iPhone 6s Plus backlight - Now apparently dead

[epumer]

Hi everybody

So this happened!

1. We received this 6s Plus with half of the screen blacked apparently without reason.
2. Diode mode on PP_LCM_BL_ANODE_CONN showed O.L.
3. Removing the underfill to check filters, FL4211 was loose with 1 whole pad attached to one side of the filter. SO NO MORE PAD on the board.
4. We put FL4211 back, using the side still with a pad, and a jumper between C4211 and the side of FL4211 with no pad. Tested OK for continuity between PIN 6 of the conector (PP_LCM_BL_ANODE_CONN) and C4022.
6. At this point we double check the phone and the problem still there (Upper Right side is dimmed to black)
7. We measured diode mode for PP_LCM_BL_ANODE_CONN again and (not SURPRISINGLY) still O.L
8. Next step we do a visual inspection of the backlight circuit and found no signs of damage.
9. We proceed to the next suspect, U4020. So once U4020 was removed and the pads flattened, We cleaned the area with IPA to remove burned Flux.
10. We put more FLUX, positioned the chip and apply heat as usual (Quick on 360 ºC and 82 Air)
11. When the chip was in position already we hear a sound, like water on hot oil. (Maybe some drop of IPA in the Hot Flux) ?
12. We conected the phone to the DCPS and when prompted to boot the current oscilates betwen 0.06 Ma and 0.25 Ma. No short either

We dont have a clue of what went wrong or if there is any chance to resolve this. So any thoughts on the matter will be really really apreciated

[Jessa_the_Professa]

Here is where you went wrong on this one. You should have checked for continuity between the big cap (the one that is usually called C4023-sorry, answering from home with no 6sp schematic available) and the connector. It would have been OL. You could have run a wire from the big cap to the test point under the sticker (in lieu of getting a wire out to the filter) and it would have been fine. Or you could have done "jessa's little 6sp trick" and just ran a wire from backlight testpoint to the other backlight testpoint and let one backlight driver serve both sides of the screen.

The problem is that when that filter FL4211 blows up in the 6sp it is such a violent death that when the pad pulls, it also severs the connection to the little cap. That cap is sitting on nothing, so having continuity between the cap and the filter is nothing--it is not connected to backlight anode line. (hence the wire from the little cap back to the big cap would have solved).

In any event--how did we get a brain dead phone after backlight driver change? Something is bridged or knocked or overheated. Start by eliminating i2c lines bridged/short under backlight driver. Just take it off again and try to boot. If not that, then take off the shield and look for solder balls/mickeymouse ears and let heat be your guide.

[epumer]

So long story short
1. We removed the two backlight drivers and they were perfect (no bridges at all). Also we checked for shorts in the i2c lines in the area and everything was good.
2. We realized that when we were installing the first backlight driver, maybe the heat was not so focalized because of the distance I was holding the quick so we might have overheated other areas of the board.
3. Because of this we checked for bridged lines under U3500 and u3400. Nothing there
4. We checked the same area on the other side of the board and we found some solder balls in the C2042 area. I will post a picture of one of them between C2043 and C4040

We hit rock bottom again so any tips on how to proceed will be SUPER DUPER appreciated!

[Jessa_the_Professa]

When you took off the backlight drivers, does the phone boot or change dcps behavior?

Feel free to post a video of the behavior on dcps and to post more pictures of the board so we can see what you see.