QFN? QFP? QFwhat?
The QFN (quad flat pack, no leads) has become my favorite integrated circuit package. It's very compact, yet is easier to use than a micro BGA.
Micro BGAs of 0.5mm and smaller pitch become a bit more difficult and costly with more than two rows of pins. At those geometries, escape routing can involve plugged and plated vias which adds complexity and cost to the board fab. QFNs can be almost as small, but have all of the pins exposed around the edges - so, no need for escape routing.
One thing that's important to note, is that despite sharing the first two letters (Q and F), the QFP and QFN footprints are not interchangeable. We do, from time to time, see boards laid out for one along with the other form packaged part.
Take a look at this PCB layout clip from the Arduino Leonardo. It has both footprints on the board. You can see how much bigger the QFP package is.
They put down both footprints because the Atmega32U4 chip used in the Leonardo sometimes has supply issues in one package or the other. This gives them the flexibility to use either without making any changes on the board.
You might consider this as an option if you have the space for a QFP and are concerned about the available of one package variant or the other.
If you do, there are some very important things to check out:
- Make sure the pin-outs match. Some parts will vary the pin-out a bit between packages or have extra pins on one or the other.
- Make sure the extra space won't cause noise problems. Generally, you want bypass caps as close as possible to the supply pins. This amount of extra space probably won't be a problem when using a QFN, but in some designs, it might.
- Make sure the board won't be in an environment where unsoldered pads will be a problem. Some harsh environments could attack the unsoldered pads. If that's the case, consider conformal coat.
We're always being pushed and shoved by people trying to beat the clock
But we like it - it's what we do