Hi Kerry,
The direct cause is a separation force too large for the adhsion present.
Now, there are three underlying reasons that could be individually 
responsible or acting in combination: (1) low quality, (2) bad design/material choices, 
and (3) loads higher than normal [whatever normal is].
(1) Most of the responses so far have dealt quite adequately with 'low 
quality' issues.
(2) You have indicated FR-4 with Tg=170 [which is good if measured by TMA], 
but you say nothing about CTE's and Td—which are very important in this 
context. You indicated that you saw thin Cu barrel plating—that will cause excessive 
expansion of the resin into the hole space causing plastic deformation of the 
Cu plating—on cooling, it simply cannot come back to its former geometry. It 
will depend on how thin the Cu actually is and whether or not you have removed 
non-functional lands [You may want to check out my Reliability Column in 
Global SMT & Packaging magazine, August 2006 issue]. You are actually lucky that 
you did not see barrel cracking under the circumstances [depends on drilled hole 
diameter, of course]. I am dealing with many of these issues in my recent 
White Paper on the changes needed for PCB FAB notes and specifications for 
lead-free soldering [I will send the nfo by separate mail]
(3) Higher loads are certainly being generated with the higher lead-free 
soldering temperatures. You also could get higher loads with entraped moisture 
creating excessive internal vapor pressure. 

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

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