Thickness should be <1.5mm (incl. solder balls and some reserve etc.) or the board risks to be pulled by bending forces out of the B2B connectors.That brings up an interesting point in these types of, "lets consider using X chip," discussions. What are the physical constraints of the Pyra's SoC daughter board?
Max physical dimensions in X,Y and Z axis?
Max current draw?
Max thermal output?
Available area depends on what else is needed around the SoC chip.
There is also a limitation by one of the magnetic nub sensors on the mainboard so it is kind of a puzzle game.
Roughly we have for the OMAP5 board:
* 20x20mm for OMAP5 plus some circuits around (oscillator)
* 40x15mm for RAM chips
* 15x13mm for eMMC
* 16x15mm for Palmas power controller plus inductors and capacitors
* 12x12mm for twl6040 audio codec
There is also some random space which can be used for glue logic.
Max. current draw: less than ca. 2A @ 3.7V (battery voltage). Peak current can be higher (ca. 3A from battery), but then the system is discharged faster than it can be charged.
Thermal: not too hot
Another important factor is the number of available interfaces - here is some minimum:
* 3xSDIO (plus something internal for eMMC)
* 3xUSB2 (the OMAP5 boards has an on-board 3-port hub for that purpose)
* 1xSATA or USB3 (then 2xUSB2 is sufficient)
* Audio: 2x Handsfree speaker, Microphone, Line/Headset in/out, 1x vibramotor
* ~30-40 GPIOs
Also important: 1.8V I/O levels (unless defined otherwise by standard, e.g. SDIO, MIPI, HDMI, USB, SATA).
If a SoC does not provide them all, it must have some expanders on the board.
I did an evaluation ca. 2 years ago and a Z8550 would almost fit (except the power controller and the audio subsystem), but it is an Atom...