Thermomigration of Ti in flip-chip solder joints

doi:10.1016/j.scriptamat.2012.01.020

Scripta Materialia

Volume 66, Issue 9, May 2012, Pages 694-697

https://doi.org/10.1016/j.scriptamat.2012.01.020 Get rights and content

Titanium has a very large heat of transport value of −768 kJ mol^–1. However, thermomigration of Ti in solder joints has not been examined. In this study, void formation was observed in non-current-stressed Al traces during electromigration and thermomigration tests of SnAg solder joints. Voids formed inside the traces located above the solder bumps with no current flow. We found that Ti thermomigration occurred in the joint, resulting in the reaction of Al and Cu, leaving voids behind in the Al trace.

References (22)

C. Chen et al.
Annu. Rev. Mater. Res.
(2010)
M.O. Alam et al.
Acta Mater.
(2006)
Y.C. Chan et al.
Prog. Mater. Sci.
(2010)
J.H. Matlock et al.
Acta Mater.
(1971)
W.J. Choi et al.
J. Appl. Phys.
(2003)
E.C.C. Yeh et al.
Appl. Phys. Lett.
(2002)
H. Ye et al.
Appl. Phys. Lett.
(2003)
S.H. Chiu et al.
Appl. Phys. Lett.
(2006)
A.T. Huang et al.
Appl. Phys. Lett.
(2006)
H.Y. Hsiao et al.
Appl. Phys. Lett.
(2007)

X. Gu et al.

J. Mater. Sci. Mater. Electron.

(2011)

Cited by (11)

