Browsing by Author "Liang, S. W."

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Blocking hillock and whisker growth by intermetallic compound formation in Sn-0.7Cu flip chip solder joints under electromigration
(2010-05-01)
Liang, S. W.
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Chen, Chih
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Han, J. K.
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Xu, Luhua
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Tu, K. N.
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. (C) 2010 American Institute of Physics. [doi:10.1063/1.3410796]
Direct measurement of hot-spot temperature in flip-chip solder joints under current stressing using infrared microscopy
(2008-08-01)
Hsiao, Hsiang-Yao
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Liang, S. W.
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Ku, Min-Feng
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Chen, Chih
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Yao, Da-Jeng
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 1.06x10(4) A/cm(2), the temperature in the two hot spots are 161.7 and 167.8 degrees C, respectively, which surpass the average bump temperature of 150.5 degrees 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. (c) 2008 American Institute of Physics.
Effect of Al-trace degradation on Joule heating during electromigration in flip-chip solder joints
(2007-02-19)
Liang, S. W.
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Chiu, S. H.
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Chen, Chih
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材料科學與工程學系
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Department of Materials Science and Engineering
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 0.6 A at 100 degrees 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. (c) 2007 American Institute of Physics.
Effect of Al-Trace Width on the Electromigration Failure Time of Flip-Chip Solder Joints
(2010-10-01)
Liang, S. W.
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Hsiao, H. Y.
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Chen, Chih
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材料科學與工程學系
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Department of Materials Science and Engineering
The effect of Al-trace width on electromigration (EM) in flip-chip solder joints was investigated experimentally. EM tests were performed on eutectic Sn-Ag solders with 40-mu m- and 100-mu m-wide Al traces. Under the same stressing conditions (0.5 A at 165 degrees C), the failure time was 44.1 h for solder joints with 40-mu m-wide traces and 250.1 h for solder joints with 100-mu m-wide traces. The Al-trace width influenced both the current crowding and the Joule heating effects. Thus, both effects are responsible for the significant difference in failure time. Finite-element analysis was used to examine the current crowding effect in solder bumps with Al traces of the two different widths. The results showed that the current crowding effect was slightly higher in joints with 40-mu m-wide traces. In addition, the temperature coefficient was used to measure the real temperatures in the solder bumps during EM. The results indicated that the width of the Al traces had a substantial influence on the Joule heating effect. The measured temperature in the solder bump was 218.2 degrees C and 172.2 degrees C for the bump with 40-mu m- and 100-mu m-wide Al traces, respectively. This difference in the Joule heating effect plays a crucial role in causing the difference in the failure time of solder joints with the two different widths.
Effect of Al-Trace Width on the Electromigration Failure Time of Flip-Chip Solder Joints
(2010-10-01)
Liang, S. W.
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Hsiao, H. Y.
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Chen, Chih
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材料科學與工程學系
;
Department of Materials Science and Engineering
The effect of Al-trace width on electromigration (EM) in flip-chip solder joints was investigated experimentally. EM tests were performed on eutectic Sn-Ag solders with 40-mu m- and 100-mu m-wide Al traces. Under the same stressing conditions (0.5 A at 165 degrees C), the failure time was 44.1 h for solder joints with 40-mu m-wide traces and 250.1 h for solder joints with 100-mu m-wide traces. The Al-trace width influenced both the current crowding and the Joule heating effects. Thus, both effects are responsible for the significant difference in failure time. Finite-element analysis was used to examine the current crowding effect in solder bumps with Al traces of the two different widths. The results showed that the current crowding effect was slightly higher in joints with 40-mu m-wide traces. In addition, the temperature coefficient was used to measure the real temperatures in the solder bumps during EM. The results indicated that the width of the Al traces had a substantial influence on the Joule heating effect. The measured temperature in the solder bump was 218.2 degrees C and 172.2 degrees C for the bump with 40-mu m- and 100-mu m-wide Al traces, respectively. This difference in the Joule heating effect plays a crucial role in causing the difference in the failure time of solder joints with the two different widths.
Effect of bump size on current density and temperature distributions in flip-chip solder joints
(2009-05-01)
Kuan, Wei-Chih
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Liang, S. W.
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Chen, Chih
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材料科學與工程學系
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Department of Materials Science and Engineering
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 mu m bump height is 103.15 degrees C which is only 3.15 degrees C higher than the substrate temperature due to joule heating effect. However, upon reducing the bump height to 28.9 mu m, the maximum temperature in the solder increased to 181.26 degrees 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. (C) 2009 Published by Elsevier Ltd.
Effect of migration and condensation of pre-existing voids on increase in bump resistance of flip chips on flexible substrates during electromigration
(2008-07-01)
Liang, S. W.
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Chang, Y. W.
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Chen, Chih
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Preciado, Jackie
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Tu, K. N.
