Electromigration In Flip Chip Solder Joints Research Articles

Influence of Cu column under-bump-metallizations on current crowding and Joule heating effects of electromigration in flip-chip solder joints

The electromigration behavior of SnAg solder bumps with and without Cu column under-bump-metallizations (UBMs) has been investigated under a current density of 2.16 × 104 A/cm2 at 150 °C. Different failure modes were observed for the two types of samples. In those without Cu column UBMs, when SnAg solder bumps that had implemented 2 μm Ni UBMs were current stressed at 2.16 × 104 A/cm2, open failure occurred in the bump that had an electron flow direction from the chip side to the substrate side. However, in those with Cu column UBMs, cracks formed along the interface of Cu6Sn5 intermetallic compounds and the solder on the substrate side in the Sn-3.0Ag–0.5Cu solder bump that had an electron flow direction from the substrate side to the chip side. A three-dimensional simulation of the current density distribution was performed in order to obtain a better understanding of the current crowding behavior in solder bumps. The current crowding effect was found to account for the void formation on both the chip and the substrate side for the two kinds of solder bumps. One more important finding, as confirmed by infrared microscopy, is that the alleviation of current crowding by Cu column UBMs also helped decrease the Joule heating effect in solder bumps during current stressing. Therefore, the measured failure time for the solder joints with Cu column UBMs appears to be much longer than that of the ones with the 2 μm Ni UBMs.

Effect of Si-die dimensions on electromigration failure time of flip-chip solder joints

Electromigration tests were performed for solder joints with various Si dies thickness and area. For the electro-migration in flip-chip solder joints with a 60, 100, 250 and 760 μm thick Si die, it is found that Si-die thickness has profound influence on electro-migration lifetime. The average failure time was 1608.0 h for joints with a 760-μm-thick die when they are stressed by 1.0 A at 100 °C. However, it decreased significantly to 0.6 h for joints with a 60-μm-thick die. According to the temperature measured by infrared microscopy, solder joints with a thinner die has a higher Joule heating effect, which results in a shorter electromigration lifetime. In addition, the die area has a considerable influence on the electromigration failure time. The electromigration failure time decreases as the die area decreases. The average failure time is 1608.0, 28.0, 10.6, 5.0 and 0.3 h for the solder joints with die area of 5350 × 4350 μm 2, 5350 × 3600 μm 2, 5350 × 3000 μm 2, 5350 × 2000 μm 2 and 5350 × 1000 μm 2, respectively. The Joule heating effect becomes more serious in smaller dies, which causes a shorter electromigration lifetime.

Evaluation of electromigration in flip-chip solder joints

The flip-chip solder joint has become one of the most important technologies of high-density packaging in the microelectronics industry. But, electromigration has become a critical reliability issue in flip-chip technology. Because the dimensions of solder joints are expected to decrease and current density is expected to increase. This study is about electromigration of flip-chip solder joints, we evaluated many kinds of solder balls such as SnAgCu, SnCu and so on in flip chip package. The lifetime against electromigration was defined the fail from the value of resistance with electric current reaches 1.5 times of that of initial resistance with electric current for. In solder bumps with electric current, since the atoms composed of the solder bump and UBM move in the direction of electron flows, the IMC was accumulated on the anode side. Meanwhile, the IMC disappeared in the cathode side, and the voids were formed. In the solder bumps without electric current, the IMC gradually grew on both sides. SnAgCu had better lifetime than SnCu, and different time-to-failure caused by different crystallographic orientation of Sn. And various dopants in SnCu had a different EM lifetime each other.

Investigation of Void Nucleation and Propagation in the Joule Heating Effect During Electromigration in Flip-Chip Solder Joints

This study analyzes the effect of void propagation on the temperature increase of solder joints by using x-ray microscopy, Kelvin probes, and infrared microscopy. It was found that the temperature rise due to void formation was less than 1.3°C when the voids depleted about 75% of the contact opening, even though bump resistance had increased to 10.40 times its initial value. However, the temperature rose abruptly with an increase of up to 8.0°C when the voids depleted 96.2% of the contact opening. A hot spot was observed immediately before the occurrence of open failure in the solder bump. The local increase in temperature was about 30.2°C at the spot. This spot may be the remaining contact area immediately before the occurrence of open failure.

Investigation of Joule Heating Effect in Various Stages of Electromigration in Flip-chip Solder Joints by Infrared Microscopy

AbstractThe Joule heating effect at various stages under electromigration of flip-chip Sn3.5Ag solder joints was investigated under a current of 0.5 A at 100°C. During various stages of electromigration, voids may form and propagate and Joule heating effect may vary at different void sizes. To verify the void nucleation and propagation on Joule heating effect during electromigration process, the solder bump was stressed for different lengths of time and then examined by Kelvin bump probes and infrared microscopy. We found that voids started to form at approximately 1.2 times of the initial bump resistance. Then the voids propagated when the bump resistance increased. In addition, the temperature of the solder joints increased with the bump resistance and the increase of current stressing time. It increased very slowly in initial stages. In the last stage, the temperature of the solder bump increased rapidly due to the increase of the bump resistance and the local Joule heating effect.

