How to diagnose little step down and power management BGA chips never shorted

[corrize]

Hello,
I have different sort of those little BGA chips (step down DC-DC, power management battery, …) I replaced 10 on differents mobile devices (no schematics) but I noticed often they can be dead but never shorted. I see some signs : burn marks, resistance drop by comparison with good board (40 KOhms to 1 KOhms for example)… but not easy to be sure of the diagnose. Because I can't access on all tracks (balls) with the meter.
Sometime, I know only after replacing chips.
Do you have tips for diagnose please ?

[sam_sam_sam]

Please post some pictures of the front and back of the boards so we can see what you are dealing and talk about

[corrize]

Here it's a simple board, DC-DC down convertor so the chip is easy to find as guilty in this case because the other components are easy to check, but this chip is never shorted. I have this one, and in 4 or 5 dead boards : no short. Even if the chip is physicaly broken. It's harder/not possible to diagnose when that sort of chip is on other big board.
The most of the time, I can't diagnose with power on because the boards are inside box with cables. Sometime, I diagnose like this : if no voltages out of this chip and no short, so chip dead... Because I never saw those ones shorted. But it's not reliable, the chip could be not enabled because something else is faulty.
In diode mode, I can't find a big obvious differences. In resistance mode, I can see differences but on big board, I can't be sure.
(I have no thermal camera).

[sam_sam_sam]

Try doing it in diode mode instead ohms mode it might reveal more results

I had posted on another post about this topic before

Do the same exact testing in the diode mode on your meter and compare them this way

Here is a video that explain exactly how to it this way and why she thinks it better for troubleshooting boards

https://www.youtube.com/watch?v=pRpyHF2dfq0

I have tried this method once briefly and it I think it doable but it more time consuming

This is if you have two exactly the same boards

I hope this helps one note the video is a little bit long but if you have the patience she does explain why she thinks it's better than using ohms mode on your meter when comparing values from one board to another board this might be helpful to you trying troubleshooting this device