Four-Point-Bending Experiments on Multilayer Ceramic Capacitors: Microstructural Details on Crack Initiation and Propagation

Surface-mounted multilayer ceramic capacitors (MLCC) are one of the most widely used type components in modern electronic assemblies. The components reliability is often limited by failures related to flex cracking. This particularly failure mode is predominantly created by forces transmitted from the PCB trough the component's terminals. Such forces can be created by deformations of the board itself (bending, vibrations) or by differences in thermal expansion of the components ceramic body compared to the organic board material, when the assembly undergoes thermal cycles. In order to evaluate the problem of flex cracking, the paper will present images of cross sections and the cracked capacitors that were metallographically prepared for microscopic examination. The multilayer ceramic capacitors were cracked by using a specially set up four-point-bending experiment. The specimens consisted of MLCCs that were soldered to pcb stripes having the same width as the capacitors. The study focuses on MLCC 1825 capacitors having a brick shift design with X7R dielectrics. The capacitors were soldered to the PCB by using either an undereutectic SnAg3.0Cu0.7 solder or an eutetic SnAg3.8Cu0.7 or SnPbAg2 alloy. The metallographic preparation was done in a way, that the soft parts of the section (solder, copper metallization etc.) and the hard parts (ceramic body) could be well observed for the same sample. The cracks in the samples were examined using light microscopy with brightfield contrast. The paper will present the results of these microscopic studies, with regard to the crack initiation points and the crack propagation angles.