Improving the thermomechanical behavior of lead free solder joints by controlling the microstructure

Effects of solder alloy, volume and pad finishes on various aspects of microstructure and the corresponding thermomechanical properties of SnAgCu solder joints were investigated. Particular attention was focused on the behavior of solder joints with interlaced Sn grain morphologies. Crossed polarizer microscopy and scanning electron microscopy (SEM) were used to characterize Sn grain structures. Precipitate sizes and distributions were measured using backscattered scanning electron microscopy and quantified using image analysis software. Mechanical properties including hardness and indentation creep were measured. Results show that the amount and frequency of interlacing increased as the joint size decreased, as the amount of Ag in the solder increased, and if the joint was reflowed on ENIG substrates. The interlaced structure was harder and more creep resistant compared to the common beach ball morphology. Image analysis results showed this to be related to much higher densities of secondary precipitates in the interlaced regions. A mechanistic understanding of the microstructure is discussed and recommendations are made as to the design of more reliable solder joints.