Elsevier

Journal of Alloys and Compounds

Volume 673, 15 July 2016, Pages 372-382
Journal of Alloys and Compounds

Phase segregation, interfacial intermetallic growth and electromigration-induced failure in Cu/In–48Sn/Cu solder interconnects under current stressing

https://doi.org/10.1016/j.jallcom.2016.02.244Get rights and content

Highlights

  • Sn migrates to the anode, while In migrates to the cathode, during EM in Cu/In–48Sn/Cu.

  • The atomic flux of Sn has been calculated.

  • The interfacial IMCs were identified as: Cu6(Sn,In)5 + Cu(In,Sn)2.

  • The interface evolution is strongly influenced by the migration of Cu and the accumulation of Sn-rich and In-rich layers.

  • The failure is attributable to the cathode Cu dissolution-induced liquid-state EM.

Abstract

The evolution of microstructure in Cu/In–48Sn/Cu solder bump interconnects at a current density of 0.7 × 104 A/cm2 and ambient temperature of 55 °C has been investigated. During electromigration, tin (Sn) atoms migrated from cathode to anode, while indium (In) atoms migrated from anode to cathode. As a result, the segregation of the Sn-rich phase and the In-rich phase occurred. A Sn-rich layer and an In-rich layer were formed at the anode and the cathode, respectively. The accumulation rate of the Sn-rich layer was 1.98 × 10−9 cm/s. The atomic flux of Sn was calculated to be approximately 1.83 × 1013 atoms/cm2s. The product of the diffusivity and the effective charge number of Sn was determined to be approximately 3.13 × 10−10 cm2/s. The In–48Sn/Cu IMC showed a two layer structure of Cu6(Sn,In)5, adjacent to the Cu, and Cu(In,Sn)2, adjacent to the solder. Both the cathode IMC and the anode IMC thickened with increasing electromigration time. The IMC evolution during electromigration was strongly influenced by the migration of Cu atoms from cathode to anode and the accumulation of Sn-rich and In-rich layers. During electromigration, the Cu(In,Sn)2 at the cathode interface thickened significantly, with a spalling characteristic, due to the accumulation of In-rich layer and the migration of Cu atoms - while the Cu(In,Sn)2 at the anode interface reduced obviously, due to the accumulation of Sn-rich layer. The mechanism of electromigration-induced failure in Cu/In–48Sn/Cu interconnects was the cathode Cu dissolution-induced solder melt, which led to the rapid consumption of Cu in the cathode pad during liquid-state electromigration and this finally led to the failure.

Introduction

Recently, eutectic indium-tin (In-48 wt.% Sn) solder has attracted considerable attention - due to its low melting point (118 °C), great ductility, long fatigue life and excellent wettability [1], [2], which makes it a promising candidate for Pb-free solders. Considering its excellent properties, eutectic In–48Sn solder is promising for applications in advanced and specialized electronic products, such as heat sensitive devices, optoelectronics (OE) modules [3], micro-opto-electromechanical systems (MOEMS) packaging [4], laser die bonding [5] and chip-on-glass bonding [6]. In recent years, studies on the microstructure and properties of In–48Sn solder joints on various substrates have been widely reported [7], [8], [9]. Copper (Cu) is the most commonly used substrate material in the electronics industry. In the interfacial reaction between In–48Sn and Cu substrates, both In and Sn may react with Cu to form several types of interfacial intermetallic compounds (IMCs) [10], which makes the In–48Sn/Cu solder joints exhibit particular and complicated microstructure. Jung et al. [11] reported that the IMCs at an In–48Sn/Cu interface were composed of two phases - Cu(In,Sn)2, adjacent to the solder, and Cu6(Sn,In)5, adjacent to the Cu, during aging at temperatures between 70 °C and 100 °C. Chuang et al. [12] identified that a planar layer of Cu3(Sn,In) at the Cu side and a scalloped layer of Cu6(Sn,In)5 at the solder side were formed at the eutectic In–Sn/Cu interface, during reflow at temperatures ranging from 150 °C to 400 °C. Shang et al. [13] reported that the Cu2(In,Sn) was identified as the only IMC at the In–48Sn/Cu interface during solid-state aging at 100 °C. Liu et al. [14], [15] investigated the IMC growth between In–48Sn solder and single crystalline Cu substrates. The interfacial IMC showed a two layer structure of Cu(In,Sn)2 at the solder side and the Cu2(In,Sn) at the Cu substrate side. It is notable that the phase transformation between Cu(In,Sn)2 and Cu2(In,Sn) was observed [16]. The growth orientation relationships between Cu substrate and Cu2(In,Sn) IMC and the growth mechanism of Cu2(In,Sn) were reported [17]. Wojewoda-Budka et al. [18] found that the shear strength of Cu/In–48Sn/Cu thin film interconnects was related to the type of IMC filling the solder interconnect region. The solder interconnects with Cu41(Sn,In)11 IMC showed higher strength than those with Cu6(Sn,In)5.

