Earl

Well this really a very long sentence and I had to red it twice to get it. However, it sais a lot and nothing. Especially regarding the IMC layers.
The problem is, that IMC's are strong and brittle. This means that usually IMC's don't deform in thermal cycling. However, as in all brittle materials, a notch or crack propagates very fast through the material ( in fact almost instantaneously) due the stress concentration occurring at the tip of the crack. To do so a certain stress is needed. Note, degradation in solder is strain driven, in the IMC it is stress driven. But, a brittle material with a notch needs less load to crack than one without a notch.
The problem of the thickness of an IMC is quite tricky. That's why the IPC is somewhat wishy washy about that. 
- Firstly, the thicker the IMC the  more structured is the interface solder- IMC, this means the more notches you have. 
- Secondly, in the gap between a component and the pad you have a limited thickness of solder to accommodate the strain forced into the solder joint during a thermal cycle. Since stress and strain rate ( It's getting academic did you note it? ) are in a direct relationship in creep, this results in higher stress  the thinner this layer is. Considering the fact that IMC's are formed with the solder available one can say that  the thicker the IMC the thinner the remaining solder gap and thus the higher the stress occurring in thermal cycling.

However, who can say how much solder you have between your component and the pad. I feel, if 30um of solder between the IMC and the connector of the component is left no problem is to expected in an environment above 0 deg.C and temperature ramps below 4 deg.C per minute.
Regarding the thickness, well as I said before: I believe the IMC should be as thin as possible but there must be one because that's the alloying of Sn and Cu we need when soldering but I have no facts. Would be fun to look after it.

Have a great day

Guenter

EMPA
Swiss Federal Institute for Materials Testing and Research
Centre for Reliability
Dipl. Eng. Guenter Grossmann

8600 Duebendorf
Switzerland

Phone: xx41 1 823 4279
Fax :     xx41 1823 4054
mail:     [log in to unmask]

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