PCB Delayering

I was hoping to find marking codes on the BGA chips while decapping the main chip(SiP), but found nothing. So I decided to look inside the PCB layers and hopefully get some clues.

The delayering process was straightforward, using Sandpaper, Double-sided tape, and a piece of glass on which I mounted an extremely thin PCB.

PCB delayering was a success, I uncovered 3 part numbers:

U3 - IMU ICM-20600
U7 - flash memory MT25QU256ABA
U13 - battery charger BQ25125

PCB has 5 copper layers, with the ground plane in the middle.

Mics and strange Caps

PCB reverse-engineering begins with a weird discovery: a capacitor from flex PCB that is connected to the pin on BLE SOC U9, unusually far away from U9.

Button

Button connected to the main PCB with 2 traces from flex PCB, another strange design choice.

i2c

The chip design continues to get weirder: i2c lines are not connected to the pads on the right side! This means the second i2c temperature sensor located on flex PCB is useless.