Effect of highly increased current density in the microstructure of lead-free solder joints

In this paper we focus on the effect of highly increased current density and resulting temperatures in the microstructure of lead-free solder joints of chip-scale components in order to investigate quality and reliability aspects of these widely used components in surface mount technology (SMT). We prepared a daisy chain of zero-ohm resistors, where the current loading was set to specific values (2 A, 2,5 A) for specific components (0402, 0603) and specific trace-to-pad connections. The substrate was RO4000 type epoxy-ceramic, to increase possible thermal load on the substrate. The assembly process was performed with batch-type SMT machinery and vapour phase soldering with classic SAC305 alloy. The loading was performed on ambient atmosphere, and in drying ovens on elevated temperatures for 2500 hours. We also prepared control samples without load on ambient atmosphere. We found with cross-sections, that dissolved intermetallic compound structures (Cu6Sn5) were found in specific regions of the meniscus of surface mounted components, in the possible route of the current from the pad to the functional resistive layer of the SM resistor. The cross-section analysis was also further extended with polarized optical image investigations, where different microstructures are presented in function of the sample types. The paper also presents joint quality aspects such as shear-strength analysis. It was found that due to the current and the increased load, the microstructure of the joint changed, but extensive proof of electromigration was not present.