Explore MLCC failure modes, reliability, and degradation models to enhance manufacturing excellence and improve electronic device performance.
Understanding MLCC Failure Modes
Multilayer Ceramic Capacitors (MLCCs) are vital components in modern electronic devices, but they are not impervious to failure. Understanding the failure modes of MLCCs is crucial for improving their reliability and performance. Common failure modes include cracking due to mechanical stress, dielectric breakdown, and migration of conductive materials.
Cracking often occurs due to improper handling during manufacturing or assembly, thermal cycling, and mechanical vibrations. Dielectric breakdown can be caused by excessive voltage(leakage) or the presence of impurities within the dielectric material. Conductive material migration, particularly in high-humidity environments, can lead to short circuits and device failure.
The Cause of Leakage in MLCCs
Dielectric Defects:
Manufacturing defects or microscopic cracks in the ceramic material can lead to reduced insulation resistance and higher leakage. These defects may arise from inconsistencies in the raw materials or errors during the sintering process, where the ceramic layers are fused together. Such imperfections can create weak points in the dielectric structure, making it more susceptible to electrical stress and ultimately leading to premature failure of the capacitor.
Contaminants:
Impurities during the manufacturing or assembly process can cause paths for leakage currents. These contaminants might include foreign particles, such as dust or void, that become embedded in the dielectric layers. Additionally, chemical residues from cleaning agents or flux used during assembly can also contribute to contamination. These impurities can disrupt the uniformity of the dielectric material, creating conductive paths that facilitate leakage currents and compromise the capacitor's performance.
High Humidity/Environment:
MLCCs are sensitive to moisture ingress, which can degrade the dielectric properties. In environments with high humidity, water molecules can penetrate the protective coatings and reach the dielectric layers. This moisture can alter the dielectric constant and increase the likelihood of dielectric breakdown. Over time, exposure to humid conditions can lead to swelling or delamination of the layers, further exacerbating the risk of failure.
Overvoltage/Stress:
Applying voltages higher than the rated limit can damage the capacitor, causing leakage or even failure. When an MLCC is subjected to overvoltage, the electric field across the dielectric layers intensifies, potentially exceeding the material's breakdown strength. This can result in the formation of conductive paths through the dielectric, leading to increased leakage currents or catastrophic failure. Repeated exposure to overvoltage conditions can also accelerate the aging process, reducing the overall lifespan of the capacitor.
Aging:
Prolonged use under stress (e.g., temperature, voltage cycling) can degrade the dielectric, increasing leakage. As MLCCs age, the repeated thermal and electrical stresses can cause gradual changes in the dielectric material, such as the growth of microcracks or the migration of ions. These changes can weaken the dielectric's insulating properties, leading to higher leakage currents and reduced reliability. Understanding the aging mechanisms is crucial for predicting the long-term performance of MLCCs and ensuring their reliability in critical applications.
Improper Soldering:
Excessive heat during soldering can crack the ceramic or degrade the material, leading to electrical leakage. The soldering process involves applying heat to attach the MLCC to a circuit board, and if not carefully controlled, this heat can cause thermal expansion and contraction of the ceramic layers. Such thermal stress can induce cracks or delamination, compromising the integrity of the dielectric. Additionally, prolonged exposure to high temperatures can alter the chemical composition of the dielectric material, further increasing the risk of leakage and failure. Proper soldering techniques and temperature control are essential to prevent these issues and maintain the reliability of MLCCs.
Pores and Discontinuities in MLCC Nickel Electrode Layers
Discontinuities in the internal electrodes during the manufacturing process often result from pores. The presence of these pores can be identified using SEM and EDS, and for more information on the impact of these discontinuities, please refer to the previous article.
https://www.deepblock.net/blog/the-role-of-ligament-structure-in-mlcc-electrodes
Automatic detection of pore regions in SEM images of MLCC cross-sections using deepblock.net. The black areas detected in green polygons indicate pores.
