Avoiding failures in ceramic chip capacitors, also known as multilayer ceramic capacitors (MLCCs), is strongly driven by the ability of the designer, both electrical and mechanical, to follow guidelines based on an understanding on how surface mount ceramic capacitors fail.
As ceramic capacitors are the most common component in today’s modern electronics, designers should be made aware of appropriate design rules and potential modifications necessary with the introduction of Pb-free solder to ensure suficient reliability. Figure 1. Cross-section of a multilayer ceramic chip capacitor (MLCC)
Flex cracking in ceramic capacitors occurs when there is excessive flexure of the printed circuit board. An example of a flex crack is displayed in Figure 10. Once the flex crack initiates, it tends to propagate at a 45-degree angle from the edge of the termination to the dielectric/termination interface.
Experimental studies by Ansys and Kemet demonstrated that ceramic capacitors assembled with Pb-free solders consistently showed similar or improved robustness to flex cracking compared to capacitors assembled with SnPb solder (displayed in Figure 13). Further investigations identified two potential rationales for the deviation from prediction.
Failure mechanisms for ceramic capacitors are presented and the drivers for failure occurrence, mechanical, thermal, chemical, and electrical, are tabulated. The influence of Pb-free solder on each driver is then analyzed, with the subsequent output changes in current design guidelines with justification when appropriate.
Thermal shock cracks occur due to the inability of the ceramic capacitor to temporarily relieve stresses during transient conditions. The most common signature of thermal shock is a 45-degree microcrack emanating from the termination of the end cap (see Figure 5).