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Flex Cracking of Multilayer Ceramic Capacitors: Experiments on Fracture Propagation

Cracking of the brittle X7R BaTiO3 ceramic dielectric material is a severe problem in areas where large sized multilayer ceramic capacitors (MLCC) are needed to provide larger capacities or higher dielectric strength for high voltage applications. Therefore the understanding of the crack formation within multilayer ceramic capacitors (MLCC) is an important issue. The paper will describe four-point-bending experiment on MLCCs, which were soldered on a pcb. The experimental design considered existing tests for the qualification of MLCC components. Basing on these considerations a specimen was designed that is able to detect the crack event via an in situ capacitance measurement. For the fabrication of the specimens two types of capacitors were chosen: MLCC 1206 and MLCC 1812. Both were made from an X7R BaTiO3 ceramic dielectric material. The substrate consisted on a 1.6 mm thick FR 4 pcb stripe having the same width as the capacitors. The capacitors were soldered using SnPbAg2, SnAg0.3Cu0.7 and SnAg3.8Cu0.7 solder alloys. After testing all samples were metallographically prepared, to analyze the cracks within the ceramic body of the capacitor by light microscopy. The paper will present the results of these microscopic studies, with regard to the crack shape that was found in the microsections of the tested specimens. The dependence of crack shape on the employed capacitor geometry and on the used solder alloy will be discussed.