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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