Multiple driving forces may trigger migration of atoms in the SnAg joints during current stressing. According to electromigration theory, voids form at the cathode and extrusions or hillocks accumulate at the anode. Thus, open failure occurs at the cathode side. However, in SnAg joints, open failure happens not only in the cathode end, but also in the anode side, which implies that other failure mechanisms may take place during current stressing. During current stressing in flip chip solder joints, the following two driving forces may operate due to their unique geometry and composition, and thus influence their electromigration behavior.
5.1 Electrical: effect of current crowding on electromigration
Current crowding plays a critical role in the electromigration failure of the solder joint, since the current density in the Al trace is typically one or two orders in magnitude larger than that in solder.[15] Current crowding occurs in the junction of the Al trace and the solder bump. However, current density distribution in flip-chip joints is a three dimensional issue.
Figure 23 depicts the current density distribution by simulation of the solder joint. The 3-D tilt view of the current density distribution is shown in Fig. 23(a), in which 0.567ampere was applied. The corresponding current density in the Al trace is 1.11 x 106 A/cm2, and the calculated average current density in the contact opening of the chip side is 1.0 × 104 A/cm2. However, current crowding occurs at the entrance point of Al trace into the solder bump.
The maximum current density is as high as 1.24 × 105 A/cm2, as seen in the cross-sectional view along the Z axis in Fig. 23(b). On the substrate side, the current crowding phenomenon is much less due to a larger contact opening. Figure 23(a) illustrates the current density distribution on the contact opening in the chip side. The red area close to the entrance into the solder bump represents the region with high current density, and which is the most vulnerable area in the joint during electromigration testing.
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Void formation caused by current crowding can be observed in Fig. 15(a), in which the electron flow entered the bump from the right-back side. Significant flux divergence occurred in this area, causing void formation in the vicinity of the entrance point. In addition, the IMC formation in Fig. 17(b) also demonstrates the current-crowding effect on electromigration. The electrons entered the bump from the location of the root of the IMCs situated on the chip side, going vertically to the paper plane toward the substrate, and then flowed downward to the Cu line. Thus the growth direction of the IMCs matches the current crowding path. This path can be seen in Fig. 23 and indicates that it is from the top-left corner downward to the bottom-left corner. Thus current crowding plays a key role in the damage formation in cathode/chip end.
5.2 Thermal: effect of Joule heating on electromigration
During current stressing in flip chip package, Joule heating may cause temperature increase and thermal gradient. The former raises the testing temperature higher than the ambient temperature, thus cause higher diffusion rate for the materials in the bumps. The later produces another driving force for atomic migration, which may trigger a failure mechanism other than current-crowding induced failure. As shown in Fig. 20(a), the temperature increase when stressed at 1 × 104 A/cm2 was as high as 54.5 °C, which means that the real testing temperature may be over 200°C. Thus, formation of many IMCs during the current stressing occurred on the anode/chip side, as seen in Fig. 17(a). Furthermore, prior to failure, the temperature in the joint may exceed its melting point, causing local melting or the melting of the whole joint. It is inferred that the dendrite IMCs in Fig. 17(b) might be formed in the liquid state, since dendrite growth occurs only in the liquid state.
Hence, it is worthy to note that the testing condition of 150 °C 1 × 104 A/cm2 may be too stringent for SnAg solder, which has a melting point of 221°C.
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In addition, this temperature increase will affect the measurement of mean-time-to-failure (MTTF), which is typically expressed as below:[28]
)k T
where A is constant, j is current density, n is a model parameter for current density, Q is activation energy, k is Boltzmann’s constant, and T is the average bump temperature.
Since the real temperature in solder during high current stressing is much higher than the set testing temperature, the measured value of MTTF would be underestimated.
On the other hand, the thermal gradient drives atoms from the chip to the substrate side, leaving voids near the UBM on the chip side. Hopkins et al reported that thermal migration may assist electromigration in the cathode/chip side, and may oppose electromigration in the anode/chip side.[8] The measured thermal gradient was 365 ℃/cm when stressed at 1 × 104 A/cm2, which may contribute to the formation of voids in the anode/chip side, as seen in Fig. 15(b). On the other hand, when the joints were stressed by 5 × 103 A/cm2 at 150°C, failure occurred only in cathode/cathode side. The average temperature increase was detected to be 9.1 °C at the current density, and its thermal gradient decreased to 127 ℃/cm.
Therefore, both the temperature increase and thermal gradient effects became less profound and thus current crowding dominated the failure mechanism at the lower current density.
5.3 Chemical: effect of IMC formation on electromigration
Unlike electromigration in Al and Cu, chemical reaction in solder joints occurs during current stressing.[29] It is known that Cu and Sn react at room temperature to form Cu6Sn5
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intermetallic compounds.[30-32] Furthermore, Kao reported that Cu, Ni, and Sn prefer to form ternary IMC to lower free energy.[33] Therefore, in the SnAg solder joints, Sn may react with the Cu in the UBM, and Ni atoms in the substrate side may diffuse spontaneously to the chip side to form Cu-Ni-Sn ternary IMCs. As seen in Fig. 16(b), few Cu-Ni-Sn ternary IMCs were found when the joints were thermally annealed at 150°C for 20 hours.
During current stressing, the formation of the Cu-Ni-Sn ternary IMCs on the anode/chip side may expedite by the electron flow and Joule heating. Thus a large amount of Cu-Ni-Sn IMCs formed at the anode/chip side when stressed by 1 × 104 A/cm2, as seen in Fig. 19(a).
In addition, the formation of the IMCs may be accompanied by volume expansion, which can trigger crack formation and propagation. It is speculated that the voids/cracks in Fig. 17(b) might be partially due to the formation of IMCs, since a crack formed at the interface of the IMCs and the solder. However, when the joints were stressed by 5 × 103 A/cm2, no obvious IMCs were observed. Due to the lower Joule heating and lower current density, IMC did not form at this stressing condition.
Thus this unique failure mechanism may play an important role in the electromigration of Pb-free solders at higher current stressing, since most of the Pb-free solders contain over 95% Sn, and a Ni/Cu pad is the most popular metallization on the substrate side.
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