Coupling model of electromigration and experimental verification – Part II: Impact of thermomigration
2023, Journal of the Mechanics and Physics of Solids
This paper presented a comprehensive experimental and simulation study for thermomigration (TM) accompanying electromigration (EM) at elevated current densities. Both Blech and standard wafer-level electromigration acceleration test (SWEAT)-like test structures, with aluminum (Al) as a carrier, were used for testing and analysis. In Part I of our study (Cui et al., 2023a), the experimental and numerical results with the current density of 1 MA/cm² were presented. We observed that Al stripes with a SWEAT structure did not show damage in the entire length, while Blech structures showed void and hillock formations only at the cathode and anode, respectively. The temperature gradient owing to Joule heating was neglected in our previous simulations, and the predicted results agreed well with the experimental observations. However, we have not theoretically verified the effect of the temperature gradient. In this paper, we first reported the new experimental data under the elevated current densities of 3 and 5 MA/cm². In both Blech and SWEAT structures, the spreading of voids in the middle region of conductors was observed. Moreover, in Blech structures, voiding in the middle region occurred after a period of time when voids/hillocks were formed at the cathode and anode, while the SWEAT structures did not show damage at the two ends. Next, based on the coupled 3D theory (Cui et al., 2023a), new analytical one-dimensional (1D) solutions were derived for the Blech and SWEAT structures in the un-passivated configuration considering TM. We found that TM played a significant role in the EM development in the middle of conductors under the elevated current density. The numerical results were in excellent agreement with the experimental data with the consideration of TM. We further established new EM failure's threshold criteria for the SWEAT structures in the form of the product of current density and square of conductor length. This is a major departure from the original Blech's theory in which only mechanical stress gradient was considered. We also studied the acceleration factor of the current density exponent and presented an insight into failure mechanisms associated with TM.
Comparative study on the hourglass-like joint of electroplated Sn-base solder reinforced by adding Ag<inf>3</inf>Sn nanoparticles and Ag micro-alloying elements
2020, Materialia
Citation Excerpt :
With the miniaturization of electronic devices, the area array interconnection of Sn-base solder balls is a widely used technology in integrated circuit packages due to its low cost, high-density input/output (I/O), and high interconnection performance [1–3]. However, traditional drum-shaped solder joints of Sn-based solder joints formed by controlled collapse chip connection technology (C4) make two adjacent I/O solder bumps in touch easily, especially in the structure of small bump pitch, and lead to the short circuit [4–6]. Compared with the drum-shaped solder joints, the hourglass-shaped solder joints can greatly increase the spacing of the adjacent solder bumps, reduce the current crowding effect at the entrance of electrons and prolong the mean-time-to-failure (MTTF) of solder joints enhancing the reliability of solder joints under stressing current [7,8].
In this work, two types of tin (Sn)-based solder bumps reinforced by Ag₃Sn nanoparticles (nano-Ag₃Sn), and Ag microalloying elements were successfully deposited on copper-clad laminates (CCL) by electroplating. Hourglass-shaped Sn-based solder joints were formed by precise control of the solder volume and the joint height during the reflowing process in solder bonding. Results show that when shaping the solder joints during solder reflowing, Ag₃Sn nanoparticles would uniformly disperse in the Sn-nano-Ag₃Sn solder joint, while microalloying Ag preferentially binds to Sn elements enriching at Sn-Ag solder/CCL interfaces and producing large interfacial intermetallic compounds (IMCs). After the solder reflowing, the nano-Ag₃Sn will redistribute in the solder joints, where the nano-Ag₃Sn adjacent to the solder/CCL interface would advance and adhere to the IMC surface. Compared with the solder joints containing Ag microalloying elements, nano-Ag₃Sn can aggregate on the surface IMC surfaces and hinder the interdiffusion of Sn and Cu ions across the solder/CCL interface. Mechanical shear strength tests of solder joints illustrate the enhancement of mechanical robustness of nano- Ag₃Sn reinforced solder, which inherently correlates to the absorption of nano- Ag₃Sn at the interfaces. Thus, we conclude that the nano-Ag₃Sn will inhibit the overgrowth of the interfacial IMC, specifically Cu₆Sn₅, which ultimately improves the mechanical strength and interconnect reliability of the hourglass-shaped solder joints.
In situ study on interfacial reactions of Cu/Sn-9Zn/Cu solder joints under temperature gradient
2016, Journal of Alloys and Compounds
Citation Excerpt :
Presently, thermomigration (TM) induced by temperature gradient across solid solder joints is recognized as a crucial concern in the reliability evaluation of micro solder joints in service. It has been reported that TM can result in asymmetrical growth of interfacial IMCs [4], recrystallization and redistribution of IMCs in solder [5], formation of interfacial voids [6,7], and consequent reliability issues [8,9]. Generally, solder bumps need to go through several reflows [1].
Synchrotron radiation real-time imaging technology was used to in situ study the interfacial reactions of Cu/Sn–9Zn/Cu solder joints during reflow under a temperature gradient. It was clarified that both Zn and Cu atoms were driven to migrate towards the cold end by the temperature gradient, which enhanced the intermetallic compounds (IMCs) growth locally and the Cu consumption at the hot end. A Cu₅Zn₈ layer adhered to the hot end interface could drastically restrict the consumption of the neighboring Cu substrate till it spalled from the interface. Additionally, the crystallization of β-Sn grains was observed to follow a highly preferred orientation with the c-axis of β-Sn grains departing from the direction of temperature gradient by about 30°–40°. These peculiar phenomena may inspire future exploration of interfacial reaction and grain orientation control by temperature gradient in 3D IC packaging technology.
Asymmetrical growth of Cu <inf>6</inf>Sn <inf>5</inf> intermetallic compounds due to rapid thermomigration of Cu in molten SnAg solder joints
2012, Intermetallics
Citation Excerpt :
Because of the low microbump height, the cross interaction across the two ends of a microbump becomes very serious. On thermomigration, many studies reported that it occurs during accelerated electromigration tests in flip chip solder joints and it may cause damage in the neighboring joints which carried no current [2–10]. Huang et al. calculated that a thermal gradient of 1000 °C/cm is needed to observe thermomigration of Sn and Pb atoms during electromigration tests [4].
We observed asymmetrical growth of Cu₆Sn₅ intermetallic compounds (IMCs) on the two interfaces of Cu/SnAg/Cu solder joints during reflow at 260 °C on a hot plate. The IMCs grew to 12.3 μm on the cold end and 3.5 μm on the hot end after reflow for 40 min. However, the consumption of Cu on the cold end is less than that on the hot end. We propose that rapid thermomigration of Cu is responsible for the asymmetrical growth of the IMCs. With the simulated thermal gradient of 51 °C/cm across the liquid solder, the heat of transport of Cu is calculated as 20 kJ/mol.
Thermomigration in solder joints
2012, Materials Science and Engineering R: Reports
Citation Excerpt :
Then, the Si chip is flipped over to align to a polymer substrate, and then they are reflowed for chip-join [19]. For microbump fabrication, a Si chip is aligned to another Si chip and they are reflowed for about 1 min [54,55]. During the joining processes, the solder needs to be melted to react with UBM to form IMCs.
In 3D IC technology, the vertical interconnection consists of through-Si-vias (TSV) and micro solder bumps. The size of the micro-bump is approaching 10 μm, which is the diameter of TSV. Since joule heating is expected to be the most serious issue in 3D IC, heat flux must be conducted away by temperature gradient. If there is a temperature difference of 1 °C across a micro-bump, the temperature gradient will be 1000 °C/cm, which can cause thermomigration at the device operation temperature around 100 °C. Thus thermomigration will become a very serious reliability problem in 3D IC technology. We review here the fundamentals of thermomigration of atoms in microbump materials; both molten state and solid state thermomigration in solder alloys will be considered. The thermomigration in Pb-containing solder joints is discussed first. The Pb atoms move to the cold end while Sn atoms move to the hot end. Then thermomigration in Pb-free SnAg solder joints is reviewed. The Sn atoms move to the hot end, but the Ag atoms migrate to the cold end. Thermomigration of other metallization elements, such as Cu, Ti and Ni is also presented in this paper. In solid state, copper atoms diffuse rapidly via interstitially to the cold end, forming voids in the hot end. In molten state, Cu thermomigration affects the formation of intermetallic compounds.
Temperature Gradient Induced Orientation Change of Bi Grains in Sn–Bi57–Ag0.7 Solder Joint
2022, Acta Metallurgica Sinica (English Letters)

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Thermomigration of Ti in flip-chip solder joints

Annu. Rev. Mater. Res.

Acta Mater.

Prog. Mater. Sci.

Acta Mater.

J. Appl. Phys.

Appl. Phys. Lett.

Appl. Phys. Lett.

Appl. Phys. Lett.

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J. Mater. Sci. Mater. Electron.