Temperature Measurement of Solder Joints under Current Stressing

and thermal gradient were measured for the above two stressing conditions. Figure 22(a) and 22(b) show the temperature increase and the thermal gradient respectively when the package was stressed at 1 × 10⁴ A/cm² (0.59 ampere). The measured temperature increase in the solder was as high as 54.5 °C. In addition, the temperature increase due to Joule heating slightly depended on the testing temperature. It decreased approximately 5°C when the testing temperature increased from 70 °C to 100 °C. Therefore, although the testing temperature was maintained at 150 °C, the real temperature inside the solder might be over 200 °C. The thermal gradient under the stressing condition was measured to be 365 /cm℃ . In contrast, when the package was stressed at 5 × 10³ A/cm², the average temperature increase was detected to be 9.1 °C, and its thermal gradient became 127 /cm℃ .

4.3 3-D Current Density Simulation of Flip Chip Solder Joints

Fig. 23(a) shows the simulated 3-D current density distribution in the solder joint with the Ti/Cr-Cu/Cu thin film UBM. The majority of current crowds into the joint in a small volume near the Al trace, causing very high current density up to 3.32 × 10⁵ A/cm² in the UBM. Since the current density in the Al trace is as high as 1.11 × 10⁶ A/cm², and the resistance of the solder joint is only few mini-ohms, current crowding occurred at the entrance of the Al trace into the solder joint. However, once the current enters the solder joint, it drifts down vertically toward the substrate (Y-axis direction), and also spreads out laterally at the same time (X-axis and Z-axis directions). Thus, the solder close to the entrance carries high density of current. Figure 23(b) displays the current density distribution at the cross-section

^{- 22 -}

along the Z axis in Fig. 23(a). The maximum current density is as high as 1.24 × 10⁵ A/cm² at the solder near the entrance of the Al trace. On the substrate side, the current crowding phenomenon is much less due to a larger contact opening and a thicker Cu line. The average current density in the Cu line is 2.84 × 10⁴ A/cm², which is 34 times less than that in the Al trace. In order to investigate the 3-D current density distribution inside the solder joint, different cross-sections of the solder joint were examined. Figure 23(b) specifies the locations of the different cross-sections perpendicular to Y-axis. Cross-section Y1 is located inside the UBM layer, cross-section Y2 represents the IMC layer between the UBM and the solder, cross-section Y3 is located in the top layer of the solder joint connecting to the IMC, and cross-section Y4 is situated near the middle of the solder joint, which has the largest cross-section of 184.2 µm in diameter. Cross-section Y5 is situated between the middle and the bottom of the solder, which has a necking due to solder mask. Cross-section Y6 represents the bottom of the solder joint close to the Ni3Sn4 IMC in the substrate side.

The current density distributions at the six cross sections are shown in Fig. 24(a) through 24(f), respectively. Figure 24(a) shows the current density distribution in the UBM layer, in which the maximum current density reaches 3.32 × 10⁵ A/cm² near the entrance of the Al trace. However, the current density at the other end of UBM is only about 1 × 10³ A/cm². Current crowding occurred at the entrance of the Al trace. Fig. 24(b) shows current density distribution in the IMC layer. The distribution behaves similarly to that in the UBM layer. The maximum current density decreased slightly, and its value is 2.58 × 10⁵ A/cm² near the entrance of the Al trace. The current density distribution inside the solder near the UBM layer is shown in Fig. 24(c). The maximum current density of 1.24 × 10⁵ A/cm² occurred at the upper-left corner of the solder joint, which is near the entrance of the Al trace. This small volume of solder experiences about 25 times larger than the average value of this cross-section. Therefore, it is the most vulnerable site of the solder joint during current

^{- 23 -}

stressing, since the solder has a much lower melting point than the UBM materials and Al.

Solder may migrate away much easily from the volume and form voids at this site.[6] In addition, the gradient of the current density reaches approximately 5.40 × 10⁷ A/cm³ along the Z-axis direction at this cross-section.

