I recently had a New 3DS XL come in for repair with a sort of weird issue, and I wasn't having much luck finding information on it anywhere online, so I thought this would be a good one to document in case it helps anyone out in the future.
In addition to some charging issues which were corrected separately, I had a New 3DS XL brought in because when powered on, the console would only show the blue power light (no other lights) and would remain in that state until the battery was removed or the power button was held down.
What made this weirder was that sometimes the console would turn on perfectly fine, but freeze after a few minutes of power on time, then return to being stuck on the blue light. From the research I was doing online plenty of things can cause this behavior, but none of it should have let to intermittent boots. The other thing is that since the system would sometimes turn on, I was able to test everything, such as the screens, touch, 3D, and cameras, and had no issues with any of them, which indicated to me that these were all working fine. So what gives?
The New 3DS needs a few things in order to start up; Both of the screens, their backlights, and the speakers. If these aren't connected then the system will turn on, make a loud pop and then turn back off. Since this wasn't happening, I was more inclined to believe the issue was somewhere on the mainboard, but I wasn't quite sure what. I quickly buzzed around the board with my multimeter but didn't come across any obvious issues. So I did what any sane person would do and totally gutted my own 3DS to start swapping parts around and narrow down the issue. I first swapped the mainboard from the bad 3DS into the chassis of my own. At first, the system consistently booted perfectly fine, which prompted me to swap my "good" board into the "bad" chassis to test the screens etc. Imagine my confusion when it turned on perfectly fine. Confused, I went back to my chassis with the bad board, and after further testing was able to do replicate the fault, so at this point we know that the issue was on the main board somewhere.
I took a deeper look with the multimeter but still wasn't finding any weird readings that jumped out at me. At this point I was worried it was something with one of the ICs, but without any shorts reading, how could I figured out which one?
I decided to take a step back and think about the issue I was encountering; it almost seemed like it was trying to start up but just couldn't for whatever reason. So what if the issue is with the storage?
I took a look around the board for the NAND chip, and after some. googling was able to identify it. Conveniently, this is also the ONLY of the ICs without any under-fill, which started to provide me with some confidence that I was on the right track. Its very possible this system was dropped, and one of the balls under the chip was broken or only sometimes making contact with the board.
The NAND chip on a New 3DS XL, highlighted in yellow. Borrowed from 3dbrew.org.
At this point I decided to reflow the chip. I applied some flux around the outside of the chip, fired up the hot air station to around 380c @ 50% airflow, and went to it. After a minute, the chip was flowing, and I let it cool down and cleaned up.
I put the board back into its original chassis, and honestly to my surprise, the system came right on. After extensive testing I was confident the issue was resolved.
So there you go, TLDR if you run into this issue and are beating your head on the wall trying to find what could be causing it, try reflowing the NAND chip! Please only do this if you have experience with doing this type of soldering, if you mess it up and you don't have all your NAND keys, the 3DS will be cooked permanently. Even if you have the backups, restoring it will not be an easy job!