Size Effect On Tensile Properties Of Cu/Sn-9Zn(SAC305)/Cu Solder Interconnects Under Aging And Current Stressing

With the rapid development along the trend of miniaturization, high performance and lead-free in microelectronic packaging manufacturing, the solder joint size decreases continusly. Size effect on the microstructural evolution becomes more important, which is a key factor to the reliability of solder joints. At the same time, the current desity through solder joints increases rapaidly and electromigration (EM) damage becomes more severely, which would deteriorate the reliability of solder joints. Tensile test is one of the approaches to assess the degradation of solder joints, especially that tensile test can be used to reveal the weakest layer or interface since the stress distribution in all interface layers is the same. Therefore, in this article, tensile test is used to evaluate the effect of aging (without current) and EM (5× 103 A/cm2) on the tensile properities of Cu/Sn-9Zn/Cu and Cu/Sn3.OAgO.5Cu (SAC305)/Cu solder interconnects with different sizes at 150 ℃.The main conclusions are drawn in the present work:(1) Cu5Zn8 intermetallic compounds (IMCs) formed at the interfaces of Cu/Sn-9Zn/Cu solder interconnects after aging and EM. With increasing aging time, the finer Zn particles in the solder disappeared gradually and the thickness of the interfacial IMCs increased. The fracture site shifted from the interface to the solder and the tensile strength decreased, which was mainly determined by the strength of solder. With increasing EM time, the reverse polarity effect that the CusZn8 interfacial IMC at the cathode was thicker than that at the anode was found and it was more obvious in the larger solder interconnects. The fracture site shifted from the solder to the cathode interface and the tensile strength decreased, which was mainly determined by the bonding strength of the interface.(2) Solder size had a significant effect on the tensile strength and fracture mode in the Cu/Sn-9Zn/Cu solder interconnects, due to the different amount and different decreasing rate of Zn content in the solder interconnects with different dimensions. Compared with the 300 μm solder interconnects, the decreasing rate of residual Zn content in the 200 μm solder interconnects was higher during aging and EM. The lower residual Zn content led to the lower strength of solder martix and the lower anti-electormigration properities in the 200 μm solder interconnects. Therefore, the tensile strength of the 200 μm solder interconnects was lower and it was easier to fracture in the bulk solder during aging and at the cathode interface during EM.(3) In the Cu/SAC305/Cu solder interconnects, the thickness of interfacial IMCs increased during aging. All the interconnects fractured inside the solder and the tensile strength decreased after aging. During EM, the orientation of Sn grains had a significant effect on the microstructural evolution and tensile properties of the solder interconnects, while size effect was relatively weakened. When the c-axis of the cathode Sn grain paralleled with the electron flow direction, the cathode Cu substrates consumed severely and large holes formed at the cathode interface. The tensile strength decreased significantly and the interconnects fractured at the cathode interface; When the c-axis of the cathode Sn grain was vertical to the electron flow direction, the consumption of Cu substrates at cathode was inhibited, and the tensile strength decreased slowly. And with the EM time increasing, voids formed at the cathode as the vacancies accumulated due to Sn atoms were driven to the anode and the fracture site shifted from the solder to the cathode interface.