3-D simulation on current density distribution in flip-chip solder joints with thick Cu UBM under current stressing

Abstract

In flip-chip solder joints, Cu has been used as a underbump metallization (UBM) for its excellent wettability with solders. In addition, electromigration has become an crucial reliability concerns for fine-pitch flip chip solder joints. In this paper, 3-D finite element method was employed to simulate the current density distribution for the eutectic SnPb solder joints with 5 µm, 10 µm, and 20 µm thick Cu UBM. It was found that the thicker the UBM is, the lower the maximum current density inside the solder. The maximum current density decreased from 4.37 × 10 4 A/cm 2 to 7.54 × 10 3 A/cm 2 when the thickness of the UBM changed from 5 µm to 20 µm. Thicker Cu UBM can effectively relieve the current crowding effect inside the solder.

Flip chip back end design parameters to reduce bump electromigration

“…This agrees with the result reported by Liang et al [13] and Su et al [16]. As mentioned earlier, variation in current density with respect to PO diameter and UBM diameter is insignificant.…”

“…Liang et al [13] studied effect of 5 µm, 10 µm and 20 µm thick copper UBM on the current density distribution for the Sn63Pb37 solder joint. They used 3D Finite Element Analysis (FEA) for their analysis.…”

Finite element modeling on electromigration of solder joints in wafer level packages

“…The current density at the corner is approximately one order of the magnitude higher than the average current density in solder bumps. The third bump in Table 1 Electromigration basic parameters [3,[9][10][11][12][13][14].…”

“…The electromigration parameters for SnAgCu solder bump are listed in Table 1 [3,[9][10][11][12][13][14] where E A is activation energy, Z Ã is effective charge number, D 0 is self diffusion-coefficient, Q Ã is heat of transport, q 0 is initial electrical resistivity, a is temperature coefficient resistance, X is atomic volume, Boltzmann constant K B is 1.380662eÀ23, the electron charge e is 1.60219eÀ19, and the room temperature T 0 is 303.…”

Some remarks on finite element modeling of electromigration in solder joints

“…The epoxy, silicon chip and PCB in this model do not conduct electricity, thus very high resistance values are assigned to these materials. The electromigration parameters used in calculating the atomic fluxes by Equations 5-11 for SnAgCu solder bump are listed in Table 1 [10][11][12][13][14][15][16]. …”