One way to solve the problem is changeing

thermal pad to copper shim. I mean the thermal pad between CPU and heat sink.

Thermal pad heat conductivity: 1-3

Copper shim heat conductivity: 277

Before changing it, my netbook fan turns on in 14 minutes (read pdf files); After changing it, the fan turns on after one and half hour (read pdf files), big difference. Now the fan is quiet almost all the time.

The possible problem: thermal pad is soft, copper shim is hard, now my netbook has to handled with care, shake too much may cause problem.