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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