Apple has changed the Power system after releasing the iPhone6S and iPhone6Splus. You can easily notes that when you compare between the (iPhone 6) & (iPhone 6s) board. Apple used new Power IC. iPhone (6S / 6Splus) power IC is thiner so they used anti shock material to protect the IC from cracking.
iPhone 6S has less number of ICs and the first two ICs you can directly notes that they are not existed anymore are the touch controller ICs. Read No Touch ICs in the IPhone 6S/6Splus . And by reducing the number of ICs, the phone will consume less power in active mode which will increase the working time and the battery life in the iPhone.
we have a new Backlight IC also ( LM3539A0YFFR ) instead of ( LM3534TMX-A1 ) in the iPhone 6. The new one has (16 pads) with two diodes and two coils allowing powerful and flexible Backlight controlling.