Hi Pradeep,
To answer your questions:
> 1.    The separation is mostly on the adjacent layer to the outer layers.
> A: The inner-layer separations (ILS) occur primarily at layers 1, 2, n-1 and 
n-2 because there the stresses from the radial tension from the resin thermal 
expansion and the torque load due to land rotation combine.
> 2.    Why is that it is happening only for hand soldered boards
> A: The reason for this is that the maximum temperatures experience by the PCB 
are in manual soldering.
> 3.    Somebody can help us with some reference articles on the advantages 
> and disadvantages of both hand soldering as well as machine soldering.
> A: No reference articles come to mind; but I talk about it at length in my 
workshop "Interconnect Failures and Design for Reliability for Plated-Through 
Holes/Vias (PTHs/PTVs) (incl. Lead-Free Soldering Impact)" next tentatively 
scheduled for IPCWorks in Dallas in Semptember.
> 4.    The customer keeps telling that the soldering operator is trained and 
> with 'years of experience' and can not make a mistake!
> A: I wished I could tell you that were true and meaningful—manual soldering 
by its very nature is uncontrolled no matter how good the experience and 
training [which by the way is totally useless with lead-free soldering]. but even 
under the best control, hand soldering is still hotter than either reflow or 
wave soldering.

Regards,
Werner Engelmaier
Engelmaier Associates, L.C.
Electronic Packaging, Interconnection and Reliability Consulting
7 Jasmine Run
Ormond Beach, FL 32174 USA
Phone: 386-437-8747, Fax: 386-437-8737, Cell: 386-316-5904
E-mail: [log in to unmask], Website: www.engelmaier.com