Electromigration In Solder Joints Research Articles

Effect of Stress Interaction on Multi-Stress Accelerated Life Test Plan: Assessment Based on Particle Swarm Optimization

Sustainability of products that seek to maintain ecosystem balance, such as electric vehicles or solar system inverters, often require extensive testing during their developmental stages in a manner that minimizes wastage and drives creativity. Multi-stress accelerated test planning is often used for these products, their subsystems and components if their in-service failures are driven by multiple stress factors. Multi-stress accelerated life testing (ALT) often expedites time to failure for highly reliable products. Many studies assume model parameters that may not be appropriate for the considered stress factors. Most importantly, the effect stress interaction has on the ALT plan is often ignored, especially for cases where historical data are lacking. To address this gap, in this work, a technique based on a combination of rapid experimental data collection and heuristic-based optimization is proposed for ALT planning. In addition, the effect of stress interaction on the ALT plan was also evaluated. Specifically, the Arrhenius model was used to develop a maximum likelihood mathematical expression for multi-stress factor scenarios with and without interaction. Subsequently, two optimization stages based on particle swarm optimization (PSO) were carried out using time varying inertia weight constants to drive early and late global and local searches, respectively. In the first stage, model parameters were estimated, while, in the second stage, an ALT optimal plan was generated based on a D-optimality criterion. Verification of stress factor interactions was carried out using graphical response analysis. An experiment, designed to investigate electromigration in solder joints under three stress factors (temperature, current density and mechanical load), was used to validate the study. The variation in the choice of Latin hypercube design (LHD) results in disparity in the levels of stress within each stress combination as well as sample allocation. Our results clearly show the need to investigate stress interactions prior to multi-stress acceleration planning.

Electromigration in Solder Joints Using Sn-3.0Cu Solder in Power Modules

パワーモジュールのはんだ接合部エレクトロマイグレーション(EM)が信頼性問題と認識されている。本論文では，高温はんだSn-3.0Cuを用いたCu/Ni-P/Sn-3.0Cu/Ni-P/Cu接合部のEM基本現象(175°C, 50 A/mm2)を明らかにする。リフロー後の電極界面には，厚くかつCuリッチな(Cu,Ni)6Sn5層と薄いPリッチ層が形成された。この構造的特徴が，カソード側でのEM起因の断線原因である(Cu,Ni)6Sn5層消失とPリッチ層成長を抑制することを明らかにした。

Effect of Grain Structure and Ni/Au-UBM Layer on Electromigration-Induced Failure Mechanism in Sn-3.0Ag-0.5Cu Solder Joints

The development of advanced electronic devices leads to highly miniaturized interconnect circuits (ICs), which significantly increases the electromigration (EM) phenomenon of solder and circuits due to higher current density. The electromigration of solder joints under high current density has become a severe reliability concern in terms of microelectronic product reliability. The microstructure of the solder plays an important role in the electromigration induced degradation. In this study, Sn-3.0Ag-0.5Cu solder bumps with Ni/Au under bump metallization (UBM) layer were fabricated and electromigration acceleration tests were conducted under current density of 1.4 × 104 A/cm2 and 120 °C to investigate the effect of grain structure and Ni/Au-UBM layer on EM-induced failure. Grain structures of solder bumps were determined by utilizing the Electron Backscatter Diffraction (EBSD) technique, and single-crystal solder, single-crystal dominated solder, and polycrystalline solder are observed in different test samples. According to the Scanning Electron Microscope (SEM) images, it is observed that the Ni/Au-UBM layer of the Cu pad can inhibit atom diffusion between solder bump and Cu pad, which reduces the consumption of Cu pad but causes a large void and crack at the interface. The EM lifetime of single crystal solder bumps is lower than that of polycrystalline solder bumps when the c-axis of single crystal solder bumps is perpendicular to the electron flow direction. Additionally, the single crystal structure will increase the brittleness of the solder bump, and cracks are easily generated and expanded under the stress caused by the mismatch of thermal expansion coefficients between the solder bump and Ni/Au-UBM layer near Cu pad. Polycrystalline solder bumps with a higher misorientation angle (15–55°) have a higher atom diffusion rate, which will result in the acceleration of the EM-induced failure.

