Browsing by Author "Cheng, Yin"

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Electromigration Mechanism of Failure in Flip-Chip Solder Joints Based on Discrete Void Formation
(2017-12-20)
Chang, Yuan-Wei
;
Cheng, Yin
;
Helfen, Lukas
;
Xu, Feng
;
Tian, Tian
In this investigation, SnAgCu and SN100C solders were electromigration (EM) tested, and the 3D laminography imaging technique was employed for in-situ observation of the microstructure evolution during testing. We found that discrete voids nucleate, grow and coalesce along the intermetallic compound/solder interface during EM testing. A systematic analysis yields quantitative information on the number, volume, and growth rate of voids, and the EM parameter of DZ*. We observe that fast intrinsic diffusion in SnAgCu solder causes void growth and coalescence, while in the SN100C solder this coalescence was not significant. To deduce the current density distribution, finite-element models were constructed on the basis of the laminography images. The discrete voids do not change the global current density distribution, but they induce the local current crowding around the voids: this local current crowding enhances the lateral void growth and coalescence. The correlation between the current density and the probability of void formation indicates that a threshold current density exists for the activation of void formation. There is a significant increase in the probability of void formation when the current density exceeds half of the maximum value.
Study of Discrete Voids Formation in Flip-Chip Solder Joints due to Electromigration Using In-Situ 3D Laminography and Finite-Element Modeling
(2016-01-01)
Chang, Yuan-Wei
;
Cheng, Yin
;
Xu, Feng
;
Helfen, Lukas
;
Tian, Tian
Nowadays, the microelectronics industry broadly uses the flipchip technology to enhance the packaging density. However, the small size and the unique geometry of the flip-chip solder joints induce the electromigration (EM) reliability issue. In this study, a Pb-free solder joints (SAC1205) was EM tested by a current of 7.5x10(3) A/cm(2). During the tests, a three-dimensional (3D) X-ray laminography method was applied to in-situ observe the microstructure evolution. The laminography method allows for the non-destructive observation and provides the quantitative analysis among three dimensions. After EM testing for 650 hr, a new EM failure mechanism was found rather than the well-known models, the pancake void propagation and the under-bump-metallization dissolution. According to the laminography images at different testing stages, many voids simultaneously formed and grew during the entire procedure of testing. Most of them distributed in the current crowding region, but a few also located in the low-currentdensity region. As the testing time increased, voids grew bigger, coalesced with each other, and finally became large voids which occupied the interface and caused EM failure. The finite-element (FE) method was also applied to analyze the interplay between the microstructure evolution and current density redistribution. A series of 3D FE models were built based on the laminography images at different testing stages. The current density distribution from the FE analysis indicates that the multiple voids formation does not affect the global current density distribution until the voids merged together and became very large voids in the late stage of EM testing. The relieving of the global current crowding in the pancake void model was not found in this new EM failure mechanism. It was the local current crowding found in the new model that responsible for the EM retardation.
Study of electromigration-induced formation of discrete voids in flip-chip solder joints by in-situ 3D laminography observation and finite-element modeling
(2016-09-15)
Chang, Yuan-Wei
;
Cheng, Yin
;
Xu, Feng
;
Helfen, Lukas
;
Tian, Tian
In the microelectronics industry the flip-chip (FC) technology is broadly used to enhance the packaging density. However, the small size and the unique geometry of the FC solder joints induce the electro-migration (EM) reliability issue. In this study, a pair of lead-free solder joints (SAC1205) was EM tested by a current of 7.5 x 10(3) A/cm(2). During the tests, X-ray laminography was applied to observe the microstructure evolution in-situ. Laminography enables the non-destructive observation of the bump microstructure and allows for a quantitative three-dimensional (3D) analysis. After EM testing for 650 h, a new EM failure mechanism was found, differing from the two well-known models, the pancake void propagation and the under-bump-metallization dissolution. Here, a few pre-existing small voids grew and simultaneously many new voids formed and grew over the entire EM testing period. Most of the nucleating voids were distributed in the current crowding region, a few also located in the low-current density region. As the testing time increased, voids increasingly coalesced with each other, forming a porous network which occupied a large part of the interface area and caused the EM failure. A finite element (FE) method was then applied to analyze the interplay between the microstructure evolution and current density redistribution. A series of 3D FE models was built based on the laminography images for the different testing stages. The current density distribution from the FE analysis indicates that the formation of discrete voids did not affect the global current density distribution until a major coalescence of the voids occurred. The relieving of the global current crowding in the pancake void model was not found in this new EM failure mechanism. It was the local current crowding around individual void found in the new mechanism that is held responsible for the EM retardation. (C) 2016 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.