No bites on this one yet so I'll give it a try.

I don't think anyone has got a good handle on this one yet, however the
solderability is much more robust than leaded components so the lack of
solderability spec hasn't cause major heartburn yet(?). This issue also
depends on whether you are using all eutectic balls or dual alloy structure
(CBGA).

With all eutectic solder balls, any oxidation (even very thick ones) will be 

broken up and displaced when the joint collapses during reflow, even if the
flux doesn't have enough activity to reduce all the oxide.  And obviously
none of the standard methods used today would work. And I don't think anyone 

is ready to use ellipsometry methods to measure sub-micron oxide thickness'
yet and I would have to guess the effectiveness would not be that great. The 

only risk is if a large non-wetting oxide particle is trapped at the Cu pad
interface which will act as a starting crack reducing the fatigue life of
the joint.

My past experience (at another employer) with dual alloy structures
indicates they are very robust also. When we were attempting to define
solderability specs, we tried using steam aging to produce poorly solderable 

parts. We had a very difficult time trying to produce unsolderable parts.
 At the time, we were looking a tensile pull methods for evaluating
solderability, but this proved to be an unreliable method due to the large
scatter inherent in the method and the lack of producing parts with known
poor solderability. (The tensile pull method we looked at mimicked the board 

attach process using WS-605 flux, pretty active stuff). We never came up
with a spec.

I did have one experience that was very bizarre which lead to a high temp
tensile pull method that allowed examination of the 90/10 ball , eutectic
interface without the use of metallurgical x-sectioning or C-SAM methods. We 

noticed a growth of contaminant on the solder balls that was a result of
tray outgassing (due to improper processing of the tray) and a catalytic
interaction of the ball surface. It took about 3 weeks to grow the material
to a point in which it cause solderability problems. The main problem was
poor wetting and outgassing which caused excessive voiding. If you solder
 to the ball using a tensile pull stud and pull at temps above 100C at low
strain rates, you can fracture at the ball / eutectic interface. One can
directly see the level of voiding in the eutectic part of the joint and can
see non-wetted parts of the solder ball.
I left this employer before this work was finished, but I don't think it
ever developed into a spec.

Don Foster
Symbios Logic
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