I just have a theory, the one that fails more often is the closest to the CPU at the back side of the PCB. The PCB eventually gets too hot in this area due to CPU inefficient cooling solution. It's a well known issue and I didn't invent the solution. This failure results in LAN 4 port LED light and a non-responsive router. Chinese guys in their forums share information where to get the IC from. I just used donor boards to swap it. The other VRM I've seen failed is the one right next to the power jack. I believe it was IT76630 DC/DC converter. Failure of this one results in half lit power LED and obviously dead router. The router also suffers from 2.4GHz RF issues. I don't know the exact reason why, but I reflowed the RF ICs on one PCB and it came back to life. Failed RF circuit keeps the router going, but with Channel 0 in UI and inability to change the settings for corresponding band. The possible cause of failure is thermal stress and microcracks in solder balls under the ICs. This is also a well known RoHS standards created issue in thermally stressed PCBs. I don't know how useful this information is for you, but keep in mind I only see defective units. This doesn't mean every AC86U will fail in a short period of time. There are folks here with 2017 production year AC86U routers and still going strong.
I think its very cool that you know all of this. Even if it does not help me I appreciate the information. That micro crack theory sounds plausible. I don't have any experience with it on a Router but I did on a Plasma TV board. Had a $3500 60" Plasma that died out of warranty. And I could not get anyone to fix it and I'm not great with solder or tracking down broken traces. But i tore it down anyway as I suspected faulty power supply.
I looked and failed, never found the issue. I found a guy that was good with TVs but he was out of state, Had to make educated guess and send him the components (not entire TV). I don't know how he found the broken traces or soldered them I had to ask him to take photo because I could not even see what he soldered. I could barely see it even circled it on a photo. I was like How the heck did you even see that and then solder it so well? Very impressed and thankful, not wanting to trash a TV that was 99 percent perfect but a few tiny broken traces.
He said It was due to heat built up and the type of solder used that tends to crack on expansion with repeating cooling/heating cycles. Its held for like 5 years and counting.
I have always been impressed by guys good with soldering. I can do it, but not like you or the guy I hired.
The guy that looked at my TV seemed to say they are using solder that is built to sort of fail. Maybe its the same on Asus. I thought great he fixed a few cracks but not the entire board. I think he fixed areas that he knew were likely to fail and the other areas even though they used the same solder were not as likely to crack.
I liked that you see only defective units. I don't know what you do but sound like you have some good experience with these. I don't remember all the the fellow I hired told me but It seemed to me he said manufacturers had changed the type of solder used for a reason. I don't know if it was cost or why but it seemed that he said older units had used better solder. Like if I sent him two routers and OLD one and New he would look at it and pick it apart saying this one has much better solder joints etc. Maybe you could clear that up for me, I dont look at enough to know. But what you said reminded me of that guy. I don't think I am good enough to do what you do to fix it.
I think I asked him if I should put a fan behind the TV assuming its going to fail again but he seemed confident that it was not needed. Like active cooling would not have stopped it from failing. He had done quite a few before mine. I guess the issues was the few known weak areas like using the better solder was needed only there and not everywhere.
have you seen other models besides the 86U? I mean the boards. Did something change on the 86u were heat is more likely to cause issues vs earlier models? Just trying to understand if the layout has been changed where it created a new potential area of failure that other models did not have.
