I'd love to hear more about how you tracked down the schematic and how you used it to find the right joint. What kind of problem was the machine exhibiting?
Remove the WiFi board and all was fine.
Turns out the WiFi card is PCI express and one of the signalling lines associated with the bus sleep functionality had a pull up resistor that was open circuit.
Replaced the resistor and now all is fine.
I found the schematic by typing random text strings from the board into baidu. Ended up finding the 500+ page schematic direct from Apple (presumably leaked from a supplier). It ended up coming from one of those silly websites where you have to earn coins by uploading other leaked documents to download the one you want - luckily they're easily fooled by uploading opensource projects. It is interesting to see the ways Apple design things - for example every component, even down to resistors and capacitors, is marked which production test covers it (or if it isn't covered at all by tests, like the one I had that was broken!).
There's another channel on youtube, louis rossman, a really entertaining new yorker who fixes mac books and explains how he goes about doing it. He uses Paul Daniels software.
Louis is fighting for the right to repair as well. He's always ranting about how apple makes it impossible for others to repair macs. Apple themselves don't really repair macs, if a product is out of warranty they usually quote a repair price that makes it cheaper to buy a new mac.
I have no idea how to track down the schematics, though.
His series on electronics basics is the best I’ve seen, for the reasons he states. Simple language, no college level theory and big words.