Experience something similar before. It is due to PCB design which
caused thermal stress at certain location's. 

-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Wenger, George M.
Sent: Friday, 1 July 2011 9:30 AM
To: [log in to unmask]
Subject: Re: [TN] BGA ball dewet from substrate

Julie,

I don't believe that the component sagging has anything to do with the
"dewetting".  If this is only happening with one vendor's component then
I would be very suspicious that the electrolytic Ni/Au surface finish on
the substrate has problems.  The electrolytic Ni on some lands may not
have been properly cleaned or protected by the Au over plating allowing
passivation of the Ni especially around the perimeter of the land
resulting in a solder attachment but poor wetting.  During the second
reflow the Ni at the edges of the poorly wet land oxidizes further and
the solder "dewets" from the land.  

Regards,
George
George M. Wenger
Senior Principal Reliability / FMA Engineer
Andrew Corporation - Wireless Network Solutions
40 Technology Drive, Warren, NJ 07059
(908) 546-4531 Office (732) 309-8964 Mobile
E-mail: [log in to unmask]


-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Julie Silk
Sent: Thursday, June 30, 2011 8:57 PM
To: [log in to unmask]
Subject: Re: [TN] BGA ball dewet from substrate

I agree it's very strange.  I don't have cross-sections after the first
reflow yet.  The raw parts show very nice joints between ball and
substrate.  The faulty balls are mostly at the interior rows (empty
center, no balls there).  It is one part, one supplier -- but nothing
fishy on the raw part.  Looks like electrolytic Ni/Au on the substrate.
Steve Gregory is posting a picture.  Could the upside-down part sag so
much in the middle during reflow that it pulls away??

--------------------------------
Julie Silk
Environmental Compliance Technical Program Manager
Agilent Technologies
707-577-4296
[log in to unmask]


-----Original Message-----
From: [log in to unmask] [mailto:[log in to unmask]] 
Sent: Thursday, June 30, 2011 3:49 PM
To: TechNet E-Mail Forum; SILK,JULIE (A-Sonoma,ex1)
Subject: RE: [TN] BGA ball dewet from substrate

Is this random with respect to location?  Is it particular to one
part/supplier?

It seems very strange that a properly reflowed BGA solderball connection
would "dewet" itself during a subsequent reflow operation.

A cross section after the first reflow will probably explain what
happened.




-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Julie Silk
Sent: Thursday, June 30, 2011 3:29 PM
To: [log in to unmask]
Subject: Re: [TN] BGA ball dewet from substrate

The substrate pad shows an intermetallic layer, demonstrating that there
had been a metallurgical bond.  Also, the surface of both pad and ball
are smooth -- not a fracture, but as if reflowed.  I have a good picture
from the FA report to share once I figure out how.  I believe that if
you were to analyze the very first layer on the BGA substrate pad, it
would have a layer of solder.

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