In Pb-free solder joints formed by reflowing a bump of solder paste, voids are formed within the solder due to the residue of flux in the reflow process. These voids migrate toward the cathode contact during electromigration under current stressing. Accompanying the electromigration, resistance jumps of a few 100 m Omega were observed. It was postulated that a jump occurs when a void touches the cathode contact. This study investigated the effect of the void migration and condensation on the change in bump resistance using three-dimensional (3D) simulations and finite element analysis. It was found that there was negligible change in bump resistance during void migration towards the high-current-density region before touching the cathode contact opening. When a small void condensed on the contact opening and depleted 18.4% of the area, the bump resistance increased only 0.4 m Omega. Even when a large void depleted 81.6% of the opening, the increase in bump resistance was 3.3 m Omega. These values are approximately two orders of magnitude smaller than those reported in the literature for the change in resistance due to void migration in flip chips on flexible substrates. We conclude that the reported change in resistance was most likely that of the Al or Cu interconnection in the flip-chip samples.
Effect of Passivation Opening Design on Electromigration Reliability Issue in Flip-Chip Solder Joints
(2009)
Chen, Tzu Yu
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Liang, S. W.
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Chen, Chih
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材料科學與工程學系
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Department of Materials Science and Engineering
This study investigates the effect of passivation opening design on electromigration reliability issue in flip-chip solder joints by using statistical analysis. Eutectic SnAg solder joints were used in this study with two different filling condition of polyimide (PI), ie with or without contact opening. The bump height is 50 gm. The four point probe method was used to monitor the bump resistance of a specific solder joint with downward election flow, under 0.8 A at 155 degrees C. The failure criteria are defined as the resistances rose to 1.2 times and 2 times of their initial values. In addition, three-dimensional finite element analysis (3D-FEA) was employed to show the distribution of current density, and the different failure modes were also discussed. The solder joints without PI performed longer failure time from the statistical analysis results. In other words, the set without PI shows better reliability than that with PI.
Effect of three-dimensional current and temperature distributions on void formation and propagation in flip-chip solder joints during electromigration
(2006-07-10)
Liang, S. W.
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Chang, Y. W.
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Shao, T. L.
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Chen, Chih
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Tu, K. N.
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. (c) 2006 American Institute of Physics.
Effects of current crowding and Joule heating on reliability of solder joints
(2007)
Liang, S. W.
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Chiu, S. H.
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Chen, Chih
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材料科學與工程學系
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Department of Materials Science and Engineering
This study employs three-dimensional simulation to investigate the Joule heating effect under accelerated electromigration tests in flip-chip solder joints. It was found that the Joule heating effect was very seriously during high current stressing, and a hot spot exists in the solder bump. The hot-spot may play important role in the void formation and thermomigration in solder bumps during electromigration.
Electromigration Failure Mechanism in Sn-Cu Solder Alloys with OSP Cu Surface Finish
(2012-09-01)
Chu, Ming-Hui
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Liang, S. W.
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Chen, Chih
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Huang, Annie T.
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材料科學與工程學系
"Organic solderable preservative (OSP) has been adopted as the Cu substrate surface finish in flip-chip solder joints for many years. In this study, the electromigration behavior of lead-free Sn-Cu solder alloys with thin-film under bump metallization and OSP surface finish was investigated. The results showed that severe damage occurred on the substrate side (cathode side), whereas the damage on the chip side (cathode side) was not severe. The damage on the substrate side included void formation, copper dissolution, and formation of intermetallic compounds (IMCs). The OSP Cu interface on the substrate side became the weakest point in the solder joint even when thin-film metallization was used on the chip side. Three-dimensional simulations were employed to investigate the current density distribution in the area between the OSP Cu surface finish and the solder. The results indicated that the current density was higher along the periphery of the bonding area between the solder and the Cu pad, consistent with the area of IMC and void formation in our experimental results."
Electromigration issues in lead-free solder joints
(2007-03-01)
Chen, Chih
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Liang, S. W.
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材料科學與工程學系
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Department of Materials Science and Engineering
As the microelectronic industry advances to Pb-free solders due to environmental concerns, electromigration (EM) has become a critical issue for fine-pitch packaging as the diameter of the solder bump continues decreasing and the current that each bump carries keeps rising owing to higher performance requirement of electronic devices. As stated in 2003 International Technology Roadmap for Semiconductors (ITRS), the EM is expected to be the limiting factor for high-density packages. This paper reviews general background of EM, current understanding of EM in solder joints, and technical hurdles to be addressed as well as possible solutions. It is found that the EM lifetimes of Pb-free solder bumps are between the high-Pb and the eutectic composition under the same testing condition. However, our simulation results show that the electrical and thermal characteristics remain essentially almost the same during accelerated EM tests when the Pb-containing solders are replaced by Pb-free solders, suggesting that the melting points of the solders are likely the dominant factor in determining EM lifetimes. The EM behavior in Pb-free solder is a complicated phenomenon as multiple driving forces coexist in the joints and each joint contains more than four elements with distinct susceptibility to each driving force. Therefore, atomic transport due to electrical and thermal driving forces during EM is also investigated. In addition, several approaches are presented to reduce undesirable current crowding and Joule heating effects to improve EM resistance.
Geometrical effect of bump resistance for flip-clip solder joints: Finite-element modeling and experimental results
(2006-08-01)
Liang, S. W.
