Electrochemical Migration Process Research Articles

A phase-field model of electrochemical migration for silver-based conductive adhesives

Electrochemical migration failure is the main reliability problem that silver conductive adhesives face in the field of power electronics. However, there is no prior existing study that describes the electrochemical migration process of silver conductive adhesive using the phase-field model. In this study, a phase-field model is proposed to study the electrochemical migration process of silver-based conductive adhesive. The model involves the analysis of electrochemical reactions, diffusion of metal ions, applied electrode potential and overpotential and the morphology of electrochemically migrating dendrites. Model-building is based on the water drop (WD) test of silver conductive adhesive. The simulation results of the phase-field model not only reproduce the morphology of the silver dendrite, but also realize the critical voltage phenomenon in the WD experiments. In addition, by comparing the results of WD experiments as well as the model simulations, we demonstrate that the phase-field model is able to predict the electrochemical migration failure time of silver conductive adhesives effectively. The proposed model may be a helpful tool for understanding and predicting the evolution of dendrites during the electrochemical migration of silver-based conductive adhesives.

Electrochemical corrosion and electrochemical migration characteristics of SAC-1Bi-xMn solder alloys in NaCl solution

The effects of Ag and Mn content were investigated from the electrochemical corrosion (ECC) and electrochemical migration (ECM) point of view in the case of SAC0307–1Bi-xMn and SAC305 lead-free solder alloys in NaCl solution. On the one hand, long-term results showed that lower silver content showed higher corrosion resistance. On the other hand, the microstructure and elemental composition of the dendrites formed after the ECM test were investigated by transmission electron microscopy (TEM) and energy dispersive X-ray spectrometer (EDS) which demonstrated that micro-alloying elements (i.e. Mn) also contributed to the ECM processes. The ECM model of Mn was also established.

ELECTROCHEMICAL MIGRATION FAILURE OF PURE TIN UNDER BROMIDE-POLLUTED THIN ELECTROLYTE LAYER

The electrochemical migration (ECM) time to failure (TTF) tests for pure tin in the bromide-containing environment were carried out using the thin electrolyte layer (TEL) method. In-situ electrochemical measurement was made to monitor the ECM behavior while the changes of surface morphology and localized pH during the ECM process were real-time observed using the 3D optical digital microscope. Microstructure and composition of products obtained after the ECM tests were characterized using the scanning electron microscope, energy-dispersive spectrometer and X-ray diffractometer. Results indicated that the growth rate of dendrite increased with bromide ion concentration at low bromide concentration levels, and it decreased at medium concentration levels. Tree-like tin dendrites covered with white precipitates are formed after the ECM tests. As for the high bromide ion concentration condition, no dendrite can be found and a thick layer of precipitate is formed between the twin electrodes. This precipitate layer has a higher thickness than the electrolyte layer, which can act as a barrier layer retarding ionic migration and thus inhibiting the ECM failure. Relevant mechanism has been proposed to clarify the effect of bromide ions on the ECM of pure tin under the thin electrolyte layer.

Nanopath-Beacons for Directed Silver Dendrites’ Migration across Graphene Oxide Terrain

With their special hierarchical fractal and highly symmetric formation, silver dendrites have a large surface area and plentiful active sites at edges, which have allowed them to exhibit unique properties ranging from superhydrophobic surfaces to biosensors. Yet, many suggested synthesis processes either require a long reaction time or risk contamination from sacrificial elements. Limited research in directing while enhancing the growth of these silver dendrites also hinders the application of these unique microstructures as site-selective hydrophobicity of surfaces and location-dependent SERS (surface-enhanced Raman spectroscopy). A possible solution to this is to utilize WO3 nanocubes as beacons to accelerate and conduct the growth of these silver dendrites through the electrochemical migration process. These nanocubes effortlessly altered the applied electric field distributed between the electrodes, depending on their orientations and positions. As the silver dendrites branched from the nanocubes, the dendrites themselves further concentrated the electric field to encourage the growth of more loose fractal silver dendrites. The combinatory effect successfully directs the growth of silver dendrites along the concentrated electric field paths. Both changes to the electric field and directed growth of silver dendrites are underscored using Multiphysics COMSOL simulations and time-lapse microscopy. This work provided insight into the possibility of designing microstructures to direct and accelerate the growth of silver dendrites.

