Synchrotron radiation real-time in situ study on dissolution and precipitation of Ag3Sn plates in sub-50 μm Sn–Ag–Cu solder bumps

doi:10.1016/j.jallcom.2014.03.047

Journal of Alloys and Compounds

Volume 602, 25 July 2014, Pages 281-284

https://doi.org/10.1016/j.jallcom.2014.03.047 Get rights and content

Abstract

Synchrotron radiation real-time imaging technology was used to in situ study the dissolution and precipitation behavior of Ag₃Sn plates in sub-50 μm Sn–4.0Ag–0.5Cu flip chip solder bumps on Ni–P under bump metallization (UBM). The dissolution rate of large Ag₃Sn plates was as slow as 0.1–0.2 μm/s at the heating stage, while the precipitation rate exceeded 5 μm/s at the cooling stage with an undercooling of 31–55 °C. The random precipitation behavior of Ag₃Sn plates was in situ observed and explained from the perspective of cluster. This study shows an effective method to investigate the microstructural evolution and the undercooling of phases in real ultra-fine solder joints.

Introduction

Sn–Ag–Cu solders have become the most promising alternatives to the conventional Sn–Pb solders. High Ag concentration in solder has beneficial effects on the mechanical, anti-electromigration and corrosion properties of solder joints [1], [2], [3]. Furthermore, the addition of Ag can eliminate the formation of voids in fine solder joints and suppress the formation of Kirkendall voids at the interface [4], [5]. However, large Ag₃Sn plates usually precipitate at the interface and in the solder matrix [6], [7], which is detrimental to the mechanical properties of solder joints, since cracks tend to propagate along the surface of the inherently brittle Ag₃Sn plates [8], [9]. Furthermore, the probability of the precipitation of large Ag₃Sn plates is higher in smaller volume solder joints [10]. With continuous miniaturization of solder joints, the influence of large Ag₃Sn plates on the reliability of ultra-fine solder joints becomes a concern.

Therefore, in order to achieve high reliability solder interconnects, many efforts have been made to prevent the formation of large Ag₃Sn plates. One way is to control the cooling rate of solder joints [7], [11], [12], since Ag₃Sn plates precipitate in the middle of cooling stage [13]; the second way is to control the concentration of Ag to be less than 2.7 wt.% [14]; the third way is to dope solders with nanoparticles, such as ZnO, SiC and TiO₂, to refine Ag₃Sn phases [15], [16], [17] or alloying elements, such as Ni, Pr and Zn, to inhibit the formation of Ag₃Sn plates [18], [19], [20]. However, the case of ultra-fine solder joints is more complicated as the consumption of Sn to form interfacial intermetallic compounds (IMCs) will cause an increase in Ag concentration, which facilitates the formation of large Ag₃Sn plates [21]. Thus, it is necessary to better understand the formation of large Ag₃Sn plates in ultra-fine solder joints. Furthermore, synchrotron radiation imaging technology was proved to be an effective method to observe the interfacial reaction in solder joints [22], [23], [24].

In the present work, the dissolution and precipitation behavior of Ag₃Sn plates in Sn–4.0Ag–0.5Cu/Ni–P ultra-fine solder bumps, as well as the undercoolings of Ag₃Sn plates and solder bumps, was in situ investigated using synchrotron radiation X-ray real-time imaging technology. The nucleation mechanism of the Ag₃Sn phase was also discussed.

