Hi Ingemar,
And there I always thought Swedish women were different...
The reason solder balls are called solder balls is because that is what you
start out with on the component. What happens during reflow will depend on
other things, of course.
I have not heard anything special about Ni3Sn4 IMC layers--IMCs are all
brittle, but with some exceptions also strong. However, your crystal-analogy
may be applicable. Do you get fractures actually in the IMC? Typically
Ni-IMCs are too thin for observation. The NIST-researchers did some work with
IMCs, as did the Swedish Metal Research Institute--maybe we can get some
answers there.

Werner Engelmaier

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