Effect of Sb Addition on Electromigration in Solder Joints

Effect of Sb Addition on Electromigration in Solder Joints

Electromigration in solder joints: A cross-sectioned model system for real-time observation

With the increasing demands and consequent miniaturization of electronic devices, electromigration is posing a challenge to the reliability not only of integrated circuits but also of the electronic packages. We report a methodology that allows for quasi-continuous in-situ optical microscopic imaging of a cross-sectioned solder joint (sample) during testing for electromigration. A SAC305 solder joint in a flip configuration was coated in epoxy and cross-sectioned. Current stressing was applied to producing electromigration, with a theoretical current density of 1.37 × 104 A/cm2 for 5 A of current. In-situ optical microscopic imaging and resistance measurements were performed simultaneously until an open circuit was observed.A crack and void started to form in an example experiment, and their evolution was recorded using the real-time imaging. The void and the crack continued growing until the end of current stressing. The void area at the cross-section grew to 1500 μm2 while the crack grew to a length of 110 μm. After 330 h, the experiment was stopped as an open circuit occurred, which was preceded by a 10% rise of the resistance, indicating failure due to electromigration. During the end of the test, the test vehicle temperature increased by 20 °C as a result of increased Joule heating due to the loss in cross-sectional area of the solder joint. The location of the void did not agree with the expected current crowding location at the solder joint corner. It was inferred that the formation of cracks due to internal stresses prompted the formation of the void at that location that resulted in the open circuit, demonstrating the interconnection between the various failure mechanisms caused by EM. Microstructural changes were also monitored after the open circuit and were possibly due to the thermal stress occurring once the sample cooled down. Hillock formation and additional cracks were observed 30 min after the open circuit. The results highlight the unique capacity of real-time imaging to support more detailed electromigration research in solder joints by tracing the source of specific failure events.

Electromigration in Solder Joints

Electromigration in Solder Joints

Electromigration behavior in Cu/Ni–P/Sn–Cu based joint system with low current density

Although electromigration in solder joints has great influence on reliability, few study has been reported on the Cu/Ni–P/Sn–Cu based joint system electromigration with realistic current density range lower than 10kA/cm2. We investigated a Cu/Ni–P/Sn–0.7Cu/Ni–P/Cu joint with current densities of 5.0 and 7.5kA/cm2 at 423K. Solder joint breakdown at the cathode side was detected for both stress conditions. Ni–P plating disappeared completely at the cathode side and a Cu–Sn intermetallic compound (IMC) formed at the interface. Cu–P IMC formed on the solder breakdown interface. Ni diffusion in Ni–P plating at the cathode was accelerated and the P-rich layer grew thicker than at the anode side before breaking down under electromigration stress. The P-rich layer reached the Cu electrode resulting in cracking along the interface between solder layer and Cu. Sn was diffused from the Ni3SnP IMC to the P-rich layer cracks and formed Cu3Sn IMC with the Cu electrode. Thus, the electromigration mechanism in an electroless Ni–P plating/Sn–Cu based joint system with low current density was clarified.

Solder electromigration behavior in Cu/electroless Ni–P plating/Sn–Cu based joint system at low current densities

Electromigration (EM) in solder joints has great influence on their reliability. Nevertheless, few reports have been published on the EM in solder joints with Ni–P barrier layers at lower current densities less than 10 kA/cm2. In the present study, EM in Cu/Ni–P/Sn–0.7Cu/Ni–P/Cu joints was investigated at 150 °C with current densities of 5.0 and 7.5 kA/cm2. The breakdown mode was open failure of the solder joint on the cathode. It was found that Ni in the Ni–P barrier layer diffused toward the anode, resulting in a thicker P-rich layer, which caused the cracks and the delamination of the P-rich layer. Additionally, the diffusion of Sn detached the solder from the Ni3SnP intermetallic compound on the cathode.