Previous studies have focused mainly on the interfacial IMC identification, the kinetics of the interfacial reaction and the mechanical properties of In–48Sn/Cu solder joints. The investigation of the electromigration of In–48Sn solder interconnects has been little reported. Electromigration refers to an enhanced diffusion process of metal atoms under the impact of high direct current stressing [19]. During electromigration, atoms of different materials inside the solder interconnect may migrate downwind to the anode side or upwind to the cathode side, respectively, under the effect of current stressing, which may lead to microstructure evolution in the interconnect and finally lead to electromigration failure [20]. The electromigration process is strongly influenced by the current density and temperature carried by the solder interconnects [21], which makes it one of the most serious reliability issues for advanced electronic packaging associated with the miniaturization trend of electronic products. In the past several decades, the size of electronic solder interconnects has down-scaled continuously. For example, the diameter of a flip-chip solder bump is currently about 100 μm - and it will possibly be reduced to 1 μm in the future [22]. The downsizing of the solder interconnects has led to a massive increase in the current density within the solder interconnects and has further increased the operating temperature - due to serious Joule heating, which has significantly accelerated the electromigration process and increased the potential for electromigration failure [20].

Understanding the electromigration behavior of In–48Sn solder interconnects is vital for their application. Very few studies reported the electromigration of In–48Sn solder interconnects. The opposite migration directions for Sn and In atoms was reported [23], [24], [25], in which Sn atoms preferentially migrated toward the anode while In atoms migrated toward the cathode. Our previous research [24] investigated the electromigration in In–48Sn interconnects with Au/Ni/Cu pads - under 0.7 × 104 A/cm2 at room temperature. The investigation revealed that the migration of Sn atoms leads to the accumulation of a Sn-rich layer at the anode side. The accumulation rate of the Sn-rich layer and the atomic flux of Sn were calculated to be 7.85 × 10−10 cm/s and 7.27 × 1012 atoms/cm2s, respectively. Huang et al. [25] analyzed the interfacial IMC growth in Cu/In–48Sn/Cu line-type interconnects during electromigration under 2.0 × 104 A/cm2 at 90 °C. The IMCs at both the anode and the cathode interfaces were composed of Cu2(In,Sn), adjacent to the Cu - and Cu(In,Sn)2, adjacent to the solder, during the early stages of electromigration. With increasing electromigration time, the anode IMC was entirely transformed into Cu2(In,Sn), due to the net loss of In atoms.

Previous studies lacked accurate identification of In–48Sn/Cu IMCs. In addition, investigations of the microstructure evolutions during electromigration and the mechanisms of electromigration failure in Cu/In–48Sn/Cu solder bump interconnect have never been reported. In the present study, the effect of electromigration on the microstructure evolution of Cu/In–48Sn/Cu solder bump interconnects was investigated. The focus was on phase segregation, interfacial IMC identification, IMC growth and electromigration failure analysis.

Section snippets

Experimental procedures

To prepare the solder bump interconnect sample, two FR4 printed circuit boards (PCBs), with dimensions of 10 × 10 × 1.5 mm3 and 24 × 10 × 1.5 mm3, were employed as the chip and the substrate, respectively. The Cu trace on the surface of both the PCBs was 35 μm in thickness and 620 μm in width. A thin layer of solder mask covered the Cu trace and the PCB. The solder-mask-opening on the Cu trace defined pads with a diameter of 220 μm and pitch of 1027 μm. In order to form the interconnections

Current density and temperature distributions

During electromigration, the atomic migration inside a solder interconnect is strongly influenced by the current density and the temperature. In this study, the average current density in the solder bumps was 0.7 × 104 A/cm2. Due to current crowding effects [29], a non-uniform distribution of current density existed in the daisy-chain interconnection structure, as shown in Fig. 2a. As the electrons passed through the Cu trace and solder bump interconnects, Joule heat was generated and elevated

Conclusions

The microstructural evolution in Cu/In–48Sn/Cu solder bump interconnects during electromigration under current stressing of 0.7 × 104 A/cm2 and at an ambient temperature of 55 °C, for various time periods, has been investigated. The main conclusions obtained are as follows:

1. During electromigration, Sn and In atoms migrate along opposite directions and the segregation of Sn-rich phase and In-rich phase occurs. Sn is the “dominant diffusing species” - and migrates along the direction of

Acknowledgments

The authors would like to acknowledge the financial support provided by the National Natural Science Foundation of China and the Research Grants Council of Hong Kong (NSFC/RGC Joint Research Scheme): Project Ref. No. N_CityU 101/12; CityU Ref. No. 9054008; Electromigration and Thermomigration Studies in Nanostructured Composite Electronic Interconnects for Nanoelectronics Applications. The authors would like to acknowledge the School of Materials Science & Engineering at Nanyang Technological

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