Flex cracking, of multilayer ceramic capacitors assembled with Pb-free and tin-lead solders

In this paper, an experimental study was conducted to study susceptibility to flex cracking of multilayer ceramic capacitors (MLCCs), in which a comparison was made between identical samples which were assembled using either Pb-free (Sn3.0Ag0.5Cu) or eutectic tin-lead (Sn37Pb) solder. Flex testing was performed on MLCCs with different sizes (1812 and 0805) and on different dielectric materials (COG and X7R) obtained from three different manufacturers. Experimental results showed that MLCCs mounted on printed circuit boards (PCBs) with Pb-free solder crack less with board flexing than those mounted on boards with eutectic tin-lead solder. For 1812-size MLCCs assembled with tin-lead solder, the PCB strain at 10% failure ranged between 1700 and 2000 microstrains. The PCB strain at 10% failure for 1812-size MLCCs assembled with Pb-free solder varied between 2360 and 9600 microstrains, depending on the MLCC manufacturer. COG MLCCs are more resistant to flex cracking than X7R MLCCs. Out of 96 samples tested, none of the COG MLCCs showed evidence of flex cracking up to the maximum strain level on board of about 137 000 microstrains. In contrast, in the case of X7R MLCCs, from the same manufacturer, assembled with tin-lead solder, 94 out of 96 capacitors failed.