Microstructure evolution during sintering of multilayer ceramic capacitors: nanotomography and discrete simulations

Multi-Layer Ceramic Capacitors (MLCCs) are key passive components in modern electronics. MLCCs consist of alternating metal electrode and ceramic dielectrics layers. In ultrathin MLCC chips, the micrometric layers are composed of submicrometric metal and ceramic powders and nano sized ceramic additives (to retard the sintering of electrode and minimize the sintering mismatch). A number of defects such as cracks, delamination of layers and electrode discontinuity and homogeneity, may arise in the processing of these ultrathin MLCCs. The cracks and delamination result in product rejection. Electrode discontinuities (uncovered areas) and thickness homogeneity generate a number of problems including capacitance loss, electrical short, leakage current and decreased reliability. It is generally recognized that these defects are linked to the sintering kinetics mismatch between electrode and dielectric materials, during the co-firing (co-sintering) process of MLCCs. However, when it comes to the origin of these defects and to their evolution during the sintering process, little knowledge is available. Conventional post-sintering and 2-dimensional (2D) imaging methods suffer limitations. In this context, in-situ synchrotron X-ray imaging and Discrete Element Method (DEM) have been carried out to explore the origin and the evolution of defects during the co-sintering process. X-ray imaging including 2D radiography and 3-dimensional (3D) nano computed tomography (X-ray nCT) enable non-destructive in-situ observation of the microstructure change in 2D and 3D. In parallel, DEM can simulate the sintering of MLCCs by taking into account the powders’ particulate nature (particle size, packing, etc.) Synchrotron (Advanced Photon Source, Argonne National Laboratory, IL, USA) X-ray based Transmission X-ray Microscope (TXM) with spatial resolution of 30 nm was used to characterize a representative cylindrical volume of Ø 20 µm × 20 µm extracted from a 0603 (1.6 mm×0.8 mm) case size Nickel (Ni)-electrode Barium Titanate (BaTiO3, or BT)-based MLCC before and after sintering under 2H2%+Ar atmosphere. 3D tomographic microstructure imaging shows that the final electrode discontinuity is linked to the initial heterogeneity in the electrode layers. In situ X-ray radiography of sintering (heating ramp of 10 oC, holding at 1200 oC for 1 hour, cooling ramp -15 oC) of a Palladium (Pd) electrode BNT (Barium-neodymium-titanate) based MLCC representative volume was also carried out. It confirmed that discontinuities in the electrode originate from the initial heterogeneities, which are linked to the very particulate nature of the powder material. The discontinuity occurs at the early stage of the sintering cycle. At this stage, the electrode starts to sinter while the dielectric material may be considered as a constraining substrate. Correlative studies using Focused Ion Beam - Scanning Electron Microscope (FIB - SEM) tomography were conducted on green and sintered MLCC samples at high resolution (5 × 5 × 5 nm3). FIB images confirmed that the resolution of the X-ray nCT is sufficient to deal with these heterogeneity evolutions. Still, FIB tomography allows the X-ray nCT to be re-interpreted more accurately. Also, it provides detailed particulate parameters for the DEM simulations. The DEM was used to simulate the microstructure of a multilayer system during sintering. These simulations operate at the particle length scale and thus recognize the particulate nature of the multilayers at the early stage of sintering. First, the sintering of Ni matrix with BT inclusions was simulated using the dp3D codes (developed at SIMaP/GPM2, Université de Grenoble, France). The retarding effect of BT inclusions on the sintering of Nickel matrix was predicted by varying the size, the amount and the homogeneity of inclusions. It is found that the densification rate of the matrix decreases with increasing volume fraction of inclusions and with decreasing size of inclusions. For a given volume fraction and size of inclusions, a better dispersion of the inclusions results in a stronger retardation of the densification kinetics of the nickel matrix. Co-sintering of BT/Ni/BT multilayers was simulated with DEM by taking into account the particulate nature collected from the high resolution FIB nanotomography (FIB-nT) data, such as particle size, size distribution, heterogeneities, pores, and geometry. The temperature profile was also reproduced in these simulations. It is found that the electrode discontinuities originate from the initial heterogeneities in the green compact and form at the early stage of sintering under constraint, in good correspondence to the experimental observations. Parametric studies suggest that electrode discontinuities can be minimized by homogenizing the packing density and thickness of the electrodes and using a fast heating rate. Based on both experimental and DEM simulation results, a general conclusion is reached: the final discontinuity originates from the initial heterogeneity in the electrode layers and occurs at the early stage of sintering when the dielectric layers constrain the electrode layers. A defect evolution mechanism is proposed: after the lamination of BT sheets, there exist inevitably heterogeneous regions in the electrodes. Below 950-1000 oC, the nickel powder densifies except in heterogeneous zones for which desintering has been observed. At this stage, the Ni layers are under tensile stress. Tensile stresses in the thinner sections induce matter flow towards the thicker sections until the thinner sections are disrupted and discontinuities form. Once nickel is fully dense, electrodes are subjected to compressive stress at high temperature (1100 oC) due to BT densification. The compressive stress causes contraction of the viscous nickel, resulting in swelling of electrodes and hence a further increase in electrode discontinuity. Meanwhile, the nano-sized BT additives are expelled due to their unwettability with Ni at high temperature. The aggregated BT additives sinter, possibly forming percolation between two adjacent BT layers and enhancing the mechanical adhesion between Ni and BT layers in the MLCCs