Joule heating effect in solder joints was investigated using thermal infrared microscopy and modeling in this study. With the increase of applied current, the temperature increased rapidly due to Joule heating. Furthermore, modeling results indicated that a hot spot existed in the solder near the entrance point of the Al trace, and it became more pronounced as the applied current increased. The temperature difference between the hot spot and the solder was as large as 9.4°C when the solder joint was powered by 0.8A. This hot spot may play an important role in the initial void formation during electromigration.

The electromigration behavior of SnAg3.5 solder bumps is investigated under the current densities of 1 ϫ 10 4 A/cm 2 and 5 ϫ 10 3 A/cm 2 at 150°C. Different failure modes were observed for the two stressing conditions. When stressed at 1 ϫ 10 4 A/cm 2 , damage occurred in both the anode/chip side and the cathode/chip side. However, failure happened only in the cathode/chip side under the stressing of 5 ϫ 10 3 A/cm 2. A three-dimensional simulation of the current-density distribution was performed to provide a better understanding of the current-crowding behavior in the solder bump. The current-crowding effect was found to account for the failure in the cathode/chip side. In addition, both the temperature increase and the thermal gradients were measured during the two stressing conditions. The measured temperature increase due to Joule heating was as high as 54.5°C, and the thermal gradient reached 365°C/cm when stressed by 1 ϫ 10 4 A/cm 2. This induced thermal gradient may cause atoms to migrate from the chip side to the substrate side, contributing to the failure in the anode/chip side. Moreover, the formation of intermetallic compounds in the anode/chip side may also be responsible for the failure in the anode/chip side.

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 three-dimensional current distribution on void formation in flip-chip solder joints during electromigration was investigated using thermoelectrical coupled modeling, in which the current and temperature redistributions were coupled and simulated at different stages of void growth. Simulation results show that a thin underbump metallization of low resistance in the periphery of the solder joint can serve as a conducting path, leading to void propagation in the periphery of the low current density region. In addition, the temperature of the solder did not rise significantly until 95% of the contact opening was eclipsed by the propagating void.

No 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.

While the dimension of solder bumps keeps shrinking to meet higher performance requirements, the formation of interfacial compounds may be affected more profoundly by the other side of metallization layer due to a smaller bump height. In this study, cross interactions on the formation of intermetallic compounds (IMCs) were investigated in eutectic SnPb, SnAg3.5, SnAg3.8Cu0.7, and SnSb5 solders jointed to Cu/Cr–Cu/Ti on the chip side and Au/Ni metallization on the substrate side. It is found that the Cu atoms on the chip side diffused to the substrate side to form (Cux,Ni1−x)6Sn5 or (Niy,Cu1−y)3Sn4 for the four solders during the reflow for joining flip chip packages. For the SnPb solder, Au atoms were observed on the chip side after the reflow, yet few Ni atoms were detected on the chip side. In addition, for SnAg3.5 and SnSn5 solders, the Ni atoms on the substrate side migrated to the chip side during the reflow to change binary Cu6Sn5 into ternary (Cux,Ni1−x)6Sn5 IMCs, in which the Ni weighed approximately 21%. Furthermore, it is intriguing that no Ni atoms were detected on the chip side of the SnAg3.8Cu0.7 joint. The possible driving forces responsible for the diffusion of Au, Ni, and Cu atoms are discussed in this paper.

Three-dimensional simulations for relieving the current crowding effect in solder joints under current stressing were carried out using the finite element method. Three possible approaches were examined in this study, including varying the size of the passivation opening, increasing the thickness of Cu underbump metallization (UBM), and adopting or inserting a thin highly resistive UBM layer. It was found that the current crowding effect in the solder bump could be successfully relieved with the thick Cu UBM or with the highly resistive UBM. Compared to the solder joint with Al/Ni(V)/Cu UBM, for instance, the maximum current density in a solder bump decreased dramatically by a factor of fifteen, say from 1.11 × 10 5 A/cm 2 to 7.54 × 10 3 A/cm 2 when a 20-m-thick Cu UBM was used. It could be lowered by a factor of seven, say to 1.55 × 10 4 A/cm 2 , when a 0.7-m UBM of 14770 ⍀ cm was adopted. It is worth noting that although a resistive UBM layer was used, the penalty on overall resistance increase was negligible because the total resistance was dominated by the Al trace instead of the solder bump. Thermal simulation showed that the average temperature increase due to Joule heating effect was only 2.8°C when the solder joints with UBM of 14770 ⍀ cm were applied by 0.2 A.