Instead of using bulb, I used a heat blower to re-flow the chip.
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From my observation, there is a the gap between the GPU and the heat sink was join by a burned thermal conductive silicone sponge.
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I replace this with an aluminum foil folded a couple of times to get just the right thickness.
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After reassemble it back together and now, the moment of truth, Hey! it works! :-)
Thanks a lot jasonshay2. You really save my day.