Study of Thermal Aging Effects on Intermetallic Growth Mechanisms and Shear Strength Behavior of Wafer-Level Solder Bumps

The effect of wafer-level solder bump size on solder bump shear strength and intermetallic growth as a function of time, at different storage temperatures is studied, using solder bumps of two different heights: 95 mum (SB95) and 142 mum (SB142). Smaller solder bumps (SB95) lead to a faster decrease in bump shear strength. Increasing aging temperature and time brings about an increase in the final fracture area of the solder, from a ductile to a quasi-brittle failure mode. It is observed that the smaller solder bump (SB95) exhibits a faster rate of increase in final fracture area, than the bigger bump. A linear decrease of solder bump shear strength with intermetallic thickness is established for the systems studied. Degradation of solder bump shear strength is found to be affected by the relative volume of the solder bump with respect to Ni/Au, such that a lower solder bump to Ni/Au volume ratio leads to faster shear strength degradation, due to the consumption of Cu in the bulk solder bump and the formation of brittle intermetallics. The overall intermetallic growth activation energies for the two solder bump sizes are found to be comparable, at 0.85- 0.86 eV. For this study, (Cu,Ni)₆ Sn₅, is initially formed, at the solder bump-to-under bump metallization (UBM) interface, followed by the formation of (Ni,Cu)₃ Sn₄, between the UBM and the (Cu,Ni)₆ Sn₅ intermetallic layer.