Numerical simulation of electrochemical migration of Cu based on the Nernst-Plank equation

Electrochemical Migration (ECM) is getting more attention in the microelectronics industry due to the continuing miniaturization, which increases the possibility of short circuit formation caused by the ECM-induced dendrites. This work presents a 2D numerical model of the ECM based on the Nernst-Plank equation. The model contains the deterministic description of the metal dissolution, the changes of electrolyte properties, and the ion transport in the electrolyte. However, the reduction of the ions and the dendrite growth is described stochastically. The capability of the model was tested in the case of pure copper electrodes with a gap distance 200 µm, 10 VDC bias, 20 °C temperature, and a contaminant-free electrolyte. The results of the model were validated by experimental water-drop tests. The results showed very good agreement between the experimental and the calculated mean time to failure values, dendrite morphologies, and the kinetics of the dendrite formation. The model showed that the developing dendrites consume most of the Cu2+ ions around them, which answers why only some dominant dendrites can develop in a given area. The model proved that not only the electric field but the diffusion of the ions is also dominant in given phases of the ECM process. Our model could be a useful tool for ECM failure prediction and for further ECM researches as the digital twin of the ECM process. Also the approach can be applied in various aspects of failure-prediction in modern reflow-soldering.

On the Validity of the Null Current Assumption for Modeling Sorptive Reactive Transport and Electro-Diffusion in Porous Media

In multispecies electrolyte solutions, each individual species can migrate according to its specific ionic properties. This process is called electrochemical migration or electro-diffusion and is well-described by the Nernst–Planck equation. The common approach for solving the corresponding mathematical system is based on the null current (NC) assumption, which expresses the electric potential in terms of charges and concentrations of chemical components. This assumption has a great advantage as it eliminates the electric potential from the Nernst–Planck equation. However, the NC assumption has limited capacities in describing electro-diffusion processes when the domain is subjected to an external electric field. The validity of the NC assumption could be questionable, even in the absence of an external electric field. This topic has never been investigated in the past. The main goal of this work is to evaluate the validity of the NC assumption and to understand its effect on the model outputs. Thus, we present a new reactive transport model that allows for a reliable representation of the electrochemical migration process. This model is based on the Nernst–Planck and Poisson (NPP) equations which are solved together. We also implement a model based on the NC assumption. Both models have been validated by comparison with CrunchFlow, based on several benchmarks. The results show that in the case of high sorptivity, the NC assumption is no longer valid. Therefore, in the case of sorption processes, the NPP should be used to simulate coulombic interactions.

In-situ study of electrochemical migration of tin in the presence of bromide ion

The miniaturization of electronic devices and the consequent decrease in the distance between conductive lines have increased the risk of short circuit failure due to electrochemical migration (ECM). The presence of ionic contaminants affects the ECM process. This work systematically investigates the ECM of tin (Sn) in the presence of bromide ions (Br−) in the range of 10−6 M to 1.0 M. Water drop test (WDT) was conducted in the two-probe semiconductor characterization system under an optical microscope as an in-situ observation. Polarization test was carried out to study the correlation between the corrosion properties of Sn and its ECM behaviour. The products of ECM were characterized by scanning electron microscope coupled with an energy dispersive X-rays spectrometer (SEM/EDX) and X-ray photoelectron spectrometer (XPS). The results confirm that the rate of anodic dissolution of Sn monotonously increases with the Br− concentration. However, the probability of ECM failure follows a normal distribution initially, but later increases with the Br− concentration. The main products of the ECM reactions are identified as Sn dendrites and tin hydroxide precipitates. The mechanisms of the ECM process of Sn in the presence of Br− are also suggested.

Electrochemical Migration on Lead-Free Soldering of PCBs

It is well known that in printed circuits boards assembled by SMT technology may occur Electrochemical Migration (ECM). This phenomenon appears mainly because the new packaging has the terminals very close. Also the Electrochemical Migration may become a potential reliability problem in electronic soldering when lead free technology is used in soldering electronic devices. Electrochemical Migration is an electrochemical process where metal on an insulating material, in a humid environment and under an applied electric field, leaves its initial location in ionic form and redeposit. In a PCB two adjacent terminals may behave as electrodes so the dendrites grow from cathode to anode. It can show different morphologies with the different migration elements involved depending on the solder paste composition or PCB surface finishing. A structure with a comb shape printed on FR4 substrate was used in the experiments. The distance between the fingers in the structure was 102 or 254 micron, in order to simulate a real distance between dispositive terminals. The factors considered during the experiments were surface finishing (ENIG or HASL), solder paste composition, distance between terminals (102 or 254 micron) and applied voltage (2 or 3 V). All the experiments were done two times. It was observed that the solder paste and the surface finishing don’t influences the ECM process. Tin was the main metal that migrates. All the results obtained in these study agrees with the literature.

