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RTO2000 RAM Artifacts

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jjoonathan:
As for how the LVDS broke in the first place, the symptoms appeared over a few boot cycles so I'm not 100% sure but either I zapped it with static or I accidentally plugged the screen into the instrument while powered and it took poorly to being hot-plugged. It could also be that I accidentally summoned some demons while performing the usual satanic ritual before every BGA removal, as one does. My guess is static, because I'd expect a hot plug problem to have enough power to insta-kill the common mode drivers, but it's hard to tell.

In any case, problem solved. I highly recommend the horned differential probes for chasing demons out of oscilloscopes.

tv84:
 :clap: :clap: :clap:

When there's a will there's a way!

jjoonathan:
That's the spirit! I'm not out of the woods yet though.

The RTC Chip

In parallel to the Intel PCH drama, I ordered a BGA reball jig for the RTC Chip with the poor contact beneath it. Before it arrived, I decided to try a reflow. This is where things went wrong.

When I booted the scope after the reflow, it went into "leaf-blower mode" and the mainboard stopped showing up on PCIe. Further inspection revealed that none of the power rails were coming up, and that the 1.2V core rail was ohmic short (vs 4 ohms for the other chip IIRC). I must have bridged something during the reflow, and I suspect this happened because I tried the "gentle tap" technique to see if I had successfully melted the solder. It didn't feel like I had gone too far, and I anticipated the possibility and simply planned to wait for the reball jig if it happened, but with a dead scope on hand I clearly must have tapped too hard and not felt it. In hindsight I should have checked resistances on the main rails -- any buck converters with a big inductor and certainly any multiphase buck -- before and after reflow. Not all hope was lost: I have seen many high quality R&S protection circuits and true to form they have 9 individually fused 12V domains on the mainboard, bus transceivers between them that handle power-outs, monitor circuits... they even have LEDs on the fuses. How nice is that? (01 blown fuse.jpg) There was a good chance I shorted a power stage without pulling current through the chip and in that case everything would be fine. I removed the chip, replaced the fuse with an, erm, slightly less elegant substitute (02 replacement fuse.jpg) and confirmed that the power rails came up by injecting 12V into the main bus. They did!

12V Mainboard: 4.45A (13V OVP, 5A/2ms OCP)
12V Mainboard+Frontend: 11.45A (14V OVP, 13A/2ms OCP)

The RTO even booted and enumerated the mainboard on PCIe, although with one of the RTC chips missing it did complain about GB Transceiver errors. I wonder why  :D

I inspected the RTC chip under a microscope and noticed that a bunch of pads looked partially or completely unwetted with solder. The pad corresponding to the malfunctioning DRAM bit looked especially bad. Welp, there's my problem! Everything tinned up nicely and in not too much time the jig arrived.

jjoonathan:
Reball time! With a proper jig and a big BGA. No more of this DDR2 and $10 aluminum extrusion nonsense. The reball jig that you see in all the youtube videos is really great, but it demands more patience than might be apparent from watching youtubers who have everything dialed in because they do this professionally. Some learnings:

Spend the time to adjust all 4 corners so top of stencil is at the top of 4 test balls BEFORE adding flux. Otherwise you will almost certainly crash the stencil into the flux and have to start over, or you will lift the stencil so that balls can escape to the space between holes and have to start over. Starting over is costly enough that it is worth going to great lengths to level before adding flux.

Tape off the set screws. I thought I could just avoid bumping them. Nope. Just tape them.

Spend the time to mask off unused holes. If you are doing a one-off, it is tempting to just let the balls fall through unused spaces. Don't do this. The balls pick up flux and spread it into every crevice and it is a huge pain to clean out. It also prevents you from doing enough passes with the brush, and you will probably need a number of passes before you get the "feel" so that you can brush balls into stencil holes without bouncing them back out.

Less flux! I am usually a fan of the Louis Rossman school of "there is no such thing as too much flux" but this is an exception. If you have more than even a tiny amount of flux the surface tension draws the balls together the moment it starts to flow (06 chasing balls.jpg).

Finger method for spreding flux. I really tried to get the brush to work, but I never succeeded in avoiding small patchs of too much flux. By contrast, following northwestrepair's example and spreading with my finger, I never created a patch of too much flux, it was always all-or-nothing. I don't like messes but I can't argue with success. Finger technique is better, or at least easier.

Test your tongs BEFORE heating. None of my tweezers were wide enough to grip the chip and my tongs were too curved to do a good job. I eventually made a tool that worked by cutting a heavy silicone pad, but this was a terrible hack. This is something I should have sorted while I was ordering supplies.

As you can see below, I bought one of those new MiniHotPlates for this and it was delightful. As you heard from the lessons above, it took me a few tries but I eventually wound up with a reballed chip ready to install!

jjoonathan:
Whoops, I see now that it was 36*36-1=1295 balls, not 1296 as I claimed before. I hope you will accept my humble apology for this egregious case of stolen valor.

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