A long, long time ago, in a place pretty close to here, picking a form factor was easy. Your CPU came in a 40 pin DIP. Your logic came in 14 or 16 bit dips. You picked resistor sizes based on their current carrying needs. Transistors and other power components got a lttle more difficult, but not much. It was largely a matter of power disipation requirements.
Different story now, though. First, there's thru-hole vs smt. Then there's a plethora of options beyond that. So, what really matters? A specific resistor size may come in multiple wattages. Chips come in multiple packages - often from big DIPs all the way down to tiny QFN or BGA packages. Let's look at a few examples.
Here's a simple microcontroller: the PIC18F25K22. It's a pretty typical 8-bit PIC. You can purchase it in four different packages:
- DIP, $2.05 each, Qty 100, Tube
- SSOP, $1.86 each, Qty 100, Tube
- SSOP, $1.90 each, Qty 2,100, Tape & reel
- QFN, $1.86 each, Qty 100, Tube
- SOIC $1.89 each, Qty 1,600, Tube
- SOIC $1.93 each, Qty 1,600, Tape & reel
(DigiKey prices as of the posting date. Some are non-stock items) There's also the part presentation to consider, e.g. reel, cut tape, tube.
Next, look at a 1K resistor that might be used as a pull-up. (As listed in DigiKey) Thru-hole resistors range from 1/20th Watt up to multiple Watt packages. SMT parts range from 1/32 Watt up to lots. Simplifying a bit and just looking at 1/4 Watt, you can purchase 0402, 0603, 0805 and 1206 packages. For high volumes, price will be a factor, but for lower volumes, the price difference can be trivial.
If you have plenty of space to work with and you need to build by hand or for some reason need a socketed part, your choice is the DIP. If space is a bit of an issue and you may or may not hand build, then an SOIC is probably your pick. Some people will hand build QFNs and SSOP packages, but that's not realistic in anything but rare cases.
When size, speed, current or performance need to be at maximums, selection is still not that difficult. You'll often have far fewer options to choose from at the performance edges. But when there's headroom all over the place, how do you decide? Why an SOIC over n SSOP over an QFN? Why 0603 over 0402, 0805 or 1206?
Peter Piper picked a peck of pickled PIC packages.