A study on protection behavior of inorganic coating prepared by magnetron sputtering technology for electrochemical migration corrosion failure of SAC305 alloy

The electrochemical migration (ECM) problem caused by corrosion is one of the most serious threats to the reliability of electronic equipments in a harsh condition. In this paper, the Al2O3, TiO2 and ZrO2 inorganic coating with a thickness of 1 μm deposited by magnetron sputtering technique are used to protect SAC305 solder alloy against ECM corrosion failure. Then the surface topographies, surface roughness and thickness of the coating are observed through atomic force microscopy and scanning electron microscopy. The thin electrolyte film containing chloride experiments are used to simulate the marine atmospheric environment to inspect the protection performance of the coatings. The results show that these coatings can inhibit ECM process and extend the time to short circuit to some extent. Specifically, TiO2 has the most excellent protective properties, followed by ZrO2. Moreover, the protection performances of these coatings are also further compared through adsorption energy of chloride on the coatings surface calculation, and the result indicates that Cl− is more easily absorbed on the surface of Al2O3, which also implies that the Al2O3 has poorer protection property than TiO2 and ZrO2.

Electrochemical migration behavior of Sn–3.0Ag–0.5Cu solder alloy under SO2 polluted thin electrolyte layers

The effect of HSO3− on the electrochemical migration (ECM) of Sn–3.0Ag–0.5Cu lead-free solder alloy under thin electrolyte layers was investigated using the thin electrolyte layer method. The results showed that the migration element of Sn–3.0Ag–0.5Cu alloy was Sn and the faster-growing branch of the dendrite resulted in a sharper tip during the dendrite growth process. Increasing the amount of HSO3− decreased the probability of ECM. Due to the hydrolysis of HSO3, the pH value of electrolyte in thin electrolyte layer shifted toward a strongly acidic environment and a lower pH condition was favorable for dendrite formation. However, HSO3− reacted with tin ions to form some insoluble compounds during the ECM process, resulting in a decrease of tin ion concentration under thin electrolyte layers. A protective film formed on the anode surface, thus blocking the anodic dissolution process. Possible reactions were proposed to explain the inhibitory effect of HSO3− on the ECM of Sn–3.0Ag–0.5Cu alloy.

Electrochemical Migration of Copper Caused by Volcanic Ash Deposited on Printed Circuit Boards

When volcanic ash is suffiently fine, that it can pass through a filter and be deposited on a printed circuit board, electrochemical migration (ECM) is expected to be strongly affected by the deposited volcanic ash. In this study, to better understand the influence of volcanic ash on ECM, a water drop (WD) test is performed on two biased Cu electrodes using distilled water droplets containing five samples of volcanic ash from eruptions of different volcanos. From the results, it is seen that all of the volcanic ash droplets exhibited electrical conductivity. The ECM process was accelerated by three of the volcanic ash samples and was negligibly affected by the other two. In addition, both the acceleration and deceleration of the ECM were strongly affected by the content of volcanic ash from Mt. Shinmoe. The WD tests, performed using a commercial SiO2 powder, revealed that the growth of Cu dendrite is physically interrupted by the particles of volcanic ash. Furthermore, the WD tests, performed using the supernatant liquids of each volcanic ash and H2SO4 solutions, revealed electrical conductivity, and that the acceleration of ECM results from water-soluble components, primarily the SO42− ions dissolved from the volcanic ash. Consequently, the acceleration or deceleration of ECM is determined by the balance between the acceleration by water-soluble components and interruption by the particles.

