Screaming Circuits: Partners


Using the Newest gen ARM Microcontrollers

KL03 on stampI'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 1.6mm 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 0.4 mm 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 PC board, 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 KL03 SunstoneFF 4mil 800to 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 PC boards, 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.

Check back next week for the first set of results, and be sure to quote your assembly job at Screaming Circuits or your PC boards at Sunstone.com.

Duane Benson
"Screaming Reflowster" not sold here

QFN Solder Paste Layer

LBDCminiI've got the fab order placed with Sunstone.com for my next demo project. The little board is represented here at pretty close to actual size on screen - provided you have a 22" monitor set at 1680 pixel horizontal resolution. Give that, you might want to click on it to pop up a bigger representation of it. That makes it about 4 X life size.

When you do that, take note of the QFN / DFN parts: The processor in the middle, the LiPoly battery charger right between the upper two mounting holes and the RS232 driver in the lower left. I've followed my paste layer advice and segmented the paste stencil layer to reduce the chance for float or major voids.

I found a footprint in the library for the big processor in the middle. I just had to modify the paste layer, as shown here. I made the footprint for the charger and RS232 chips from scratch. Neither had anything close enough in the library.

The DFN has a slightly different approach to segmenting the stencil layer. Little squares like I used on the other two chips work just as well, but this is effective as well.

Another thing to take note of is the markation on the LEDs. The original footprint for the 0402 LEDs does have a polarity mark, but it's one of the types that can easily be misinterpreted or can be difficult to see. The diode symbol put down in silk screen removes any possibility of ambiguity.

Duane Benson
I'm happy I live in a split level head.