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 1.6mm 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 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 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.
"Screaming Reflowster" not sold here