Amol,

If you are joints that much more prone to failure, don't rule out your process 
just because they aren't failing during the process.

Are these ENiG plated boards?  When you did your cross sections, did you do 
any analysis of the interfacial?  Are you getting P-enriched zones?  Are there 
voids - large or micro, in the joints or balls?  Are the soldermask openings and 
registration the same on both versions?  Do you inspect the paste deposition 
to make sure the stencil is releasing the lead free solder as it did with tin/lead?

While the fractures aren't showing up until you apply mechanical stress, the 
process itself could be weakening the joints, and that doesn't always show up 
on all of the balls, or even the same ones consistently.  Though pad cratering 
indicates that at least at those pads, the joint was stronger than the laminate.

Be careful using dicy cured epoxies with lead free process temperatures.  A 
whole 'nother can o' worms.

Pete

-------------------------------------------------------------------------------Leadfee Mail List provided as a service by IPC using LISTSERV 1.8d
To unsubscribe, send a message to [log in to unmask] with following text in
the BODY (NOT the subject field): SIGNOFF Leadfree
To temporarily stop/(start) delivery of Leadree for vacation breaks send: SET Leadfree NOMAIL/(MAIL)
Search previous postings 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
-------------------------------------------------------------------------------