820-00138-A PP1V35_S3_MEM short to ground

[dudipupan]

Hello everybody,
actually i'm working on a 2015 MacBook pro 15" (820-00138-A) that is not powering on.
I detected PP1V35_S3_MEM is shorted to ground but i struggle in locating the short circuit.
I inserted 1.35V to Pin 2 of R5560 but here all memory chips are getting warm.
There are about 200 caps that could cause the short - or one of the 32 memory chips.
Has anybody an idea how to locate the faulty component?

Thank you and best regards

 

[2informaticos]

Forget the caps, the memory chips are much more sensitive.
An IR camera can help you in this case.

 

[dudipupan]

Unfortunately I'm not owning one and I don't know anybody who ownes one :-(
Already tried it with Isopropanol but i'm not getting any clear result.

 

[2informaticos]

I remember 3023 board with pure short on RAM power supply.
Couldn't detect any hoter chip than other; the short disapperead after removed 13 (from 16) chips.
I checked last 2 chips removed, no internal short found.
Finally I removed and reballed all 16 chips; machine still works after half year.
So none of the chips was shorted in fact.
I can only suspect short was caused by corrosion between balls, under several chips.

 

[dudipupan]

So you think I can try it first with some flux and hot air before starting to remove the chips?

 

[2informaticos]

You can try that.
If still the same, then remove RAM chips 2-3 at a time.
You may have luck and eliminate the short fastest.

 

[dudipupan]

I managed to get a IR-camera and took a closer look on the RAM chips.
The picture is a little bit misaligned so the thermal image is moved to the right.
As feared I see no temperature peak here. But some of the chips are not getting warm at all.
Any assessment?

 

[2informaticos]

"remove RAM chips 2-3 at a time"
Start with the hotest ones from IR image...

 

[dudipupan]

So one memory Chip was left when the short to ground disappeared. Seems like most of the Memory Chips are faulty - 4 Ω between Pins 2 and 7. I compared it to a known good one - this one had 500 Ω.
Or did i maka a wrong measurement? What could cause 16 Memory chips to get faulty?

 

[dudipupan]

So now I got 27 chips with very low resistance (<5 Ω) between GND and Power supply, 1 with 100 Ω, 2 with 4 kΩ and 2 with 0.5 kΩ. I'm confused... are they all bad?

 

[dudipupan]

I got a set of chips from a 2013 15" macbook pro. May I use them?

 

[2informaticos]

Compare the memory specs.
Datasheets may be available on Internet.

 

[dudipupan]

What are the spects I do have to pay attention to?

Thank you

 

[dudipupan]

So the second one i marked with a red arrow are the faulty ones from the original board (H5TC4G83AFR-PBA) and the first marked is the chip model from the donor board.
Should be fine?

 

[2informaticos]

Looks compatible, only manufacturing time is different...

 

[dudipupan]

So just reballed 32 RAM chips... :-D
Should I check the RAM power supply voltage before soldering them back on? I'm afraid burning the new ones :-D

 

[2informaticos]

The only reason to burn so many RAM chips, is a severe voltage spike.
That could also damaged CPU already; is connected to the same voltage rail.

If you want to continue/waste time on it, check the RAM voltage before soldering new chips...

 

[dudipupan]

I just spent so much time for this so I will try it ;-)
At least I got a fanspin now (without RAM). Is there a possibility to check if the CPU is working before soldering the RAM? I dont think so :-D

 

[2informaticos]

No way.
If RAM voltage is good now, go ahead.

 

[dudipupan]

I soldered the memory chips back on. Only got fan spin - no display and no usb activity. So I think you were right and the cpu is gone ?