Three-dimensional simulation on current-density distribution in flip-chip solder joints under electric current stressing

Abstract

Three-dimensional simulations on current-density distribution in solder joints under electric current stressing were carried out by finite element method. Five underbump metallization (UBM) structures were simulated, including Ti∕Cr–Cu∕Cu thin-film UBM, Al∕Ni(V)∕Cu thin-film UBM, Cu thick-film UBM, Ni thick-film UBM, and Cu∕Ni thick-film UBM. The maximum current density inside the solder occurs in the vicinity of the entrance of the Al trace into the solder joint, while there is no obvious current crowding effect in the substrate side of the joint. The crowding ratio, which is defined as the maximum current density inside the solder divided by the average value in the UBM opening, is as high as 24.7 for the solder with the Ti∕Cr–Cu∕Cu UBM. However, it decreases to 23.4, 13.5, 8.7, and 7.2 for the rest of the UBM structures, respectively. Solder joints with thick UBMs were found to have a better ability to relieve the current crowding effect. The simulation results are in reasonable agreement with limited published data. The solder joints with higher current crowding ratios have a shorter electromigration failure time.

Effect of surface finish on the failure mechanisms of flip-chip solder joints under electromigration

“…This are was believed to have the highest local current density and the highest degree of current crowding initially. [2][3][4][5][6][7][8][9][10] Figure 5 is a zoom-in image for the left side of the void shown in Fig. 2b.…”

“…[1][2][3][4][5][6][7][8][9][10][11][12][13][14] To counter the negative effects caused by high current density, several approaches have been proposed and deployed. Two well-known approaches involve better circuit and interconnect designs.…”

Nonuniform and Negative Marker Displacements Induced by Current Crowding During Electromigration in Flip-Chip Sn-0.7Cu Solder Joints

“…However, the marker movement at the low-current-density region (nos. [1][2][3][4][5] was in the opposite direction, against the electron flow. This interesting phenomenon will be discussed later.…”

“…By using the finite-element method to simulate current distribution, it was found that the current density in the currentcrowding area can be one order of magnitude higher than the average density in the solder bump. 3,4 The effect of current crowding on pancake-type void formation at the cathode and whisker growth at the anode of solder joints has been reported. [5][6][7] However, the effect of nonuniform distribution of electric current on diffusion in the bulk of the solder bump is unclear, and no direct measurement of the rate of electromigration in the current-crowding region versus that in the rest of the solder bump has been reported.…”

Three-Dimensional Thermoelectrical Simulation in Flip-Chip Solder Joints with Thick Underbump Metallizations during Accelerated Electromigration Testing

“…For the solder joint with Al/ Ni(V)/Cu thin-film UBM, the maximum current density is 1.11 · 10 5 A/cm 2 and the maximum temperature is 98.8°C in the solder, as reported in our previous publications. 14,20 For the solder joint with the 5-lm Cu UBM, the maximum current density was 4.37 · 10 4 A/cm 2 and the hot-spot temperature was 89.5°C in the solder. The MTTF would be 3.3 times longer than that with the Al/Ni(V)/Cu UBM under 0.567 A at 70°C.…”