Analyses and design for electrochemical migration suppression by alloying indium into silver

Silver electrochemical migration (ECM) is a serious reliability issue for fine pitch as well as power electronic devices that employ silver as interconnection materials. In this study, a method to suppress the ECM behavior of silver was proposed and implemented. Solid solutions of Ag–In and intermetallic compounds were fabricated and their ECM properties were studied through the water drop test (WDT). The WDT was performed on alumina substrate in deionized water at 3 V, where the leakage current was measured. The results show that Ag–In alloy samples have considerably longer lifetime before reaching short circuit than pure silver samples. As indium concentration in Ag–In solid solution increases, the resistance to ECM also increases. The migration process is completely inhibited when the indium concentration reaches 19 at.%. The morphologies of the dendrites and surfaces of the anode and cathode were also investigated. A model was established to explain the anti-ECM mechanism of Ag–In alloys. By forming indium oxide on the surface of the anode under electric field in a humid environment, the anode gets passivated and the silver dissolution path is completely blocked, and thus inhibiting the ECM process. It was discovered that the quality of electrode surfaces is essential in realizing the full potential of the inherent anti-ECM ability of Ag–In alloys. In conclusion, the results of this investigation have shown that Ag–In alloys can be promising and valuable candidates for conductors, metallization, and interconnect materials in fine pitch and high power electronic devices.

Effects of mould on electrochemical migration behaviour of immersion silver finished printed circuit board

The role played by mould in the electrochemical migration (ECM) behaviour of an immersion silver finished printed circuit board (PCB-ImAg) under a direct current (DC) bias was investigated. An interesting phenomenon is found whereby mould, especially Aspergillus niger, can preferentially grow well on PCB-ImAg under electrical bias and then bridge integrated circuits and form a migration path. The cooperation of the mould and DC bias aggravates the ECM process occurring on PCB-ImAg. When the bias voltage is below 15V, ECM almost does not occur for Ag coating. Mechanisms that explain the ECM processes of PCB-ImAg in the presence of mould and DC bias are proposed.

Role of mold in electrochemical migration of copper-clad laminate and electroless nickel/immersion gold printed circuit boards

The role of the mold in electrochemical migration (ECM) of printed circuit board (PCB) under DC bias was investigated using environment scanning electron microscope, 3D stereology microscopy, and scanning Kelvin probe. The mold under electrical bias can grow well and complete with the processes of growth, metabolism and proliferation. The mold can also promote metal ionization and localized corrosion and provide the ion source for ECM process. The cooperation of mold and DC bias promote ECM process on PCB and aggravate short circuit failure behavior. The proposed ECM mechanisms explain the ECM process of PCB under mold and DC bias environment.

Electrochemical migration of Ni and ENIG surface finish during Environmental test contaminated by NaCl

The electrochemical migration (ECM) mechanism occurs at the presence of moisture in the case of operating circuits and results in shorts (dendrites) between adjacent conductor lines/traces. Dendrite growth occurs as a result of metal ions being dissolved into a solution from the anode and deposited at the cathode, thereby growing in tree-like formations. In this study the water condensation process and ECM behavior of Nickel (Ni), Electroless Nickel Immersion Gold (ENIG) and pure copper were investigated using Thermal Humidity and Bias (THB) test in NaCl environment. The THB results show that Cu has higher ECM resistance than Ni and ENIG surface finishes, which was an unexpected result. The main influencing factors of the water condensation (e.g.: surface roughness, thermal parameters) and the ECM processes (precipitates and dendrites) were investigated and discussed in details. Furthermore, a novel ECM model for Ni and NiAu surface finish was established using THB test in case NaCl contaminated samples.

Electrochemical corrosion and electrochemical migration of 64Sn-35Bi-1Ag solder doping with xGe on printed circuit boards

Ge-doped effects on electrochemical corrosion (ECC) and electrochemical migration (ECM) behaviors of 64Sn-35Bi-1Ag (SBA) solder on printed circuit boards (PCBs) in 3wt% NaCl solutions were discussed. ECC was conducted by potentiodynamic polarization and electrochemical impedance (EIS) tests. Dendrites growth is a result of electrochemical migration (ECM) process, which is another factor affecting soldering reliability for electronic packagings, it was carried out as a designed schematic diagram coupled with a direct current power supply. Microstructures of products were investigated by SEM coupled with EDAX, XRD techniques. ECC results showed that anti-corrosive capacity increased as an increase of Ge addition in SBA solder which was expressed as the percent corrosion prevention (IE%), there was a maximal IE% value (83.74) when Ge addition was 0.5wt%, this obtainment was consistent to EIS results. The probable reason was that Ge had a “skin collection” effect, that was, it highly enriched and segregated at edge of SBA solder, which protected Sn in SBA solder from being corroded. Microstructure of corrosive products was distinctly different for SBA solder with and without Ge addition. ECM results showed that the dendritic grown rate of pure SBA solder was slower than those of its candidates with Ge dopings. Dendrites of SBA solder with 0.5wt% Ge addition grew to about 100–150nm in length and 20–50nm in width after ECM test for 8h, they were nanorods arranged trimly into arborization. While dendrites was shorter under the same condition for pure SBA solder without Ge doping. The dendritic growth mechanism was tentatively discussed. It showed the loaded electronic field intensity during ECM process played an important role for dendritic formation owing to its controlling the dendritical crystal anisotropic growth rate. Medium concentrations had an another key effect, which affected by controlling the ion diffusion from anode to cathode. From the point of microelectronic packagings, Ge addition was advantage to improve anti-corrosion capacity of SBA solder, but it also reduced soldering reliability if ECM process took place in surrounding of wet and thermal weather.

