820-00138-A - Advice on how to fix corroded missing / pads

[mikay786]

Working on this a MacBook i purchased, has some corrosion around what according to the schematic is SSD CURRENT SENSE circuit. I've got 3 missing pads


The pad left side of R3742, where would be best to run a jumper wire too?

I assume the other 2 missing pads should be simple enough with top C3701 pad which appears to be connected to the other components on the same plane. Pad on right side of R3742 just need to make sure is connected to trace.

Mainly struggling with left pad of R3742 and where to run a jumper, maybe a via as appears to go to multiple places. Am i correct in thinking can run a jumper to any of the yellow points?

Schematic and boardview in attached along with photos of board, plus spares board.

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[reformatt]

I assume you are referring to P3V3SSD_VMON from R3742? I'd just scrape off the mask on the track, and run a small wire to that, but you can also use the test point that's just above U3740 if that's too fiddly. How you do it is up to you, but I often put my component down first with the pad that is there, use conformal coating on the soldered end, then solder my jumper at the opposing end.

[mikay786]

I assume you are referring to P3V3SSD_VMON from R3742? I'd just scrape off the mask on the track, and run a small wire to that, but you can also use the test point that's just above U3740 if that's too fiddly. How you do it is up to you, but I often put my component down first with the pad that is there, use conformal coating on the soldered end, then solder my jumper at the opposing end.

Managed to do it and is detecting ssd now.

i left out R3742 as didn't seem to be doing anything? Just ran a jumper from broken trace straight to R3741.

R3742 just seems like it's connected to ground?

I don't want to keep fucking with it as those components are microscopic. So if I can do without R3742 would be ideal but not sure if needed?

[mikay786]

Basically done this, see attached

is that ok?

[reformatt]

It's part of a voltage monitor for the SSD 3.3V rail, nominal voltage is 1V to one input of the comparator in U3740. If the divider voltage is below 0.7V (due to PP3V3_S0SW_SSD being below 2.4V or so), the comparator output will stay low and hold the SSD in reset. Designers have put it there to force SSD to stay in reset if the SSD power rail is way below spec (assuming to prevent SSD data corruption). So under normal circumstances, it doesn't play a part. Leaving it out means that the input is tied to the rail, basically disabling that circuit.

[mikay786]

Am I safe to leave as is or should I go back and add it?

guess I could add it further down the trace so not having to risk the work already done.

[reformatt]

It's not really a circuit that is used if the 3.3V for the SSD is normal, so it's not an issue to leave it out. However, if you do want to add it for the sake of completeness, then just add the resistor to the test pad for P3V3SSD_VMON, and run a jumper to ground for the floating end if you don't want to mess with what you have done.

[piernov]

This is voltage/reset supervisor for power-on reset. I don't know if it's critical here, but even under normal circumstances it is useful as it waits for the power rail to be above a threshold + a constant delay before de-activating reset. This is done so that the SSD does not try to initialize before the power rail is stable after turning on the machine or waking up from sleep. It always take some time for a power rail to be fully stable at its nominal voltage. In some cases this kind of circuit is also used for proper power sequencing (to wait for other signals or power rails to come up).
Personally I'd fix it properly, I never like to leave out random components.