IBM ThinkPad T43 Hardware Maintenance Manual page 140

Removing and replacing a FRU
Important notice for handling the system board:
The security chip a is soldered on the underside of the
system board as in this figure.
a
The accelerometer chip for the IBM HDD Active Protection
System b is also soldered on the topside of the system
board.
b
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MT 2668/2669/2686/2687