Abnormal depletion of Cu metallization pads in line–bump–line solder joints under electron current stressing

We observed abnormal electromigration-induced Cu depletion in a traditional line–bump–line (Cu/Sn/Cu) solder joint with extremely high Cu depletion away from the current crowding region (CCR). Electron backscatter diffraction analyses showed that the β-Sn crystallographic orientation of the solder alloy is responsible for this abnormal electromigration phenomenon. A pronounced depletion in Cu occurred when the Cu neighbored the Sn grains with c-axes parallel to electron flow, despite the fact that the current density distributed in this type of area was one order-of-magnitude lower than that of the CCR. This observation indicated that the current stress and β-Sn crystallographic orientation played dominant roles in Cu depletion during electron current stressing. Finally, a new failure mechanism is proposed for electromigration in solder joints.

Experimental and simulation analysis of concave-down resistance curve during electromigration in solder joints

Resistance curves play a crucial role in detecting damage of solder joints during electromigration. In general, resistance increases slowly in the beginning, and then rises abruptly in the very late stage; i.e., the resistance curve behaves concave-up. However, several recent studies have reported concave-down resistance curves in solder joints with no satisfactory explanation for the discrepancy. In this study, electromigration failure mode in Sn2.5Ag solder joints was experimentally investigated. The bump resistance curve exhibited concave-down behavior due to formation of intermetallic compounds (IMCs). In contrast, the curve was concave-up when void formation dominated the failure mechanism. Finite element simulation was carried out to simulate resistance curves due to formation of IMCs and voids, respectively. The simulation results indicate that the main reason causing the concave-down curve is rapid formation of resistive Cu6Sn5 IMCs in the current-crowding region, which are 9 times larger than Cu IMCs. Therefore, when Cu reacted with Sn to form Cu6Sn5 IMCs, resistance increased abruptly, resulting in the concave-down resistance curve.

A Canary Design to Monitor Electromigration of Solder Joints

Electromigration-induced failure has become one of the most serious problems in solder joints. In this paper, a canary device has been designed to monitor the eletromigration of solder joints. Then the current of the canary device is calculated. The canary solder joints are identical to those used in the host circuit but suffered properly larger environmental stress to accelerate aging so they are supposed to fail before those in the host circuit. For this reason the canary device are designed to be sacrificial, and are not used in the circuit. Instead it is packaged separately in order not to influence the normal operation of the host circuit. How to apply sufficient, but not overly large, stress current to the canary device to enhance its electromigration failure without stressing other part of the circuit becomes one of the problems we need to solve. In this paper a canary circuit is designed to solve this problem. Also the increased stress we need was calculated. With the help of the canary device�E we can be warned before the solder joints fail to ensure the system health.

Electromigration in Solder Joints for High Temperature Flip-Chip Application

The electromigration effects in chip metallization and wire bonds are well known and detailed investigated. Current density could be extremely high because of the small size of the cross sectional area of conductors. This can cause a migration of metal atoms toward the electrical field, so current densities up to 106 A/cm2 are possible. In comparison with chip structures are the usual solder joints of flip chips relatively thick. But the homologue temperature of solder alloys, typically based on tin, is also much higher than for gold or aluminum wires. For instance a SAC solder alloy is naturally preheated up to 0.6 homologue temperature, for high temperature application with 125 °C operating temperature even more than 0.8. This means, that atoms are very agile and a directed movement needs only lower field strength. Additionally is the specific resistance of solder alloys tenfold higher than for aluminum, copper or silver. So is the self-heating of solder joints not negligible. This contribution shows the test results of flip-chip assemblies, loaded with different current densities and stored at 125 °C ambient temperature. At the end of life of a significant number of test chips, a metallographic analysis shows the causing failure effects and weak spots of assemblies. Accompanying simulations help to explain the interaction between current density and migration effects.