Causes of Pore and Discontinuity in Ni Electrode Layers
Material Impurities:
Contaminants in the raw materials used for electrode formation can form localized weak points. These impurities might include foreign particles, such as dust or metal fragments, that become embedded within the electrode material during the manufacturing process. Additionally, chemical residues from previous processing steps or reactions between different materials can introduce unwanted elements into the electrode. These impurities can disrupt the uniformity of the electrode's microstructure, creating areas with different electrical or mechanical properties. Such localized weak points can act as initiation sites for cracks or other forms of degradation, ultimately affecting the performance and reliability of the MLCC.
Sintering Issues:
Improper sintering conditions, such as incorrect temperature or insufficient time, can result in incomplete densification of the electrode layer, leaving pores. Sintering is a critical step in the manufacturing of MLCCs, where the ceramic and electrode materials are heated to a high temperature to promote bonding and densification. If the temperature is too low or the duration too short, the particles may not fully coalesce, leaving voids or gaps within the electrode. These pores can weaken the mechanical strength of the electrode and provide pathways for moisture or other contaminants to penetrate, increasing the risk of electrical failure. Conversely, excessive sintering temperatures can lead to grain growth or unwanted reactions, further complicating the electrode's microstructure.
Shrinkage Mismatch:
During co-sintering, mismatched shrinkage rates between the ceramic layers and the Ni electrode can create cracks or separations. As the materials are heated, they expand and contract at different rates due to their distinct thermal expansion coefficients. If these differences are not carefully managed, the resulting stresses can cause the layers to pull apart or crack, especially at the interfaces. This mismatch can be exacerbated by variations in the composition or thickness of the layers, as well as by the presence of impurities or defects. The resulting separations can compromise the electrical connectivity and mechanical stability of the MLCC, leading to premature failure.
Oxidation of Ni:
If oxygen infiltrates the sintering environment, nickel can oxidize, leading to weakened and porous layers. Nickel oxidation occurs when the metal reacts with oxygen to form nickel oxide, a process that can be accelerated by high temperatures and the presence of moisture. This oxidation can alter the electrical conductivity and mechanical properties of the electrode, making it more brittle and prone to cracking. Additionally, the formation of nickel oxide can create voids or gaps within the electrode, further reducing its effectiveness. To prevent oxidation, it is essential to maintain a controlled atmosphere during sintering, often by using inert gases or vacuum conditions.
Mechanical Stress:
Stress induced during layer stacking or pressing can create microscopic cracks or discontinuities. The process of assembling the multiple layers of an MLCC involves applying pressure to ensure proper adhesion and alignment. However, if the pressure is too high or unevenly distributed, it can cause the layers to deform or crack. These mechanical stresses can be compounded by thermal cycling or vibration during operation, leading to the propagation of existing cracks or the formation of new ones. Such discontinuities can disrupt the electrical pathways within the MLCC, increasing the risk of failure.
Thin Electrode Layers:
As designs push for miniaturization, thinner electrodes are more prone to imperfections due to handling and process limitations. The trend towards smaller and more compact electronic devices necessitates the use of thinner electrode layers, which can be more challenging to manufacture consistently. Thinner layers are more susceptible to damage from mechanical handling, such as bending or scratching, and may not provide the same level of mechanical support as thicker layers. Additionally, the reduced thickness can make it more difficult to achieve uniform deposition and sintering, increasing the likelihood of defects. These imperfections can compromise the electrical performance and reliability of the MLCC, particularly in demanding applications.
Consequences of Pores and Discontinuities
Increased ESR (Equivalent Series Resistance):
The presence of pores or breaks increases the resistance, reducing efficiency in high-frequency applications. ESR is a measure of the internal resistance of a capacitor, and it can be affected by the quality of the electrode material and the presence of defects. Pores or discontinuities within the electrode can increase the resistance by disrupting the flow of current, leading to energy losses and reduced efficiency. This is particularly problematic in high-frequency applications, where low ESR is critical for maintaining performance. High ESR can also lead to increased heat generation, further stressing the MLCC and potentially leading to failure.