Secondary Current Crowding Effect during Electromigration of Flip-Chip Solder Joints

In flip-chip interconnects under current stressing, the primary current crowding effect occurs at the entrance edge of the contact interface with the highest current density. In this study, an increased current density also occurred at the other edge of the contact interface, followed by a selective dissolution of under bump metallization. After primary current crowding, the rest of electrons flow to the metallization edge, followed by an abrupt change in direction toward the anode. Primary current crowding is attributed to the electrical field change whereas the secondary crowding effect is due to the physical blocking of the electron flow. Because this effect is not as great as that of primary current crowding, it must be assisted by thermal diffusion.

Effect of void propagation on bump resistance due to electromigration in flip-chip solder joints using Kelvin structure

Three-dimensional modeling is employed to simulate various Kelvin structures for detecting the change in bump resistance due to void formation and propagation during electromigration in flip-chip solder joints. It is found that the Kelvin structures can sense the highest voltage drop when its voltage probes are placed at the current entrance into the bump on the chip side, and it is thus the most sensitive design to monitor void formation and propagation. When the bump resistance increases 20% of its initial value, the depletion percentage of contact opening ranges from 21.0% to 65.0%, depending on the position of the probes.

Effect of Al-trace degradation on Joule heating during electromigration in flip-chip solder joints

This study investigates the mechanism for the abrupt increase in temperature at later stages of electromigration in flip-chip solder joints. It is found that electromigration also occurs in Al traces when stressed by 0.6A at 100°C. Three-dimensional thermoelectrical simulation by finite element analysis was carried out to simulate the temperature distribution in solder joints with and without degradation of the Al trace. It is found that the degradation of the Al trace has substantial effect on the Joule heating of solder joints. This model can explain the serious Joule heating effect in the later stages of electromigration.

Investigation of void nucleation and propagation during electromigration of flip-chip solder joints using x-ray microscopy

X-ray microscopy was employed to investigate void nucleation and propagation during electromigration in solder joints. The shape of the voids at various stages can be clearly observed. The voids became irregular when they propagated to deplete the contact opening. Growth velocity at the early stage was found to be 1.3μm∕h under 6.5×103A∕cm2 at 150°C, and it decreased to 0.3μm∕h at later stages. Formation of intermetallic compound (IMC) and compositional changes at the interface of solder/IMC on the chip side were attributed to the retarded growth rate at later stages.

Effect of Al trace dimension on electromigration failure time of flip-chip solder joints

The effect of Al-trace dimension on electromigration of flip-chip solder joints was investigated. The Al trace dimension was found to have a significant influence on the electromigration failure time. When joints with Al traces 100 µm wide were stressed by 1.0 A at 100°C, failure times were 35 h, 1,700 h, and >3,000 h for joints with Al traces that were 2,550 µm, 1,700 µm, and 850 µm long, respectively. Solder joints with Al traces 40 µm wide and 2,550 µm long failed instantly at 0.6 A. The Joule heating effect was found to be responsible for the huge difference in failure time.

Geometrical effect of bump resistance for flip-chip solder joints: Finite-element modeling and experimental results

The bump resistance of flip-chip solder joints was measured experimentally and analyzed by the finite-element method. Kelvin structures for flip-chip solder joints were designed and fabricated to measure the bump resistance. The measured value was only about 0.9 mΘ at room temperature, which was much lower than that expected. Three-dimensional (3-D) modeling was performed to examine the current and voltage distribution in the joint. The simulated value was 7.7 mΘ, which was about 9 times larger than the experimental value. The current crowding effect was found to be responsible for the difference in bump resistance. Therefore, the measured bump resistance strongly depended on the layout of the Kelvin structure. Various layouts were simulated to investigate the geometrical effect of bump resistance, and a significant geometrical effect was found. A proper layout was proposed to measure the bump resistance correctly. The Kelvin structure would play an important role in monitoring void formation and microstructure changes during the electromigration of flip-chip solder joints.

Study of void formation due to electromigration in flip-chip solder joints using Kelvin bump probes

Kelvin bump probes were fabricated in flip-chip solder joints, and they were employed to monitor the void formation during electromigration. We found that voids started to form at approximately 5% of the failure time under 0.8A at 150°C, and the bump resistance increased only 0.02mΩ in the initial stage of void formation. Three-dimensional simulation was performed to examine the increase in bump resistance at different stages of void formation, and it fitted the experimental results quite well. This technique provides a systematic way for investigating the void formation during electromigration.

Local melting induced by electromigration in flip-chip solder joints

A new electromigration failure mechanism in flip-chip solder joints is reported. The solder joints failed by local melting of a PbSn eutectic solder. Local melting occurred due to a sequence of events induced by the microstructure changes in the flip-chip solder joint. The formation of a depression in the current-crowding region of a solder joint induced the local electrical resistance to increase. The rising local resistance resulted in a larger Joule heating, which, in turn, raised the local temperature. When the local temperature rose above the eutectic temperature of the PbSn solder, the solder joint melted and consequently failed. The results of this study suggest that a dynamic, coupled simulation that takes into account the microstructure evolution, current density distribution, and temperature distribution may be needed to fully solve this problem.