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Date: | Sat, 16 Dec 2006 12:38:50 EST |
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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
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