Section snippets

Experimental

Sn–4.0Ag–0.5Cu/Ni–P solder bumps of 45 μm in diameter and 100 μm in pitch on flip chips of 5 mm × 5 mm in size were supplied by Fraunhofer IZM. The synchrotron radiation experiment was carried out at the BL13W1 beam line of Shanghai synchrotron radiation facility. Fig. 1 shows the schematic of the experiment configuration. X-ray beam with an energy of 25 keV transmitted through the solder bumps was collected by a charged couple device (CCD) camera with a resolution of 0.74 μm/pixel. The CCD camera

Results and discussion

Fig. 2 shows the synchrotron radiation images of the solder bumps on a chip without experiencing the reflow process and the top-view SEM images of IMC phases in the corresponding bumps with the solders etched away. Large rectangular plates with a thickness of 5 μm and a size of 30 μm grew on the interfacial IMCs. EDX analysis identified the average composition of the large plates as 74.05%Ag–25.95%Sn (in at.%), i.e., Ag₃Sn phase. In addition, the average composition of the interfacial IMCs on the

Conclusions

The dissolution and precipitation behavior of Ag₃Sn plates in sub-50 μm Sn–4.0Ag–0.5Cu flip chip solder joints, as well as the undercoolings of Ag₃Sn plates and solder bumps, were in situ studied using synchrotron radiation real-time imaging technology. The large Ag₃Sn plates had a dissolution rate of 0.1–0.2 μm/s at the heating stage, while their precipitation rate exceeded 5 μm/s at the cooling stage. The undercooling of the Ag₃Sn plates was in the range of 31–55 °C, which is slightly smaller

Acknowledgments

This work is supported by the National Natural Science Foundation of China under Grant No. 51171036 and the BL13W1 beam line of Shanghai Synchrotron Radiation Facility.

