I've been having fun with a Raspberry Pi lately, showing it to fellow engineers and asking them to "find the CPU." Most can't.
Raspberry Pi -- where's the CPU?
The big chip is RAM. The next smaller one is a USB and Ethernet controller. There are no interesting parts on the bottom of the board. No chips are under the connectors. So where's the CPU?
The arrow points to the CPU.
Though even in person it's really hard to see, the RAM chip is on top of the processor using a packaging technique common in the mobile phone business called "Package on Package" (PoP). In this case, the CPU is soldered to the board, and the RAM to 168 balls on top of the processor package. Propagation delays go down, and packaging density increases.
Smaller is better, and we're sure seeing this in the microcontroller world. Freescale's ARM Cortex M0+ MKL02Z32CAF4R is probably the smallest ARM part at either 1.994 x 1.94 mm (according to the packaging documentation) or 2 x 1.61 mm (according to the datasheet, which later references the packaging docs). It's in a wafer-level chip-scale package (WLCSP), where an entire wafer of ICs is fully formed, plastic, balls and all, and then individual devices are sawed apart. The silicon is exposed around the edges.
Read the full story on our sister site Embedded.com.