Laptop motherboard power requirements in order to switch on first are the availability of 3V and 5VALW. As a technician you have to be careful when finding 5VALW missing. Switched on first cause of several motherboard, 5VALW appear after switch on then 3V_ALW or 3VPCU which provides power supply to the EC , BIOS IC , South Bridge ( SB ) , clock generator and oscillator on ALW voltage .
VSB or switch voltage button (3.3V) will appear after Bios and EC powered. some of laptop motherboard will found 17V switch voltage on one of pin switch button but after switch -on voltage is changed to be 3.3 V this is normal.
Press power button ( short to ground ) then 3.3VSB will turn into 0V and back to 3.3V and if standby VSB 17V press power button VSB become 0V and back to 3.3V. If this happens, power button is okay.
After pressing power button, signal is being sent to the EC ( Embedded controller NPCE 885LAODX ) signal names is NBSWON # most other brands of EC signal name may SW_ON, EC_ON or whatever name of the signal as long as signal coming from SW1 sent to EC , this voltage must respond in the same manner of testing power button , BIOS IC and firmware connected to SPI interface to the EC.
In other to ensure EC BIOS is working properly, RSMRST # signal of 3.3V must be present. RSMRST# signal sometimes appear after switch on and other motherboard without switch on 3.3 V already present .Modern Motherboard has 2 or 3 IC BIOS.
Other important signal from EC is DNBSWON # ,this is PWR_BTN signal to switch on South bridge sent by the EC after receiving NBSWON # signal from power button . DNBSWON # 3.3V must be present otherwise it would not switch on.
After this signal appears then the SB will sent SUSCLK # signal back to the EC and turn on VS as trigger for all switching gate driver as VR_ON , Suson , MAINON , S5_ON or SLP_S3 , SLP_S4 and SLP_S5 . and for all the Vs voltage to the Device.