Electromigration failure site and failure mechanism change

The case without a Cu column UBM is presented here first. Figure 2-3(a) illustrates the cross-sectional SEM image of a solder bump with a Ni UBM before EM

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tests. Figures 2-3(b) and 2-3(d) show the cross-sectional SEM images of solder bumps with Ni UBMs after upward and downward current stressing of 2.16 × 10⁴ A/cm² at 150 °C for 42.7 h, respectively. The current density was calculated based on the area of passivation opening on the chip side. Open failure occurred after 42.7 h of current stressing, and the damage in the Al trace between two solder bumps can be observed by IR microscopy. When the direction of electron flow was from the substrate side to the chip side as shown in Figure 2-3(b), some of the Cu metallization layer on the substrate side was consumed and reacted with solder to form Cu6Sn5 IMCs. A slight amount of Cu6Sn5 IMCs in the solder bump near the substrate side was aligned with the direction of electron flow because of electromigration effect in the solder joint. A crack formed along the chip/solder interface, which may be attributed to polishing during sample preparation for cross-sectional SEM observation. Additionally, some voids were observed in the Al trace on the chip side, as labeled in Figure 2-3(b). Some of voids were filled with Sn atoms. Figure 2-3(c) shows the EDS analysis for the location A in Figure 3(b), and the results indicated the element there was almost pure Sn. We will discuss this interesting point later. When the direction of electron flow was from the chip side to the substrate side as shown in Figure 2-3(d), the solder melted because of serious Joule heating effect. Furthermore, underfill layer was also damaged. However, the EM failure mode of the solder joints with Cu columns is quite

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different. Figure 2-4(a) shows the cross-sectional SEM image of a solder bump with a Cu column UBM before EM tests. Figures 2-4(b) and 2-4(c) show the cross-sectional SEM images of solder bumps with Cu column UBMs after upward and downward current stressing of 2.16 × 10⁴ A/cm² at 150 °C for 286.5 h, respectively. During this period of current stressing time, the total resistance, which includes two solder joints and Al trace, is raised by 50 mΩ from 145 mΩ to 195 mΩ. When the direction of electron flow was from the substrate side to the chip side as shown in figure 2-4(b), voids of about 84.7 μm long can be observed clearly at the interface of Cu6Sn5 IMCs and the solder on the substrate side. The voids in the middle of solder may be attributed to reflow process. This crack was the main reason of resistance increase during the failure. Similar to the results in the solder joint with a 2-μm Ni UBM in Figure 2-3(b), extensive dissolution of Cu layer on the substrate side took place, resulting in the massive formation of the Cu-Sn IMCs in the solder. Yet, there was no voids found in the substrate side in Figure 2-3(b). Ke et al. reported two failure mechanisms may occur in solder joints with Cu UBM: void formation and Cu dissolution [51]. It is not clear at this moment that why voids did not form in the solder joints in Figure 2-3(b).

When the direction of electron flow was from the chip side to the substrate side as shown in figure 2-4(c), the IMCs at the interface of the Ni layer and the solder

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became thinner whereas the IMCs at the interface of the solder and the Cu metallization layer on the substrate side became thicker because of the polarity effect.

Void of about 28 μm long was formed at the top-right-hand corner in the solder bump, and some Kirkendall voids can be observed at the interface of Cu3Sn IMCs and the Cu metallization layer on the substrate side. The Cu column and the Ni layer beneath remained intact within our experimental time frame.

2.4.2 Current crowding effect

Current crowding effect plays a crucial role in the electromigration failure of solder joints. Since the current density in the Al trace is typically one or two orders in magnitude larger than that in the solder, current crowding occurs at the entrance point of the Al trace into the solder joint. Figure 2-5 depicts the 3D simulation of the current density distribution in the solder bump with Ni UBMs. The applied current was 1.5 A from the chip side to the substrate side, which resulted in a current density of 1.54 × 10⁶ A/cm² in the Al trace. The calculated average current density was 2.16 × 10⁴ A/cm² based on the area of passivation opening on the chip side. However, due to the current crowding effect, the maximum current density inside the solder bump is as high as 7.8 × 10⁴ A/cm² as shown in Figure 2-5.

With a Cu column UBM, a more uniform distribution of current density was

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obtained in the solder bump as shown in Figures 2-6(a) and 2-6(b). Same as in Figure 2-5, the applied current was 1.5 A from the chip side to the substrate side. The maximum current density in the Cu column is 1.0 × 10⁶ A/cm². It can be seen that the current crowding effect still occurs in the Cu column UBM near the entrance of the Al trace into the solder joint. However, since Cu has a much higher melting point than the SnAg solder (Cu: 1084 °C; SnAg: 221 °C), there was little EM damage in Cu in the present stressing condition. The Cu column UBM is thick enough, therefore the solder is kept away from the current crowding region, and the maximum current density in the solder bump dramatically decreases to 4.0 × 10⁴ A/cm², which is only 51% of the maximum current density inside the solder bump with the 2-μm Ni UBM.

