Using the Newest Gen Arm, Part II

I’m a bit behind in my blog work — well, way behind, actually. I started this series back in January with the intro post.

Here’s where I am right now:

  1. I have three different sets of PCBs.
  2. One set, I took home to see if it’s possible to solder a micro-BGA at home. (As someone working at a car manufacturer might want to see if they could balance a crankshaft at home, for fun)
  3. Two sets, from our partner, Sunstone Circuits, are here in my desk with parts, ready to go through our machines.

After I’ve got all three sets built, I’ll have them x-rayed to see how they look under the hood. Finally, I’ll solder through-hole headers on and fire up the chips to see if the shared escape system works.

Here’s one of the boards without access to the inner pads:

And, here’s the shared escape:

The main concern I have is that Reset is on one of the inside pins (B4). I’m not sure if I can get the chip to a state where it will operate properly without unobstructed access to reset.

The routing I’ve chosen is probably the only possible option for reset. Pin A4, right above, is used for the single-wire debug (SWD) clock. I’m assuming that can’t be shared. B5 is Vdd, so that’s out. It might be possible to go down. C4 defaults to one of the crystal pins, and D4 defaults to a disabled state.

In the route I’ve chosen, B3 is an ADC input, so it should start out high-impedance, and therefore not interfere. A3 defaults disabled, so it won’t get in the way.

Next step: solder time!

One other thing – The images above show non-solder mask defined (NSMD) pads. Those are standard for BGAs 0.5mm pitch and higher. This part is 0.4mm pitch. Some manufacturers recommend solder mask defined pads (SMD) for 0.4mm and smaller. I’m actually testing several pad styles: SMD, NSMD and solder mask opening = copper.

Duane Benson

Run it up the flag pole and see who solders

Using the Newest Gen Arm Microcontrollers

I’ve written a few times about the new Freescale KL03 ARM Cortex M0+ microcontroller. This particular part comes only in very small packages, with the smallest being a 1.6 x 2mm WLCSP (wafer level, chip scale package) 0.4mm pitch, 20 bump BGA. That’s a mouthful — albeit a very tiny mouthful. Maybe just a toothful.

On the left, here, I’ve got a pair of them on a US postage stamp.

For us, it’s not a particularly difficult part to assembly; just a garden variety 0.4mm pitch BGA, as far as we’re concerned. We place loads of them. But, it can be a very different story for a designer. Conventional wisdom says that a PCB designer has two choices with a part like this: a very expensive PCB, or don’t use the part.

Escape routing becomes very difficult (read: expensive) at 0.4 mm pitch. This part only has six connections that need to be escaped, but that can still be a problem. You can’t fit vias between the pads to escape out the back side. You can’t put vias in the pads unless you have them filled and plated over at the board house. That’s expensive in small quantities.

This blog post series is going to examine some possible ways to use these parts with more of a standard fab, such as Sunstone Quickturn. I’ve got three different process blank PCBs, each with four different land patterns.

I’ve been asked about home reflow too, so as a bonus, I’ve done my best to duplicate hobbyist conditions for one of the board sets.

Duane Benson
“Screaming Reflowster” not sold here