Hi Pradeep,
That answer is easy; zero % of your total production should pass through such 
a stringent test. These tests, even when passed, consume at least 50 to 70% 
of the love of the PCB in terms of via reliability.
These tests should be done on coupons to prove that the material in the PCB 
lay-up can survive assembly. These coupons need to include PTHs/vias with the 
smallest and largest drilled hole diameters on your PCB. The reason for this is 
that while the PCB base material may thermal degrade to cohesive 
delaminations, the functional failure indication is most likely a PTH/via interconnect 
failure.
Tg is only one of 3 critical directly measurable base material properties 
together with delamination temperature Td(5%) [Td(2%) is actually a better 
measure of degradation onset] and thermal expansion typically given as %TE from 50 
to 260°C. I have combined these material properties into a Soldering T
emperature Impact Index (STII) for easier preliminary material selection, but the 
proof-of-the-pudding is in testing coupons with tests like cT288.
By the way, you do not need etchback for successful smear removal—too much 
positive etchback creates a stress concentration geometry paretically dangerous 
for the high lead-free soldering temperatures.
I have a lengthy discussion of this in my White Paper on specifying PCBs for 
lead-free assembly. I send you the ordering information if you like.



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, Cell: 386-316-5904
E-mail: [log in to unmask], Website: www.engelmaier.com

---------------------------------------------------
Technet Mail List provided as a service by IPC using LISTSERV 1.8e
To unsubscribe, send a message to [log in to unmask] with following text in
the BODY (NOT the subject field): SIGNOFF Technet
To temporarily halt or (re-start) delivery of Technet send e-mail to [log in to unmask]: SET Technet NOMAIL or (MAIL)
To receive ONE mailing per day of all the posts: send e-mail to [log in to unmask]: SET Technet Digest
Search the archives of previous posts at: http://listserv.ipc.org/archives
Please visit IPC web site http://www.ipc.org/contentpage.asp?Pageid=4.3.16 for additional information, or contact Keach Sasamori at [log in to unmask] or 847-615-7100 ext.2815
-----------------------------------------------------