In other words, the current flow spreads out and becomes more uniform before reaching the solder bump when Cu column UBM is utilized. As a result, instead of locating on the chip side, the current crowding region in the solder bump with Cu column UBMs is actually located on the substrate side. This result agrees with the phenomenon observed in Figure 2-3(c) and Figure 2-4(b). The main electromigration-induced damage position in the solder bump moved from the chip side to the substrate side because of the existence of Cu column UBMs.

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2.4.3 Joule heating effect

The existence of Cu column UBMs also has influence on the Joule heating effect in the solder bump. Current stressing was carried out at the temperature of 100 °C on a hotplate. Figure 2-7 shows the temperature increases in solders with 2-μm Ni UBMs and Cu column UBMs during current stressing as a function of applied currents and current densities. Current stress was applied to solder joints with the current density in the range of 2.88 × 10³ A/cm² to 2.30 × 10⁴ A/cm² in the passivation opening. Figures 2-8(a) and 2-8(b) show the temperature distribution of the cross-sectioned solder bump with Ni UBMs and Cu column UBMs during current stressing of 2.30 × 10⁴ A/cm² at 100 °C, respectively. The temperature scale bars are shown at the bottom of figures. The average temperature increases were 17.9 °C and 14.4 °C in the solder bump with Ni UBMs and Cu column UBMs, respectively. In this study, the mean temperature was determined by averaging the values of 1000 pixels surrounding an approximately 65 × 40 μm² rectangle area in the center of the solder bump. Under the same current density, the alleviation of current crowding by the Cu column UBM consequently decreased the temperature rise in the solder bump. It is also known that Al trace under current stressing is a stable heating source, and the heat generated by the Joule heating of Al trace should always flow from a high temperature region toward a low temperature region. We have noticed, however, as shown in Figure 2-8(b)

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for the case with a Cu column UBM, that the temperature increase in the Cu column

UBM seems to be lower than that in the solder bump. The seemingly contradicting result actually comes from the difference between ―thermal gradients‖ and ―radiance

gradient.‖ Because Cu and solder’s surface characteristic are different, what was measured does not represent the real temperature difference between Cu column UBM and the solder joint. Nevertheless, in general, the temperature increase in various solder bumps can still been compared, since the emissivity of solder is consistent.

The Cu column also decreases the thermal gradient in the solder during high-current stressing. Figure 2-9(a) shows the temperature profile along the red line

ABin Figure 2-8(a). The temperature difference is 6.0 °C across the line and the calculated thermal gradient is 600 °C/cm. Here, the thermal gradient is defined here as the temperature difference between the two ends of the line divided by the length of the line. However, there is no obvious thermal gradient in the solder joint with the Cu column under the same stressing condition. Figure 2-9(b) presents the temperature profile along the red line CD in Figure 2-8(b). No obvious temperature difference is found across the line CD . Therefore, thermomigration may be relieved in the joints with Cu column.

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2.4.4 Mean time to failure

The solder joints with a Cu column UBM have a longer failure time than the

ones with a 2-μm Ni UBM, because the Cu column helps reduce current density in solder and Joule heating effect. As described by the Black’s equation in Eq. (2-1), the

MTTF can be prolonged when the current density and the temperature in solder can be reduced, provided the activation energy remains the same. In general, the value n in Eq. (2-1) is close to 2, which means the MTTF will double if the current density in solder reduces to half of its original value [33]. As shown in Figure 2-5, the maximum current density inside the solder bump with a 2-μm Ni UBM is as high as 7.8 × 10⁴ A/cm²; whereas the maximum current density in the solder bump with a 64-μm Cu column dramatically decreases to 4.0 × 10⁴ A/cm², which is only 51% of the maximum current density inside the solder bump with the 2-μm Ni UBM. Therefore, the lower current density renders a longer MTTF of the solder bump with the Cu column. In addition, the MTTF increase exponentially when the real temperature in solder joints decrease. The results in Figures 2-8 indicate that the Cu column reduces the Joule heating effect. Thus, the real temperature in the solder bump with the Cu column appears lower than that in the solder joint with a 2-μm Ni UBM, which also increases the MTTF of the solder joint with the Cu column. For the solder joints with 2-μm Ni UBMs, open failure took place at 42.7 h when they were subjected to a

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current density of 2.16 × 10⁴ A/cm² at 150 °C as shown in Figure 2-3. However, for the solder joint with the Cu column, the resistance increases only 50 mΩ after 286.5 h at the same stressing condition. The thick Cu column keeps the solder away from the region with a high current density. In addition, it possesses a high thermal conductivity, so the heat generated by Joule heating effect can be conducted away efficiently. Moreover, the thick Cu column also keeps the solder away from the hot-spot region [52], therefore, the temperature in the solder is reduced.

The voids in the Al trace in Figure 2-3(b) were caused by electromigration. It is reported that electromigration in the Al trace also occurs when the current density is larger than 1.0 × 10⁶ A/cm² at 150 °C [53,54]. In the present study, the current density is 1.54 × 10⁶ A/cm² in the Al trace and thus voids may form in the cathode end of the Al trace, as labeled in the figure. Once voids formed in the Al trace, the Sn atoms may be pushed into the voids by the upward electron flow.