>> There are some journal stating the differences in ESD sensitivity before
>> and after the parts were mounted on the PWBl. Could anybody explain the
>> actual difference for each ESD class?
ESD susceptibility may be increased or decreased by mounting a component on a PCB. It is not generally possible to tell what will happen to ESD susceptibility without doing an in-depth investigation - clearly most people will not want to do that because it would take an extensive research program. The general guidance is that a PCB should be considered as ESD susceptible as the most sensitive part on the PCB.
The only reference I have on this is Warren Boxleitner's paper " ESD stress on PCB mounted ICs caused by charged boards and personnel" Proc EOS/ESD Symp. EOS-12 1990 pg 54-60.