Flex Cracking and Temperature-Humidity-Bias Effects on Reliability of Multilayer Ceramic Capacitors
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Multilayer ceramic capacitors (MLCCs) are known to be susceptible to cracking when subjected to excessive printed circuit board (PCB) flexure, which is called "flex cracking". The bending of the printed circuit board causes stresses to be transmitted through the solder fillets to the surface mount capacitors. These stresses are the highest at the bottom of the capacitor, where the termination bands end. In order to reduce the amount of stress that is transmitted to the brittle ceramic body of MLCCs through end terminations, a flexible termination system which incorporates a silver-loaded epoxy in end-terminations was developed by some MLCC manufacturers. With the transition to lead-free materials in the electronics industry there is a concern that MLCCs assembled on PCBs with lead-free solder have different susceptibility to flex cracking than those assembled with eutectic tin-lead solder. In this study, the flex cracking of MLCCs assembled with lead-free solder (Sn3.0Ag0.5Cu) was compared with those assembled with eutectic tin-lead (Sn37Pb) solder and differences in the results were explained in terms of solder mechanical properties and solder solidification temperature. Tin-silver-copper lead-free solders and eutectic tin-lead solder have different mechanical properties, which affect the stresses that are transmitted to the ceramic body of the capacitor through the solder fillet. The higher solidification temperature for lead-free solder leads to increased residual compressive stresses after the reflow cool-down process for MLCCs assembled with lead-free solder compared with those assembled with tin-lead solder. In this work, the effects of dielectric material, capacitor size, solder assembly process, solder material, and end-termination type on flex cracking of MLCCs were determined for MLCCs from different manufacturers. Since some flexible- and standard-termination MLCCs are made with precious metal electrodes (silver-palladium), there is a possibility of electrochemical silver migration under bias and humidity. In this study, the effects of temperature-humidity-bias on electrical parameters of flexible-termination MLCCs were characterized and compared with standard-termination MLCCs. In addition, the effect of temperature-humidity-bias on electrical parameters of MLCCs with base metal electrodes was compared to that for precious metal electrode capacitors.