A. Caspis wrote:

I have a 3 year old laptop which has suddenly developed the infamous
spontaneous abrupt shutdown syndrome. This is usually attributed to
overheating caused by dirty fans, but I am not convinced.

- I have tested various combinations of CPU speed settings, workloads,
power-management modes, AC vs batery, ambient temperature, etc.
- Shutdowns happen in the BIOS too, so it cannot be a software issue.
- I have enabled self-tests in the BIOS and used memtest86+.
- I have fully disassembled the laptop and reseated every connector.
- I have removed all non-essential modules and peripherals: bluetooth,
wifi, modem, keyboard, LCD panel (running on VGA with a USB keyboard).
- I have cleaned the copper air ducts and operated the laptop with
external fans blowing on all large chips.

Before I call it quits I want to try one more thing: reflow the PCBs.

Check web sites with information about laptop computer repair.
Notebookreview.com is a reasonable starting place.

The Oct. Elektor Electronics (www.elektor.com) has a project for a
reflow soldering oven, but reflowing your main board is as unlikely to
fix your problem as pouring gasoline on it and setting it on fire
will.

Forget about resoldering the entire board. Instead examine it much
more thoroughly, using a 3x - 10x magnifier and strong light, and look
for hairline cracks and solder that looks rough, discolored, or balled-
up. A Google image search for "bad solder" will return many
examples. Do look at the usual suspects - areas of high mechanical
stress (under the keyboard, around external connectors and heavy
objects) and high concentrations of heat (not the CPU but voltage
regulator power MOSFETs). Measure voltages, including AC with an
oscilloscope. Excessive AC voltage is often a sign of bad
capacitors. First practice your soldering skills on worthless surface
mount boards, and don't quit until you confirm that they're restored
to normal operation.