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Chang, Y. W.
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Chen, Chih
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Liu, Y. C.
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Chen, K. H.
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 m Omega 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 m Omega, 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.
Joule heating effect under accelerated electromigration in flip-chip solder joints
(2006)
Chiu, S. H.
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Liang, S. W.
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Chen, Chili
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Yao, D. J.
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材料科學與工程學系
This study employs three-dimensional simulation to investigate the Joule heating effect under accelerated electromigration tests in flip-chip solder joints. It was found that the Joule heating effect was very seriously during high current stressing, and a hot spot exists in the solder bump. The hot-spot may play important role in the void formation and thermomigration in solder bumps during electromigration.
Nonuniform and Negative Marker Displacements Induced by Current Crowding During Electromigration in Flip-Chip Sn-0.7Cu Solder Joints
(2009-12-01)
Liang, S. W.
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Hsiao, Hsiang-Yao
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Chen, Chih
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Xu, Luhua
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Tu, K. N.
A quantitative analysis of the nonuniform distribution of current density and nonuniform rate of electromigration has been carried out by measuring the movement of an array of diffusion markers. Tiny marker arrays were fabricated by focused ion beam on the polished surface of flip-chip solder joints near the anode to measure the electromigration rate. The marker velocity at the current-crowding region was found to be at least five times larger than at locations far from the region. Some of the markers in the low-current-density region possess negative velocities, indicating that backflow occurs during the electromigration. The backflow, in which the atomic flow is against the electron flow, is explained by a constant-volume model as well as the back-stress induced by electromigration.
Optima Design of Contact Opening for Relieving Current Crowding Effect in Flip-Chip Solder Joints
(2009)
Liang, S. W.
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Chen, Chih
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材料科學與工程學系
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Department of Materials Science and Engineering
The Simulation results show that larger contact opening may not benefit on relieving current crowding effect in the nip-chip solder joints. For the solder Joints with a 10-mu m thick Cu UBM, the diameter of the contact opening for the lowest Current crowding effect is 60 mu m. It means that using larger or smaller diameter will increase the current density in the solder bumps. In addition, the optima diameter of the contact opening with a thin-film UBM is larger than that with a thick UBM. In this letter, the optimal design rule of contact opening for relieving Current crowding effect in solder joints will be provided.
Relieving hot-spot temperature during electromigration in solder and current crowding effects bumps by using Cu columns
(2007-10-01)
Liang, S. W.
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Chang, Y. W.
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Chen, Chih
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材料科學與工程學系
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Department of Materials Science and Engineering
Solder joints with Cu columns appear to be one of the best structures to resist electromigration. Three-dimensional thermoelectrical analysis was employed to simulate the current density and temperature distributions for eutectic SnPb solder bumps with 0.5, 5, 25, 50, and 100 mu m Cu under bump metallization (UBM). It was found that the hot spots and current crowding effects in the solder were reduced significantly when the Cu thickness was over 50 Pm, whereas the overall Joule heating effect remained almost unchanged. The mechanism by which the Cu column is effective in relieving the hot spot and current crowding effects is to keep the solder away from the heat source and crowding region. Simulated at a current of 0.6 A and 70 degrees C, the estimated mean time to failure of the joints with a 50-mu m-thick Cu column was 6.7 times longer than that of joints with a 0.5-mu m-thick Cu UBM.
Study of void formation due to electromigration in flip-chip solder joints using Kelvin bump probes
(2006-07-17)
Chang, Y. W.
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Liang, S. W.
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Chen, Chih
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材料科學與工程學系
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Department of Materials Science and Engineering
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 0.8 A at 150 degrees C, and the bump resistance increased only 0.02 m Omega 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. (c) 2006 American Institute of Physics.
Three-dimensional thermoelectrical simulation in flip-chip solder joints with thick underbump metallizations during accelerated electromigration testing
(2007-02-01)
Liang, S. W.
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Chang, Y. W.
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Chen, Chih
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材料科學與工程學系
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Department of Materials Science and Engineering
In flip-chip solder joints, thick Cu and Ni films have been used as under bump metallization (UBM) for Pb-free solders. In addition, electromigration has become a crucial reliability concern for fine-pitch flip-chip solder joints. In this paper, the three-dimensional (3-D) finite element method was employed to simulate the current-density and temperature distributions for the eutectic SnPb solder joints with 5-mu m Cu, 10-mu m Cu, 25-mu m Cu, and 25-mu m Ni UBMs. It was found that the thicker the UBM is the lower the maximum current density inside the solder. The maximum current density is 4.37 x 10(4) A/cm(2), 1.69 x 10(4) A/cm(2), 7.54 x 10(3) A/cm(2), and 1.34 x 10(4) A/cm(2), respectively, when the solder joints with the above four UBMs are stressed by 0.567 A. The solder joints with thick UBMs can effectively relieve the current crowding effect inside the solder. In addition, the joint with the thicker Cu UBM has a lower Joule heating effect in the solder. The joint with the 25-mu m Ni UBM has the highest Joule heating effect among the four models.