Thank you Werner for saving my time :-) I was about to put down
something similar. :-) Just to add my 2 cents, liquid balls POTENTIALLY
could separate but warpage should be enormous, which would never happen
in real life.

Vladimir  

-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Werner Engelmaier
Sent: Thursday, November 16, 2006 11:29 AM
To: [log in to unmask]
Subject: Re: [TN] Separation in BGA joints

Hi Gaby
> http://alphametals.com/products/solder_solutions/pdf/pillowhead.pd
> 
This report is interesting froma number of perspectives.
The report states as the "Failure Mechanism:"
It appears most reasonable that the opens have actually been created
during
the second soldering operation. This can be easily confirmed by
comparing 
electrical
test results on some BGA's before and after the second soldering
operation. 
During this
second soldering operation, warpage has been experienced by the BGA
package 
due
to initial package conditions (amount and direction of 'set' and
moisture 
content), and
the thermal conditions of the soldering process (preheat and reflow 
temperatures and
times).

The extensive thermal environment during the second soldering operation
has
resulted in the liquification of the previously reflowed connections,
and 
along with the
extreme warpage in certain areas, particularly around the BGA package 
periphery, has
extended the now molten solder connections beyond their cohesive limits.
The
separations thereby created, along with the lack of active flux at these

locations, have
resulted in a skin of oxide on the two surfaces resulting from the 
separation. Upon
cooling, reduction of the warpage condition, and depending upon the
exact 
timing of
solidification, these two separate solder globules have made contact and
have 
resulted
in the various conditions observed during the microscopic investigation
of 
their
microstructures.

The observed separations have occurred where the generated oxide films
on
the two solder surfaces have not fractured upon reinitiation of contact
and 
allowing
them to rejoin into one continuous globule. Instead, the significant
oxide 
film has
created both a physical and electrical barrier as detected during ICT.

The observed deformation of the larger globule on the component side, is
the
result of the slightly faster solidification of the small solder globule
left 
on the
motherboard attachment site. Upon subsequent contact of the 'oxide
enveloped'
, and
'still molten' larger globule, deformation and deflection of this larger

globule takes
place as it is pressed against the now solid mass on motherboard
attachment 
site.

Where the oxide layers have broken, the two globules have merged to
varying
degrees again, forming a number of peculiar shapes as observed during 
microscopic
examination of the microstructures and as illustrated also in the
attached
photomicrographs.

Unfortunately, this is pure technical nonsense and physically
impossible.
There is no way that a second soldering operation   can cause the
observed 
problem-separating a properly formed solder joint into 2 globules, with
the same 
volumes as the initial BGA balls and the initial solder paste volume, no

less.
If solder joints are liquid, component warpage can not separate them,
because 
they will 'stretch' because of surface tension to more than double their

thickness without separation.
This shows that even very reputable companies, like Alpha Metals, cannot

control what some of their employees publish in their name, anonymous,
no less.

Werner

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