Local shear stress-strain response of Sn-3.5Ag/Cu solder joint with high fraction of intermetallic compounds: Experimental analysis

Current trend in miniaturization of microelectronic devices is the motivating factor for size reduction of joints and interconnects. The electronic industry is expecting to reduce the scale of interconnects in the next generations microelectronic devices. Mechanical behavior of these joints will significantly be different from traditional solder joints due to the effects such as geometrical and microstructural constraints, anisotropy caused by the reduction in a number of grains, and the presence of brittle intermetallics (IMCs). Mechanical experiments at this scale are very challenging. This study was focused on investigating the effect of different volume fraction of IMCs on the shear behavior of micro-scale solder joints with a 50 μm stand-off height. A single lap-shear specimen was designed to conduct the study. Two Copper (Cu) substrates were soldered together with Sn-3.5Ag solder foil and a thickness of 50 μm was achieved. The soldering temperature of 260 °C and soldering time of 10, 30 and 60 min were utilized to achieve approximately 40%, 60% and 80% IMCs of total joint thickness. The experiments were conducted using a micro-tensile tester which is integrated with an optical microscope for monitoring and observing deformation in the testing materials. To investigate the local shear strain behavior, an optical technique along with a developed image processing computer program was used. The single lap-shear tests were conducted with the shear strain rate of 0.015s^-1 and 0.15s^-1 to observe its effect on shear stress-strain behavior. The study shows that the ultimate shear strength and strain to failure are largely dependent on the volume fraction of IMCs. On the other hand, the far-field or nominal shear strain is totally different than the local shear strain behavior at solder joint region. Fractography analysis was conducted to investigate the possible failure mechanism. A ductile to brittle fracture mode transition along with dominant interfacial delamination is witnessed when the solder joints had a higher volume fraction of IMCs than bulk solder. Effect of different shear strain rates on the shear strength, failure strain and fracture mode were also investigated. Different shear strain rates tend to have an insignificant effect on the shear strength of the joint whereas the failure strain decreases considerably at higher shear strain rate.