Hi!
For first , you must make rules for yourself step by step in diagnostic of phone ,why phone not work, if you replace ICs one by one this is long and not good way for repairing
In the most common cases the problem with phones is his customer!
Yes man! They wash with phones in bath, play ******** with phones,throw in to wall.... then go to workshop and tell to you bla bla bla.... yesterday my phone was worked, today this bu....it not work....
Try to speak with customer why phone not working
If phone was water damaged,clean PSB than to check holes between layers especially power lines, check data in flash memory(try reprogramm phone), check system clock(main oscillator 13MHz,26MHz,19.5MHz depend on phone model) if system oscillator broken you phone will never power up in all modes ,check visually under microscope the most component on PCB, try find seriously damaged than replace if find. If phone is knocked, you must try find broken soldering of component, especially solder balls under BGA(only one way - remove the component and check visually), try find mechanically damaged components, for example Nokia8210 drop network: in most cases reason is broken soldering under COBBA, try to check all ceramic component like filters, PA, antenna switch, there can be cracks.
But the first step on phones with no power always must be - Check Power! Check resistance on battery contacts, check power lines from battery to power IC, try to check voltage on secondary power lines from power IC, check soldering of this IC, for me the 80 % of all failures is bad soldering and mechanically damaged components.
Good Luck!
WBR
Max