The “crowding ratio” is denoted as the local maximum current density divided by the average current density on the UBM opening in this paper. The average current density on the UBM opening is 5.01 × 10³ A/cm² in the simulation models. The crowding ratio inside the Ti/Cr-Cu/Cu UBM is about 66.2, which means that the local current density is 66.2 times larger than the average one on the UBM opening. The maximum current density inside the solder is 1.24 × 10⁵ A/cm² and the corresponding crowding ratio is 24.7. The current density distribution at the middle cross-section Y4 is depicted in Fig. 24(d). The maximum current density at the middle cross-section is 4.07 × 10³ A/cm² and the corresponding crowding ratio is only 0.8, which is the lowest ratio in the solder joint due to its large cross-section.

It is interesting that the current density distribution at the cross-section Y5 is concave and it features a bowl-like shape, as shown in Fig. 24(e). It means that the current density on the peripheral region is larger than that in the inner region of the solder joint. This is attributed to the smaller diameter of the cross-section Y5, and that the current flowing in the vicinity of peripheral region above Y5 plane needs to be crowded into this smaller cross-section, leading to the bowl-like current density distribution. The maximum current density at this cross-section is 8.23 × 10³ A/cm² and the corresponding crowding ratio is 1.6 On the contrast, the current density distribution on the bottom of the solder joint appears convex again, as displayed in Fig. 24(f). The current density in the periphery of the joint is lower than that in the inner region of the cross-section Y6, because the current spreads out owing to a larger cross-section after passing the Y5 plane. After passing through the Y5 plane, the current drifts out of the solder joint into the Cu line in the substrate from the left-hand side in

^{- 24 -}

the figure. Thus the current density on the left-hand side is higher than that on the rest of the solder. Nevertheless, due to the thick Cu line, the crowding ratio is as low as 1.2.

Consequently, no obvious electromigration damage has ever been found in the substrate side of the solder joint. The maximum current densities and the corresponding crowding ratios for the above five cross-sections are listed in Table 6.

The effect of current crowding on the electromigration damage in the solder joint has been observed experimentally. When eutectic SnAg solder joints were stressed under the applied current of 0.567 ampere at 150 for 20 hours, they did not fail prior to ℃ microstructure observation on the chip side. Figure 25(a) shows the microstructure on the cathode/chip side after the current stressing, in which the solder was selectively etched away, and the position of the Al trace was depicted by the white dotted lines. The corresponding simulation on the current density distribution in the top layer of the SnAg solder is shown in Fig. 25(b). Although the solder used in the electromigration test is different from the SnPb solder, their current crowding behaviors are quite similar. The current flowed from the Al trace, passing through the UBM and the IMC layers, was then drifted into the solder joint. It showed that IMC/UBM dissolved much faster near the entrance of the Al trace into the solder joint, where the current crowding occurred seriously. In addition, the morphology of the damaged region matches with the shape of the high current density region, which indicates that higher current density caused more serious electromigration in the IMC/UBM layer. It is expected that the solder near this region would be migrated much easier than those in other regions, causing void formation there. Therefore, the current crowding effect plays critical role on the electromigration failure in solder joints.

Compared to the solder near the chip side, the current crowding is much less serious at the bottom of the solder. This is attributed to the thick conductive layers below the solder. The metallization layer in the substrate consists of 5 µm Ni and 25 µm Cu. Moreover, the

^{- 25 -}

cross-section of the Cu line is 25 µm × 80 µm, which is 42 times larger than that of the Al trace in the chip side. The current may drift through the IMC and the Ni layers, and keep drifting down and then flow out of the joint from the Cu line, resulting in a much lower maximum current density in the bottom of the solder than that on the chip side.

Figure 26 shows the crowding ratios at different cross-sections inside the solder for the five UBM structures. The maximum crowding ratios for the solder occurred at the Y3 cross-section, which is the top of the solder joint. They range from 7.2 to 24.7 for the five UBM structures. The joints with thin film UBM structures have higher crowding ratios.

However, the crowding ratios drop dramatically down to 1.5 ~ 2.0 at the middle cross-section, and they remain at low values of less than 3 in the substrate side.

^{- 26 -}