Using the Newest Gen Arm, Part III

The continuing saga of the 0.4 mm pitch KL03 ARM microcontroller. If you haven’t yet done so, read part I and part II.

Today, I have a look at the good, the bad, and the ugly – or more accurately, the good, and the bad and ugly. As I expected, I was quite pleased with the job done here in house. The board is nice and clean, the parts are well centered, and the solder joints are solid. No surprise here.

Here’s a top view of one we did here in Screaming Circuits:

Next, I’ve got one that I did at home. It actually surprised me and came out better than I had expected. Here’s a top-down view of the one I did at home with home-grade tools (No, I didn’t intentionally make it look bad. The board surface is just a bit shinier than the one above.):

Of course, “better” is a relative term. I didn’t say good. I could call this both bad and ugly. I did manage to center the parts quite well — that took a lot of careful nudging with sharp tweezers and and an X-Acto knife blade.

All of those little round shiny spots are solder balls. That’s what happens when you get too much solder on the board, get solder off the pads, or have the wrong reflow profile. They might look harmless, but if there are too many under the chip, the connections could be shorted.

The fillets on the 0201 capacitor are a little lean on solder in the one I did, and there’s a solder ball on the right side, but, again, it looks better than I expected.

Next time, I’ll post the x-rays and show what’s under the hood.

Duane Benson
Carburetors, man.
That’s what life is all about


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 µBGA.

µBGAs 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 add complexity and cost to the fabricated board. QFNs can be almost as small, but have all of the pins exposed around the edges, so there’s no need for escape routing.

One thing that’s important to note is that despite sharing the first two letters (Q and F), 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 changes on the board.

You might consider this as an option if there’s space for a QFP and you are concerned about the availability 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 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, bypass caps should be 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.

Duane Benson
We’re always being pushed and shoved by people trying to beat the clock
But we like it – it’s what we do

Parts Too Close For Comfort

Another tale from the sometimes kind of kooky world of prototypes. I think what happened here is that the board was originally laid out for caps with a smaller voltage rating (or even a smaller capacitance value). We’re seeing things like this more often these days due to availability issues. The part gets swapped at the last minute because the exact one is out of stock. Operationally, it won’t hurt and if the board had been laid out with more space, this wouldn’t have caused any issues at all. Of course, then, it may not have fit in the box.

The moral of this story is that with last-minute swaps, don’t forget to double check things like the part package and board spacing. This board works fine, but it won’t be meeting up with any IPC standards as long as these parts are like this.

Duane Benson
Parallel parking is hard. I’d much rather diagonal park.