The corrosion looks real bad, and affected areas as far in as the U4800 chipset (the white stuffs on the pins). There are also signs of corrosion on the traces underneath/around the ISA slots, e.g. those around R29/U25. Even the pins of the HMC chip looks oxidized. For the U8400, I would use isopropyl alcohol and a toothbrush to gentle brush following the direction of the pins. For traces underneath the ISA and RAM slots, can only hope for the best. For the 28 pins DIP sockets of the cache, if you're skilled in soldering, I would suggest replacing all the sockets, as it is not easy to remove the corrosion inside the sockets. I would also desolder the keyboard controller and discreet components/keyboard socket around the battery and check each trace for continuity. In some instances, might need to sand away the corroded solder mask, reconnect the traces and tin them to protect the copper from oxidization. I had repaired a board with much worse corrosion this way, but luckily the corrosion was localized. Your corrosion is not as bad but wide spread, so it would be hard.
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