Finite element modeling on electromigration of solder joints in wafer level packages

This work studies the electromigration of solder joints in an encapsulated copper post wafer level package (WLP) by finite element modeling. Experimental data showed that the electromigration failure occurs in solder joints on the printed circuit board (PCB) side due to the current crowding. In order to improve the electromigration performance on the PCB side with a copper post WLP, two new line-to-bump geometry designs are proposed. Coupled electro-thermal finite element modeling is performed to obtain the electrical and thermal fields simultaneously. The ionic flux from electron wind and thermal response is calculated based on finite element solutions. The divergence of the total flux, which is the sum of the divergence of electromigration and thermomigration, is extracted at the critical locations in solder joints. Results show that the new proposed design structures can reduce the maximum current density by 19%, and the divergence of the total ionic flux by 42%. Thermal gradient is very small in solder joints, therefore, the main driving force for electromigration failures comes from the electron wind. The finite element results on mesh dependency are discussed in this paper.

Study of DC and AC Electromigration Behavior in Eutectic Pb-Sn Solder Joints

In this study, direct current (DC) and alternating current (AC) electromigration experiments were carried out using solder joints with a Cu/eutectic Pb-Sn/Cu joint configuration. During stressing using DC and AC, a fixed current density of 104 A/cm2 was applied to the joints at 150°C. In the joints stressed by DC, electromigration-induced damage occurred, and the corresponding microstructural changes mainly included valley and hillock formation in the solder region, pronounced phase segregation of Pb-rich and Sn-rich domains, asymmetric growth of Cu-Sn intermetallics at the interfaces, and excessive depletion of Cu at the cathode side. In contrast, no significant electromigration was observed after the AC treatments. This was especially true for the treatment with AC frequencies higher than 1 h−1. The dependence of the damage on AC frequency suggests that electromigration in solder joints can be inhibited to a large extent when a proper reverse current is delivered.

Failure Mechanism of Electromigration in Solder Joint

In advanced electronic products, electromigration-induced failure is one of the most serious problems in fine pitch flip chip solder joints because the design rule in devices requires high current density through small solder joints for high performance and miniaturization. The failure mode induced by electromigration in the flip chip solder joint is unique, owing to the loss of under bump metallurgy (UBM) and the interfacial void formation at the cathode contact interface. In this study, Electromigration of flip chip solder joints has been investigated under a constant density of 2.45×104 A/cm2 at 120 °C. The in-situ marker displacements during the electromigration test was measured and found to show a rough linear change as a function of time. Scanning electron microscopic images of the cross section of samples showed the existence of voids at the interface between Al interconnection and under bump metallurgy. The void movement was matched with the marker displacements during the electromigration test, and voids moved to the cathode interface between Al interconnection and under bump metallurgy in the downward electron flow (from chip to substrate) joint. The mechanism of electromigration-induced void migration and failure in the flip chip are discussed. During electromigration, a flux of atoms is driven from the cathode to the anode or a flux of vacancies in the opposite direction. It can lead to two possible mechanisms of void migration. First, if we regard the void as a rigid marker of diffusion, it will be displaced towards the cathode by the atomic flux in the electromigration, Second, if we consider surface diffusion on the void surface, electromigration will drive atoms on the top surface of the void to the bottom surface of the void, and consequently the void will move towards the cathode.

Investigation of void nucleation and propagation during electromigration of flip-chip solder joints using x-ray microscopy

X-ray microscopy was employed to investigate void nucleation and propagation during electromigration in solder joints. The shape of the voids at various stages can be clearly observed. The voids became irregular when they propagated to deplete the contact opening. Growth velocity at the early stage was found to be 1.3μm∕h under 6.5×103A∕cm2 at 150°C, and it decreased to 0.3μm∕h at later stages. Formation of intermetallic compound (IMC) and compositional changes at the interface of solder/IMC on the chip side were attributed to the retarded growth rate at later stages.

Electromigration issues in lead-free solder joints

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.

Electromigration in solder joints and solder lines

Electromigration may affect the reliability of flip-chip solder joints. Eutectic solder is a two-phase alloy, so its electromigration behavior is different from that in aluminum or copper interconnects. In addition, a flipchip solder joint has a built-in currentcrowding configuration to enhance electromigration failure. To better understand electromigration in SnPb and lead-free solder alloys, the authors prepared solder lines in v-grooves etched on Si (001). This article discusses the results of those tests and compares the electromigration failure modes of eutectic SnPb and SnAgCu flip-chip solder joints along with the mean-timeto-failure.