We did not do any mechanical testing (we strictly went with the final result). We did do ICT, JTAG, functional test and microsectioning however, to ensure that there were no cracks present that may cause latent defects. so I do not have exact values to answer your question. Application and cure can be/are controlled as critical variables giving us consistent results in production. 

-----Original Message-----
From: [log in to unmask] [mailto:[log in to unmask]] 
Sent: Friday, June 12, 2009 8:08 AM
To: [log in to unmask]; Kane, Amol (349)
Subject: RE: [TN] Corner ball pad cratering

How can you predict/measure the added strength the corner BGA bonding is
providing.   Application/curing are critical elements.

Victor,

-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Kane, Amol (349)
Sent: Friday, June 12, 2009 7:03 AM
To: [log in to unmask]
Subject: Re: [TN] Corner ball pad cratering

Genny,
We have experienced a very similar phenomenon on one of our product
boards. The BGA was 1.3" square, and there was pad cratering along the
corner pads. The laminate was a high Tg, with added filler to minimize
Z-axis CTE. Chaning the laminate was not an option, so we had to come up
with a process solution to the problem. We determined that the cratering
was not occurring during reflow cool down (but was later, at a
indeterminate process step during board assembly). We did a simple set
of expts with 3 random replications of each:

1. corner bonding prior to reflow (chipbonder 3609) 
2. corner bonding immediately after reflow
3. using a SMT pallet throuout the process (so as to eliminate handling
stresses)

We made sure that at least 1/4th length of each BGA side was covered by
the bonding material. In all 3 cases we could manufacture 100% defect
free product. Then based on the simplicity, corner bonding after reflow
was instituted as the preferred stress mitigation technique.

Might be worth a shot to do something similar.

Regards,
Amol


-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Genny Gibbard
Sent: Thursday, June 11, 2009 4:10 PM
To: [log in to unmask]
Subject: [TN] Corner ball pad cratering

Hello all,
If you have identified a BGA with corner ball pad cratering, what would
be your suggestions to correct this condition?
Our thoughts:
1.  We plan to evaluate our handling processes to reduce/remove
excessive flex/strain occurring around this part
2.  We plan to evaluate corner bonding or underfilling for the BGA.

Which do you feel would be the best direction to go?  Do you have other
suggestions?
Thanks in advance for all your help

Genny 

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