Hi, Pete
Kid gloves are not ESD safe. I agree with most of your other remarks.
What do you mean by "BGA joints are especially weak in the first few
hours after thermal cycles.  Perhaps let them sit at room temperature
for a day before secondary operations." What are you expecting to happen
during that time? There is no "cure schedule" for solder after it
reaches solidus. Either you have a good solder joint or you don't, and
letting the board sit for 24 hours does nothing.

 

-----Original Message-----
From: Leadfree [mailto:[log in to unmask]] On Behalf Of Pete Houwen
Sent: Tuesday, June 24, 2008 9:25 AM
To: [log in to unmask]
Subject: Re: [LF] [TN] SN100 for Reflow Application

Amol,

Your explanation of the failures is not very clear.  You do need to get
a better idea of your failure modes.  

Soldering BGAs at lead free temperatures, without the benefit of Pb
makes it more difficult to create strong joints and introduces a number
of additional potential failures.  That's just a fact of life that we're
all learning to deal with.  
But you have a few other issues that can be addressed rather quickly and
easily easily, to at least mitigate the resulting failures.  We just
went through a similar situation at our plant.  BGAs in the corners of a
board that were cracking.  You say you need to handle the boards with
kid gloves?  Then give the operators kid gloves!  Take a walk with the
boards, find any points in the process where the boards are mechanically
stressed.  Use flat trays instead of edge racks.  Add support pins in
the screen printer.  Put them in stiffening fixtures for handling and
soldering.  Watch depanelization - if they are not using a cutter
properly, that can be an extreme amount of high strain rate stress (we
had operators breaking panels over their knee!).  How are boards handled
when they are still hot coming out of the ovens?  BGA joints are
especially weak in the first few hours after thermal cycles.  Perhaps
let them sit at room temperature for a day before secondary operations.
Are there press fit PTH parts being installed? Watch PTH lead grinding,
ICT fixture pressure (is it even?), any functional test stresses.

This isn't going to fix your soldering problem.  It will buy you time to
get a better idea of what that problem is.  If you are getting laminate
cracks, the joints are good but the boards are being overstressed, so
you had better address that anyway.  We had been getting some IMC
failures, some laminate cracks.  So we are still working on our
soldering (since we do have a field failure risk) but we have greatly
improved yields by not breaking the boards when building them.

Pete

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