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Effects of cooling rate and joint size on Sn grain features in Cu/Sn–3.5Ag/Cu solder joints
2020, Materialia
Citation Excerpt :
It is also interesting that large Ag3Sn plates tend to precipitate under a low cooling rate of 0.1 °C/s regardless of joint size, as shown in Figs. 3, 7 and 11. Note that the nucleation of Ag3Sn plate-like phase is known to be prior to Sn and once one Ag3Sn plate precipitates, the precipitation of Ag3Sn plates at the other sites nearby will be suppressed [24]. Thus, the low cooling rate, on one hand, provides Ag atoms plenty of time to diffuse towards the precipitated Ag3Sn plate to sustain it growing.
In Sn-based micro solder joints for fine pitch flip chip or 3D packaging technologies, the number of Sn grains is always limited and sometimes only one Sn grain exists in a joint, which inevitably causes anisotropy in the performance and reliability of micro interconnection. It is known that the parameters of solder joint structure and reflow process, especially cooling stage, have a great significance for regulating the features of Sn grains after soldering. Hence, in the present paper, the Sn grain features of Cu/Sn–3.5Ag/Cu solder joints with different joint sizes (300 μm, 100 μm and 50 μm) solidified under different cooling rates (1.5 °C/s, 0.5 °C/s and 0.1 °C/s) were studied. Generally, the number of Sn grains decreased as the joint size decreased. When the joint size was 50 μm, the solder joints tended to contain only one or two Sn grains irrespective of the cooling rate. Furthermore, cooling rates greatly affected the number and orientation of Sn grains as well as the growth of large Ag₃Sn intermetallic plates to varying degrees. The formation of Sn grains and Ag₃Sn plates was discussed from the viewpoint of nucleation under different conditions.
In situ observation on the solidification of Sn-10Cu hyperperitectic alloy under direct current field by synchrotron microradiography
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Citation Excerpt :
For high-temperature solders application, higher Cu content is required, while the solders beyond 7.6 wt%Cu content involve a peritectic reaction causing the formation of the IMC Cu3Sn, which has negative effects on solders performance. Up to now, the growth behavior of IMCs in the Sn/Cu [21–23], Sn-0.7Cu/Cu [24,25], Sn–Ag/Cu [26,27] soldering reaction have been in situ investigated by synchrotron radiation real-time imaging technology, which have provided direct proofs for better understanding the growth behavior of intermetallic compounds (IMCs) [26,28,29]. Synchrotron radiation imaging technology has become a powerful tool for in situ observation of alloys solidification by time-resolved high-resolution imaging over the past decade [30,31].
The structural evolution of peritectic solidification under the direct current (DC) field action, mainly on the morphological evolution of primary phase ε (Cu₃Sn) and peritectic phase η (Cu₆Sn₅), is in situ observed by high resolution X-ray real-time imaging with synchrotron radiation. The ultimate solidification microstructures after experiencing the imaging experiment are investigated by X-ray diffraction (XRD) and scanning electron microscopy (SEM) with energy dispersive X-ray spectroscopy (EDS). It is found that the η phase precipitates directly from parent liquid phase without experiencing corresponding peritectic reaction when the DC field of 20 A/cm² or 200 A/cm² is imposed during the solidification process of the Sn-10%Cu alloy. It means that the DC field has altered the phase transformation sequence and suppressed the nucleation of primary ε phase. Meanwhile, the growing and dissolving behaviors of secondary dendrite arm for intermetallic compound Cu₆Sn₅ under the DC field action are also in situ observed and discussed.
Drop failure modes of Sn–3.0Ag–0.5Cu solder joints in wafer level chip scale package
2016, Transactions of Nonferrous Metals Society of China (English Edition)
To reveal the drop failure modes of the wafer level chip scale packages (WLCSPs) with Sn–3.0Ag–0.5Cu solder joints, board level drop tests were performed according to the JEDEC standard. Six failure modes were identified, i.e., short FR-4 cracks and complete FR-4 cracks at the printing circuit board (PCB) side, split between redistribution layer (RDL) and Cu under bump metallization (UBM), RDL fracture, bulk cracks and partial bulk and intermetallic compound (IMC) cracks at the chip side. For the outmost solder joints, complete FR-4 cracks tended to occur, due to large deformation of PCB and low strength of FR-4 dielectric layer. The formation of complete FR-4 cracks largely absorbed the impact energy, resulting in the absence of other failure modes. For the inner solder joints, the absorption of impact energy by the short FR-4 cracks was limited, resulting in other failure modes at the chip side.
Thermomigration-induced asymmetrical precipitation of Ag<inf>3</inf>Sn plates in micro-scale Cu/Sn-3.5Ag/Cu interconnects
2016, Materials and Design
Citation Excerpt :
Concerning the reliability of micro-scale interconnects, it is necessary to better understand the precipitation mechanism of large Ag3Sn plates under the temperature gradient. It has been reported that Ag3Sn plates randomly precipitated at the solder/IMC interface of solder joints [13]. However, the present work shows that the precipitation of Ag3Sn plates in Cu/Sn–3.5Ag/Cu micro-scale interconnects under the temperature gradient is asymmetrical, revealed by in situ observation using synchrotron radiation X-ray real-time imaging technology.
Understanding the microstructural changes induced by thermomigration during thermal bonding process is important to micro-scale solder interconnects. In the present work, the precipitation of Ag₃Sn plates in 50 μm Cu/Sn–3.5Ag/Cu interconnects under thermomigration has been in situ characterized using synchrotron radiation real-time imaging technology. The precipitation of Ag₃Sn plates is found to be asymmetrical, i.e., Ag₃Sn plates preferentially precipitate on the cold end rather than the hot end in the interconnects; even if precipitate on the hot end, the Ag₃Sn plates tend to dissolve into the molten solder during the subsequent cooling. It is shown that the asymmetrical precipitation of Ag₃Sn plates is induced by directional thermomigration of Ag atoms toward the cold end under the temperature gradient across the interconnects, and the simulated temperature gradient is 172 °C/cm in the present work. The asymmetrical precipitation of Ag₃Sn plates is analyzed and discussed from the viewpoint of thermomigration-affected nucleation and flux-controlled growth.
Early stage growth characteristics of Ag<inf>3</inf>Sn intermetallic compounds during solid-solid and solid-liquid reactions in the Ag-Sn interlayer system: Experiments and simulations
2014, Journal of Alloys and Compounds
Citation Excerpt :
After knowing the kinetics, lifetime of these modules could be estimated by defining a critical IMC layer thickness. Huang et al. [17] performed in-situ synchrotron radiation experiments on Sn–4.0Ag–0.5Cu solder bumps at a peak temperature of 250 °C and determined the precipitation rate of Ag3Sn plates as 5 μm/s whereas the dissolution rate was only 0.1–0.2 μm/s. Once nucleation of Ag3Sn occurs, the phase continues growing rapidly and dissolution into liquid Sn can be neglected.
Transient Liquid Phase Bonding (TLPB) makes use of the isothermal solidification of a liquid interlayer through the formation of intermetallic compounds (IMC). This work investigates the Ag₃Sn IMC formation in a thin, electroplated Ag–Sn interlayer system appropriate for TLPB – while covering as-coated conditions, growth initiation up to a few minutes of holding time – with experimental and numerical methods. Both solid-state and solid–liquid reactions at 200, 250 and 300 °C, respectively, were covered under identical and near-isothermal conditions with a novel experimental setup. Morphological aspects like the lateral IMC grain size or the scallop roughness were proven to strongly depend on temperature. Growth kinetics were captured by power laws, growth exponents n were determined as 0.24, 0.36 and 0.29, parabolic growth constants k as 1.32 ⋅ 10⁻¹¹, 4.04 ⋅ 10⁻¹⁰ and 7.24 ⋅ 10⁻¹⁰ cm²/s at 200, 250 and 300 °C, respectively, and the activation energy Q as 30.6 kJ/mol. Predominant diffusion mechanisms, i.e. volume or GB diffusion, as well as coarsening effects were identified as temperature and time dependent at early stages. A mathematical model was developed based on the Arrhenius relationship including a growth correction function z_IMC_,_corr to calculate and predict IMC growth as a function of temperature and time. The study was completed by multiphase-field simulations which targeted to mimic kinetic and morphological features of Ag₃Sn IMC under various conditions. Good agreement between experiments and simulations was found for a limited set of parameters which allowed drawing conclusions about the dominant transport mechanisms for the reacting elements.
In Situ Observation of Electromigration-Induced Anomalous Precipitation of Ag<inf>3</inf>Sn Phase in Ag-Containing Solder Joints
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Journal of Alloys and Compounds

