What’s your favorite MCU package and why?

  • The DIP is big and easy to use. You can stick it in a breadboard (wireless or soldered), a socket or easily hand solder it. But, it tends to be more expensive and takes up more real estate.
  • SOIC is a good step down in size. It can be machine soldered. It’s big enough that most people can hand solder in a pinch. But, as an SMT, I’m not sure it has much purpose anymore. If there’s an SSOP available for the same part, why would you take the bigger SOIC package?
  • SSOP are nice and small so that, unless you are really tight on space, they’ll do just fine. They aren’t really any more difficult to layout than and SOIC. If you do need to hand-solder, this package is probably too small. Being smaller with everything else being equal, it might have more issues with heat dissipation than the bigger part or a smaller one with a heat slug under it.
  • QFP – these are just lie either an SOIC or SSOP, but with leads on four sides.
  • BGAs are really compact and and do a good job of keeping signals close to the PCB and to bypass caps. They can be a challenge to layout though. Many will require upping your layer count. The really fine pitch BGAs may require expensive PCB features such as blind or buried vias. CSP and wafer-scale BGAs can be more difficult to handle because of their small size. Breathing on them wrong can toss them around like dust.
  • QFN and DFNs are somewhat newcomers to the scene. The package can lead to some very tiny components. It’s great for signal cleanliness and the heat slug underneath can dissipate (with proper layout) a lot of heat. But, QFNs and DFNs seem to garner the most layout problems. Careful use of thermal vias is critical for maximum performance, but you either have to use expensive techniques, such as filled and plated vias, or you have to rationalize and get around some nearly mutually-exclusive requirements.

Yeah. They all have their pluses and minuses. Fortunately, with proper board design, our SMT machines can place all of the these types all day long without breaking a sweat. All the SMT designs, that is. We do hand place the DIPs. What’s your preference?

Duane Benson
All we are is BGAs in the wind


Picking Packages

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 little more difficult, but not much. It was largely a matter of power dissipation requirements.

Different story now, though. First, there’s through-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) through-hole resistors range from 1/20W up to multiple watt packages. SMT parts range from 1/32W up to lots. Simplifying a bit and just looking at 1/4W, 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 an SSOP over an QFN? Why 0603 over 0402, 0805 or 1206?

Duane Benson
Peter Piper picked a peck of pickled PIC packages.