Hi Jean-François,
For understanding, you need to get IPC-SM-785 “Guidelines for Accelerated 
Reliability Testing of Surface Mount Solder Attachments.” 
For 'Design for Reliability' purposes you need IPC-D-279 “Design Guidelines 
for Reliable Surface Mount Technology Printed Board Assemblies.” 
For a standard look at IPC-9701 “Performance Test Methods and Qualification 
Requirements for Surface Mount Solder Attachments.” 
You say:
> And, more precisely, after an assembly
> comes back from the field and has to be retested, are the test performed
> just the same or previous "aging" should be taken in account and how?
> What kind of 'retesting' do you have in mind--and why?

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
E-mail: [log in to unmask], Website: www.engelmaier.com