Decreased Capacitance:
Discontinuities can reduce the effective surface area for charge storage. Capacitance is directly related to the surface area of the electrode layers and the distance between them. Any discontinuities or voids within the electrode can reduce the available surface area for charge storage, decreasing the overall capacitance of the MLCC. This reduction in capacitance can affect the performance of the electronic device, particularly in applications where precise capacitance values are required. Additionally, variations in capacitance can lead to instability or drift in the device's operation, further complicating its use.
Leakage Current and Short Circuits:
Pores can allow for migration of conductive material, leading to dielectric breakdown. The presence of pores or voids within the electrode can create pathways for conductive materials, such as metal ions, to migrate through the dielectric layers. This migration can lead to the formation of conductive filaments or bridges, which can cause dielectric breakdown and short circuits. Leakage currents can also increase as a result of these pathways, reducing the efficiency and reliability of the MLCC. Preventing the formation of pores and ensuring the integrity of the electrode layers is essential for minimizing these risks.
Thermal Instability:
Poor conductivity in the electrode can lead to localized heating and eventual failure. The presence of defects or impurities within the electrode can reduce its thermal conductivity, leading to localized hotspots during operation. These hotspots can cause thermal expansion and contraction, exacerbating existing defects or creating new ones. Over time, the repeated thermal cycling can lead to mechanical fatigue and failure of the MLCC. Ensuring uniform conductivity and minimizing defects is crucial for maintaining thermal stability and preventing failure.
Reduced Lifespan:
Stress concentrations around pores and cracks make the MLCC more susceptible to mechanical and electrical fatigue. The presence of defects within the electrode can create stress concentrations, which can act as initiation sites for mechanical or electrical fatigue. These stress concentrations can be exacerbated by thermal cycling, vibration, or other operational stresses, leading to the propagation of cracks or other forms of degradation. Over time, these defects can accumulate, reducing the overall lifespan of the MLCC and increasing the likelihood of failure. Understanding and mitigating these stress concentrations is essential for improving the reliability and longevity of MLCCs.
Automated Defect Analysis in MLCC Manufacturing Processes
Defect analysis processes in MLCC manufacturing widely utilize tools like Scanning Electron Microscopy (SEM) and Energy Dispersive Spectroscopy (EDS). By leveraging false coloring techniques, researchers can enhance cross-sectional and surface images, identify the material composition of electrode surfaces, and even determine the composition of pores. These methods provide invaluable insights into defect mechanisms and material inconsistencies.
However, as modern MLCCs now involve over 600 to 700 layers in their stack, with increasingly thinner dielectric and electrode layers, the probability of defects such as pores and discontinuities rises significantly. This makes maintaining both yield rates and product reliability an ever-growing challenge.
One critical issue is the time-intensive nature of analysis. Even with advanced tools like multi-beam SEMs, which can capture high-resolution images of the entire MLCC surface and cross-sections, material scientists and engineers often spend excessive time manually analyzing these images. The growing complexity of modern MLCCs exacerbates this issue, as identifying and characterizing defects in ultra-thin layers becomes more intricate and labor-intensive.
To address these challenges, implementing Deep Learning and Computational Geometry techniques offers a promising solution. These technologies can automate the detection of defects, pinpoint their exact locations, and classify their shapes and structures with minimal human intervention. Such automation would significantly improve the efficiency of defect analysis and enhance the overall reliability of MLCC manufacturing processes.
Deep Block provides an accessible entry point for applying these advanced techniques. Deep Block is currently free to try and could revolutionize your MLCC defect analysis workflow. For inquiries about our solutions or to learn more, visit deepblock.net/contact. Let us help streamline your research and manufacturing processes.
Using deepblock.net for segmenting BT dielectric particles(red) in an MLCC cross-section micrograph
