820-00609 short on PPBUS / CPU PHASE

Narek

Member
Hi! Today my customer brought in an iMac 27” 5K 2017. As he say, computer turned off by itself and caused all electricity at home to shut down. He felt burnt component smell. I took out the board and found burnt and swollen area on PCB under one of the CPU phase mosfets (Photos of the board in attachments.) I’ve measured dead short on caps next to the swollen area. I don’t have schematic for this board, but I guess it’s PPBUS line. I’ve removed some SMD components in this swollen area and started digging into the board. After I got through all burnt parts I noticed short is gone. I’ve assembled everything for testing and computer runs fine. I haven’t stressed it yet with benchmarks, but I’m going to test it tomorrow. What I want to ask you is, what is the chance, that it will run stable with 5 of 6 CPU phases? Do you think it will have problems with CPU VCORE on high loads? Also - is there any proper way to secure such canyon on the board or just cover it with layer of UV soldermask?
 

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2informaticos

Administrator
Staff member
It can cause an issue under high graphics load; you should stress it for test.
Do you think the client will use it in such way?
 

Narek

Member
As he say, he use it for AutoCAD and SketchUP, but I’m not sure if they can load GPU so much. I will perform stress tests to find out how it behaves. In worst case it will burn PCB again or shut down because of not enough power. I’m wondering why it has burned layers of PCB instead of blowing some other components. I haven’t seen anything like this on iMac yet. Do you think it will be enough to cover the scratched area with soldermask only or I should secure it other way?
 

2informaticos

Administrator
Staff member
Don't need to protect the area if no other component mounted on top.
Only need to do this if is on the MOSFET side and need to solder it back.
In such case need to prevent a short generated by MOSFET central pad.
 

Narek

Member
I see, thank you for advices. One more question- which schematic/boardviewed can I use with this board as replacement? I can’t find anything similar
 
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