I have encounted an interesting problem during an experiment intended to
compare 95Pb/5Sn with eutectic bumping for a 20x25mm die:

Situation 1:
95Pb/5Sn solder bumping, UBM is Ni(2.5um)/Cu(0.2um)/Ti(0.2um)
Ti and Cu layer were sputtered. Ni was plated.

Situation 2:
63Sn/37Pb bumping, UBM is Ni(2.5um)/Ni(0.2um)/Ti(0.2um)
Ti and Ni were sputtered. The outlayer of Ni was plated.

Both wafers went through almost the same thermal process during bumping.
High lead bumping had one more 360 C de-oxidation process. Then cutter
peel test was done on both wafers. Interesting results came up:

For situtaion 1, most of bumps were sheared off through intermetallics,
leaving mostly Ni surface

For situtaion 2, all bumps were sheared off with surface covered by
solder.

Could anyone give me an explanation, especially from metallury and
intermetallic point of view?

Thanks,


Yuan Li
Research Associate

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