LetterSynchrotron radiation real-time in situ study on dissolution and precipitation of Ag3Sn plates in sub-50 μm Sn–Ag–Cu solder bumps

Abstract

Introduction

Section snippets

Experimental

Results and discussion

Conclusions

Acknowledgments

Mater. Des.

Int. J. Electrochem. Sci.

Scripta Mater.

Microelectron. Reliab.

J. Alloys Comp.

Microelectron. Reliab.

Mater. Sci. Eng. A

J. Alloys Comp.

Mater. Sci. Eng. A

Mater. Des.

Mater. Des.

J. Alloys Comp.

J. Alloys Comp.

Effects of cooling rate and joint size on Sn grain features in Cu/Sn–3.5Ag/Cu solder joints

In situ observation on the solidification of Sn-10Cu hyperperitectic alloy under direct current field by synchrotron microradiography

Drop failure modes of Sn–3.0Ag–0.5Cu solder joints in wafer level chip scale package

Thermomigration-induced asymmetrical precipitation of Ag<inf>3</inf>Sn plates in micro-scale Cu/Sn-3.5Ag/Cu interconnects

Early stage growth characteristics of Ag<inf>3</inf>Sn intermetallic compounds during solid-solid and solid-liquid reactions in the Ag-Sn interlayer system: Experiments and simulations

In Situ Observation of Electromigration-Induced Anomalous Precipitation of Ag<inf>3</inf>Sn Phase in Ag-Containing Solder Joints

Letter
Synchrotron radiation real-time in situ study on dissolution and precipitation of Ag₃Sn plates in sub-50 μm Sn–Ag–Cu solder bumps