S. W. Liang

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.

Effect of three-dimensional current and temperature distributions on void formation and propagation in flip-chip solder joints during electromigration

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.

Low-temperature growth of ZnO nanorods in anodic aluminum oxide on Si substrate by atomic layer deposition

Low-temperature growth of self-organized ZnO nanorods on Si substrate is achieved using anodic aluminum oxide and atomic layer deposition at 250°C without catalyst or seed layer. Photoluminescence spectrum indicates that the ZnO nanorod arrays exhibit a blue∕green luminescence at 480nm. In addition, the nanorod arrays demonstrate excellent field-emission properties with a turn-on electric field of 6.5Vμm−1 and a threshold electric field of 9.8Vμm−1, which are attributed to the perfectly perpendicular alignment of ZnO nanorods to the Si substrate.

Three-dimensional simulation on current-density distribution in flip-chip solder joints under electric current stressing

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 ...

Effect of Al-trace dimension on joule heating and current crowding in flip-chip solder joints under accelerated electromigration

Three-dimensional thermoelectrical simulation was conducted to investigate the influence of Al-trace dimension on Joule heating and current crowding in flip-chip solder joints. It is found that the dimension of the Al-trace effects significantly on the Joule heating, and thus directly determines the mean time to failure (MTTF). Simulated at a stressing current of 0.6A at 70°C, we estimate that the MTTF of the joints with Al traces in 100μm width was 6.1 times longer than that of joints with Al traces in 34μm width. Lower current crowding effect and reduced hot-spot temperature are responsible for the improved MTTF.

Effect of bump size on current density and temperature distributions in flip-chip solder joints

Three dimensional thermo-electrical analysis was employed to simulate the current density and temperature distributions for eutectic SnAg solder bumps with shrinkage bump sizes. It was found that the current crowding effects in the solder were reduced significantly for smaller solder joints. Hot-spot temperatures and thermal gradient were increased upon reducing the solder. The maximum temperature for solder joint with 144.7 lm bump height is 103.15 C which is only 3.15 C higher than the substrate temperature due to Joule heating effect. However, upon reducing the bump height to 28.9 lm, the maximum temperature in the solder increased to 181.26 C. Serious Joule heating effect was found when the solder joints shrink. The higher Joule heating effect in smaller solder joints may be attributed to two reasons, first the increase in resistance of the Al trace, which is the main heating source. Second, the average and local current densities increased in smaller bumps, causing higher temperature increase in the smaller solder bumps. 2009 Published by Elsevier Ltd.

Direct measurement of hot-spot temperature in flip-chip solder joints under current stressing using infrared microscopy

Several simulation studies reported that a hot spot exists in flip-chip solder bumps under accelerated electromigration. Yet, there are no experimental data to verify it. In this paper, the temperature distribution during electromigration in flip-chip SnAg3.5 solder bumps is directly inspected using infrared microscopy. Two clear hot spots are observed in the bump. One is located at the region with peak current density and the other one is at the bump edge under the current-feeding metallization on the chip side. Under a current stress of 1.06×104A∕cm2, the temperature in the two hot spots are 161.7 and 167.8°C, respectively, which surpass the average bump temperature of 150.5°C. In addition, the effect of under-bump-metallization (UBM) thickness on the hot spots is also examined. It indicates that the hot-spot temperature in the solder bump increases for the solder joints with a thinner UBM. Electromigration test indicates that these hot spots have significant influence on the initial failure location.

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.

Redistribution of Pb-rich phase during electromigration in eutectic SnPb solder stripes

The microstructural evolution occurring within the eutectic SnPb solder during electromigration is investigated utilizing Blech specimens. Solder stripes of about 3μm in thickness were fabricated on Cu∕Ti metallization. It was found that the Pb-rich phase ripened and aligned along the direction of the electron flow following the current stressing of 9.7×103A∕cm2 at 80°C for 24h. As the stressing time or current density increased, the redistribution of the Pb-rich phase became more significant. Three-dimensional simulation was performed to examine the change in resistance and current-density distribution during electromigration. It was found that when the Pb-rich phase aligned along the direction of the electron flow, the total resistance of the solder stripe decreased. This reduction in total resistance may provide the driving force for the redistribution of the Pb-rich phase during electromigration. In addition, current crowding occurred in the vicinity of the interface of the Pb-rich and Sn-rich phases,...

Blocking hillock and whisker growth by intermetallic compound formation in Sn-0.7Cu flip chip solder joints under electromigration

Mass extrusion occurs in electromigration at the anode in cross-sectioned Sn-0.7Cu flip-chip solder joints. In a pair of joints, the hillock squeezed out at the anode on the board side is more serious than the whisker grown at the anode on the chip side. The difference of mass extrusion has been found to be affected by the amount of intermetallic compound (IMC) formation in the solder bump. It is found that when a large amount of Cu–Sn IMCs form in the grain boundaries of Sn grains, small hillocks are extruded on the anode end. It is proposed that the excessive IMC formation may be able to block the diffusion path of Sn atoms, so the growth of both the Sn whiskers and hillocks are retarded.