Hi Dave.  eeh, FA? No, they wanted to solder to the bottom package after
separating the two parts.

The solder was SAC 305 to NiAu. I have never considered cutting as a method
of separating parts. So, I've never seen the result. I recognized the
non-solderable result but was surprised by the uniformity of the those few
that fractured right along the package. I doubted that it had ever been
properly soldered. 

It seems, from the reaction of the group, that some of the connections might
well fracture along the intermetallic leaving an non-solderable surface. FA
may be in order. But, I think we'll try desoldering before we cross-section.
We have SEM, but we are separated by 1500 miles, and a level of fear. The
project involves some pretty sensitive new product. 

If the reflowed solder connections look normal we'll assume the slicing
procedure caused the condition. 

Thanks guys.  



-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of David D. Hillman
Sent: Friday, November 20, 2009 4:13 PM
To: [log in to unmask]
Subject: Re: [TN] solder fracture solderable?

Hi Guy! One of the first rules of failure analysis is to not contribute 
any additional damage to the article under assessment or to understand 
what type of damage contribution could be contributed if something is 
unavoidable. The cutting procedure you describe most certainly contributes 
additional damage to the BGA which you would not be able to differentiate 
from damage already in place. I do not believe the knife procedure you 
described is a good practice to use.

Dave Hillman
Rockwell Collins
[log in to unmask]




Guy Ramsey <[log in to unmask]> 
Sent by: TechNet <[log in to unmask]>
11/20/2009 02:28 PM
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TechNet E-Mail Forum <[log in to unmask]>; Please respond to
Guy Ramsey <[log in to unmask]>


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Subject
[TN] solder fracture solderable?

I was visiting a customer earlier this week. They were attempting to cut a
the top part off of a package  on package, BGA on top of BGA. 

They didn't want to reflow it off because they didn't want to damage the
balls on the bottom of the bottom package. 

They had made a little fixture affair with two knives adjusted to slice
between the two packages. 

They placed the PoP in the tool and pushed it through the knives, rotated 
90
degrees and repeated the cycle. 

Most of the pads on top of the bottom package ended up with half a ball
remaining on the pad.

But some of the pads did not have any solder on them at all. The entire 
pad
was mat finish silver in color. 

None of these features would  accept solder. The number of pads without
solder varied from one or two to 30% of the pads.

 

I wondered if rather than cutting through, some of the connections has
simply fractured along the intermetallic, leaving a non-solderable surface
with this uniform appearance. 

Some of those observing conjectured that the package was bad, They
conjectured that the silver was merely solder loaded with gold and that 
the
nickel had never accepted solder. 

I asked where their SEM was and if they had EDAX equipment . . .  nope.

Would anyone care to guess  along?  How likely is it that prying off a BGA
would fracture the connection right along the intermetallic and leave a
completely non-solderable surface? 

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