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February 2008

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Subject:
From:
"David D. Hillman" <[log in to unmask]>
Reply To:
TechNet E-Mail Forum <[log in to unmask]>, [log in to unmask]
Date:
Fri, 15 Feb 2008 15:29:05 -0600
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Hi Phil! One other potential root cause to check - how balanced is your 
pwb construction? Do you have enough warp in the pwb (during reflow)  in 
combination with the SAC305 solder "mushy" zone solidification reactions 
that is causing the opens? 

Dave Hillman
Rockwell Collins
[log in to unmask]




Phillip Bavaro <[log in to unmask]> 
Sent by: TechNet <[log in to unmask]>
02/15/2008 03:22 PM
Please respond to
TechNet E-Mail Forum <[log in to unmask]>; Please respond to
Phillip Bavaro <[log in to unmask]>


To
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Subject
Re: [TN] Samsung Memory BGA Failure






The balls are SAC305.   The process is LF and the BGA balls see 237-245 
for 
30 seconds minimum.  Ramps are less than 2.5 up and down.
The crack occurs catastrophically after exiting the reflow and appears to 
be the result of CTE expansion mismatch.

Accordingly I have some CTE questions:

If the BGA component body is expanding/contracting quicker than the PWB, 
stress would be expected at the component/ball interface: True or False?

If the PWB is expanding/contracting more than the BGA body, the cracking 
would be expected at the PWB/ball interface:  True of False?

The outside rows of balls solidify first as compared to the center balls, 
and have greater natural stress with respect to mechanical forces:  True 
of 
False?

I would not expect the stress of CTE to snap all of the balls across the 
component and this might explain why the chip doesn't fall off the 
assembly 
until someone "taps" it on a bench top.

Phil


At 04:24 PM 2/13/2008 +0200, Reuven Rokah wrote:

>  Hi Philip,
>
>The Samsung Memory BGAs are Lead free balls (is it?).
>
>Are you using Sn Pb process or lead free process?
>
>Reuven
>e mail: [log in to unmask]
>
>
>-----Original Message-----
>From: TechNet [mailto:[log in to unmask]] On Behalf Of Phillip Bavaro
>Sent: Wednesday, February 13, 2008 3:59 AM
>To: [log in to unmask]
>Subject: [TN] Samsung Memory BGA Failure
>
>I am wondering if anyone else out there has run into a problem with a
>Samsung Flash memory device (in this case, the 0.8 mm pitch, 107 pin
>sac305
>BGA) where the connections are cracking right at the interface between
>the
>pwb and ball.
>
>I had read about some Samsung BGA problems but cannot find the link to
>the
>information now.
>
>We have other BGA's on the same board without any issues.  Shear
>strength
>test results are very low as well (if the flash has not already popped
>off
>by itself).
>
>I'll try to get some pictures to Steve to put up tonight.  My current
>thoughts are CTE's are grossly mis-matched as we haven't found anything
>on
>the pads to account for the gross failure.
>
>Thanks in advance,
>
>Phil
>
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