Surface failure mechanism of PCB-ENIG in typical outdoor atmospheric environments

Electroless nickel/immersion gold (ENIG)-plated printed circuit boards (PCBs) were exposed to typical atmospheric environments in China for prolonged periods to understand the mechanism of corrosion failure. Results showed that the corrosion products that covered PCB-ENIG played a protective role by filling, covering, and coating surface defects. Under Turpan atmospheric environment, the plating layer was prone to damage easily, resulting in loss of protective properties. The corrosion mechanism was mainly microporous corrosion, in which halo-shaped macropores with a raised nucleus in the center were formed. When electrical bias was applied, PCB-ENIG was prone to electrochemical migration (ECM) caused by galvanic effects in the form of localized corrosion. Both Ni and Cu elements were involved in the ECM process. The synergistic effect of contaminated medium and high humidity is the critical factor of the ECM-induced PCB-ENIG corrosion.

In Situ Study of the Electrochemical Migration of Tin under Bipolar Square Wave Voltage

The electrochemical migration (ECM) of tin under bipolar square wave voltage with various time periods was in situ studied using optical and electrochemical techniques. The results show that there is no dendrite growth but only precipitate formation for the time periods from 1 to 100 s during the same test time interval. The amount of precipitates increases with the increasing time period from 1 to 100 s. Moreover, the locations where the precipitates appear are time period dependent. As the time period increases to 200 s, both dendrite growth and precipitates formation can be observed during the ECM test and the short circuit occurs at the end of the third semi-cycle. From one semi-cycle to the next semi-cycle, the switch between the voltage and its reverse voltage derived from the bipolar square wave during the ECM of tin results in a periodic switch between cathodic polarization and anodic polarization applied on the same electrode, periodic changes in the migration direction of ions and a circulation between dendrite growth and its re-dissolution. Therefore, it is believed that the ECM processes including ion formation, ion migration, precipitate formation and dendrite growth under bipolar square wave voltage are strongly dependent upon the semi-cycle.

Initial Corrosion Behavior and Mechanism of PCB–HASL in Typical Outdoor Environments in China

A long-term (1, 3, and 6 months) outdoor exposure test was performed for lead-free hot air solder leveling printed circuit boards (PCB–HASL) in typical environments in China and the corrosion behavior and mechanism of outdoor PCB–HASL were investigated. In a dry environment PCB–HASL corroded slightly, because of the protective effect of surface oxide films. Corrosion spread from places where dust particles were deposited or mold spores were adsorbed. Under the combined effects of humidity and contamination, large amounts of granular corrosion products with a loose structure were generated, greatly reducing the protective effect of the Sn layer. Furthermore, protection of the edges of the plates was poor, and corrosion products from these regions migrated rapidly on the FR-4 board. When a 12 V electrical bias was applied, Sn and Cu migrated simultaneously. In the electrochemical migration process the effect of humidity was much more critical than that of contamination.

Microstructure and elemental composition of electrochemically formed dendrites on lead-free micro-alloyed low Ag solder alloys used in electronics

The Electrochemical Migration (ECM) behaviour of lead-free, micro-alloyed, low Ag solder alloys was investigated using Scanning Transmission Electron Microscopy (STEM), Energy Disperse X-ray Spectroscopy (EDS) and electron diffraction methods. Different solder alloys were investigated by Water Drop (WD) tests to stimulate ECM failure mechanism. After WD tests, differently structured dendrites were formed depending on the solder alloy types. The results showed that micro-alloying components (e.g. Sb) also played role during the ECM processes. The novelty of this study is the demonstration that Sb can take part in the ECM process; the ECM model of Sb is also discussed.