UBM thickness

For this study, a 3D finite-elements method was employed to simulate the current-density and temperature distributions in the solder joints with 0.5-μm, 5-μm, 25-μm, 50-μm and 100-μm Cu UBMs.

With a thicker Cu UBM, more uniform distribution of current density was obtained in the solder bumps. Figures 5-12 (a) through (e) show the current-density distribution in the solder joints with 0.5-μm, 5-μm, 25-μm, 50-μm and 100-μm Cu UBMs, respectively, when applied by 0.6 A. It can be seen that the current crowding effect still occurs in the thick Cu UBM near the entrance of Al trace into the solder joints. However, as the thickness of Cu UBM increases, the solder is kept away from the crowding region. When Cu UBM is thicker than 50 μm, the current crowding occurs mostly in Cu UBM, and the maximum current density in solder decreases dramatically. The crowding ratio in this paper is denoted as the maximum current density in the solder divided by the average value in UBM opening, which is 5.01 × 10³ A/cm². It is 19.0, 9.6, 2.9, 1.7, and 1.6 for the solder joints with 0.5-μm, 5-μm, 25-μm, 50-μm, and 100-μm Cu UBMs, respectively. We conclude that thick Cu UBM results in uniform current-density distribution and reduced maximum current density.

In short, the current flow spreads out more uniform before reaching the solder bumps with a thicker Cu UBM.

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In addition, thick Cu UBM can relieve the hot-spot issue in solder bumps. Figures 5-13 (a) and (b) show the Joule heating effect in Al trace for the solder joints with 0.5-μm and 100-μm Cu UBMs, respectively. It was found that the overall Joule heating effect in the stressing circuit did not reduce when 0.5-μm Cu UBM was replaced by 100-μm UBM. The total resistance for the circuit was about was 1330 mΩ, while the resistance decreased due to thicker Cu column was only in milli-ohm range. Thus, both models are almost the same overall Joule heating effect in Al trace. Nevertheless, Joule heating effect in solder bumps was quite different. Figure 5-14 (a) through (e) show the tile-views for the temperature distribution in the solder joints with 0.5-μm, 5-μm, 25-μm, 50-μm and 100-μm Cu UBMs, respectively, when applied by 0.6 A. For clear view of the hot spot, Cu UBMs are not shown in these figures. The top surfaces of these bumps represent the solder connecting to Cu UBMs. Hot spots exist in the solder joints with thin Cu UBMs. However, it was found that with a Cu UBM greater than 50 μm, the hot spot could be almost eliminated completely. Figure 5-15 (a) through (e) show the corresponding cross-sectional views for the temperature distribution. It is clear that the hot-spot was almost eliminated for the solder joints with 50 μm and 100 μm Cu column. The temperature difference between the hot spot and the average values is 4.5 °C, 2.5 °C, 0.7 °C, 0.3 °C, 0.1 °C for the solder joints with 0.5-μm, 5-μm, 25-μm, 50-μm and 100-μm Cu UBMs, respectively, when applied by 0.6 A. The

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difference between the hot spot and the average temperature increased as the applied current increased. Figure 5-16 (a) through (c) shows the hot spot and average temperatures as a function of applied current up to 0.6 A for the solder joints with the 25-μm, 50-μm and 100-μm Cu columns. No obvious hot spot was found after Cu column was thicker than 50 μm.

Although thick Cu UBM can relieve the hot spot, the overall Joule heating remains unchanged even for the solder joints with 100-μm Cu UBM. Figure 5-17 (a) depicts the hot-spot temperature as a function of applied current for the five models.

Compared with the solder joints with 0.5-μm-thick Cu UBM, 100-μm-thick Cu reduce the hot-spot temperature by 5.0 °C. However, the overall Joule heating effect did not change much, as illustrated in Figure 5-17 (b). It can be observed that the average temperature in solder does not decrease significantly even when Cu UBM was as thick as 100 μm. The insensitivity to Cu UBM thickness is because the primary heating source is Al trace. In these simulation models, the total resistance for the stressing circuit is about 1330 mΩ. The bump resistances are 6.1, 4.4, 3.3, 3.1 and 2.7 mΩ for the five models, respectively. Therefore, the reductions in bump resistance due to thicker Cu UBMs are negligible compared to the total resistance. Although the solder was kept away from the heating source for 100 μm Cu column, Cu is a superb heat conductor, which is expected to facilitate heat conduction. Thus, the average

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temperatures in solder for the five models were quite close. Furthermore, with thicker Cu UBM, thermal gradient is reduced considerably. The thermal gradient in this letter is determined from temperature difference between the top and bottom solder divided by the height of the solder bumps. As shown in Figure 8, the gradient reduced from 400 °C/cm to 60 °C/cm when Cu UBM is increased from 0.5 μm to 100 μm. Thus, the thermomigration in solder would be inhibited with thicker Cu UBM [37].

The elimination of the hot spot for solder joints with thick Cu UBM may be attributed to the absence of the serious current crowding since there is no serious local Joule heating for these joints. The local Joule heating power is proportional to the square of the local current density. For the above five models, the overall Joule heating were quite close. Yet, the crowding ratios for the five models are 19.0, 9.6, 2.9, 1.7 and 1.6. It is expected that the local Joule heating power in the hot spot for the bump with 100-μm Cu column is 140 times less than that of the bump with 100-μm. Therefore, the hot-spot issue could be relieved significantly in solder bumps with thick Cu columns due to reduced current crowding effect.

Furthermore, the effect of the thickness of Cu UBM on MTTF could be estimated using the equation for solder joints. Table 5-3 summarizes the maximum current density, hot-spot temperature and the ratio of estimated MTTF for the five models in this letter. Compared with the solder joint with 0.5-μm Cu UBM, MTTF for the solder

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joints with 5-μm, 25-μm, 50-μm and 100-μm Cu UBM exhibit a longer EM lifetime of 1.8, 4.6, 6.7, 7.3 times, respectively. Therefore, the solder joints with thicker Cu UBMs are likely to demonstrate better EM resistance due to lesser current crowding effect and lower hot-spot temperature. In addition, when Cu thickness is increased from 50 to 100 μm, there is no obvious increase in MTTF since there are negligible current crowding Joule heating effects when Cu UBM was thicker than 50 μm. Consequently, further thickening in Cu UBM is not expected to render longer EM lifetime.

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Figure 5-12: Current-density distribution in the solder joints with (a) 0.5-μm. (b) 5-μm. (c) 25-μm. (d) 50-μm. (e) 100-μm Cu UBM when applied by 0.6A.

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Figure 5-13: Joule heating effect in Al trace for the solder joints with (a) 0.5-μm UBM. (b) 100-μm Cu column.

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Figure 5-14: The temperature distribution in the solder bumps with (a) 0.5-μm. (b) 5-μm. (c) 25-μm. (d) 50-μm. (e) 100-μm Cu UBM when applied by 0.6 A at 100 °C.

Only solder bump was shown.

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Figure 5-15: The cross-sectional view for the temperature distribution in the solder bumps with (a) 0.5-μm. (b) 5-μm. (c) 25-μm. (d) 50-μm. (e) 100-μm Cu UBM when applied by 0.6 A at 100 °C. Only solder bumps were shown.