the current PCBs that we have, they currently all use SOIC style of chips, even when QFN parts are available alternatives. Is the reason for this because the legs on SOICs make them easier to use in manufacture? Or is it because the legs on SOICs make life easier for debugging and hand soldering etc?
We are interested in the cost of assembly in large, 100k or 1000k quantities, when produced in a production line with pick and place machines.
I haven't found any difference in the cost of the IC in the different packages, so I don't think that is the reason. But it could be that the difference packages are delivered in different methods; tape and reel vs a tray.
Is there any resource that I can use for estimating which is the cheaper packages or delivery packaging? I've tried a bit of internet searching, and not found anything yet.
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.