Evolutions Of Interfacial Structure And Morphology In Soldering And Their Effects On The Shear Behavior Of Joints

With the development of portable, reliable and lead-free electronic products, there are great challenges for the reliabilities of electronic packaging and solder interconnects. The mechanical failure under shear stress of solder bump has become one main reason for the open circuit of electronic devices. The evolution of interfacial intermetallic compound (IMC), mainly Cu6Sn5 in soldering with Cu substrate, is regarded as a critical factor. With the miniaturization of bump in size and the wild acceptation of Sn-based solders, the increasing proportion of interfacial IMC layer also aggravates the discontinuity of the intrinsic behaviors at the solder/IMC interface. However, the evolution of the interfacial IMC in service is complex, such as coarsening of grains, flattening of solder/IMC interface, increasing of defects. Moreover, the crystal structure of Cu6Sn5 would transform from one to another. During this procedure, the volume expansion might result in stress at the solder/IMC interface which is harmful to the integrity of interconnects. So far, it is not very clear that how interfacial IMC layer influences the shear deformation and fracture of solder joints. The effects of the transformation of the crystal structure of Cu6Sn5 have not been verified. Therefore, studies on the interfacial evolutions and their effects on the shear behavior of solder joints are very helpful for the development of high-density lead-free packaging technology.In this study, Sn-0.7Cu/Cu joint was selected. Minor addition of Ni was conducted to modify the interfacial structures. The evolutions of structure and morphology (including the thickness, grain size, interfacial roughness and defects) of interfacial Cn-Sn IMC with aging time and temperature were studied, respectively. Solder joint with a sandwich structure was used in shear test. The correlations between the interfacial evolutions and the shear behavior were investigated at different shear strain rates and experimental temperatures. The conclusions are as follows:(1) The solid-state transformation of interfacial Cu6Sn5 in Sn-0.7Cu/Cu was studied by XRD. Results show that after isothermal aging, the time-temperature-formation curve of the interfacialη'-Cu6Sn5 under air cooling condition presented as a "C" shape. A minimum formation time was observed at aging temperature 135-150℃. When the cooling rate of soldering decreased, the time needed for the formation ofη'phase was increased. The results also show that the addition of Ni inhibited the formation ofη'phase significantly. (2) Voids were observed at the triple grain boundary of aged interfacial Cu6Sn5 from the top morphology. The amount of the voids were the most in Sn-0.7Cu/Cu joint under air cooling condition, but fewer in those cooling with furnace and containing minor Ni. The main reason is that the growth of interfacial Cu6Sn5 during aging procedure was inhibited by the pores adsorbed on the boundaries of Cu6Sn5 grains. With the decreasing of cooling rate in soldering, there was enough time for pores to escape from the triple grain boundary. With minor addition of Ni, the ratio of length to radius and the growth rate of interfacial IMC were enhanced, and thus the pores became metastable.(3) The morphology evolutions of interfacial Cn-Sn IMC, including the thickness, grain size and interfacial roughness, were studied, respectively. With the elongation of aging time, elevation of temperature and decrease of cooling rate, the thickness and grain size were continuously increased. After minor addition of Ni, the growth of Cu3Sn was significantly inhibited. Moreover, the Cu6Sn5 grains in soldering and those aged at high temperatures were refined.(4) The shear strength and fracture mechanism of solder joints were analyzed. Results indicate that under as-soldered state, low strain rate or high testing temperature condition, ductile fracture was observed of which the crack propagated in the solder matrix near the interface due to the mismatch of the mechanical properties between solder and interfacial IMC. The shear strength of the solder joint was controlled by the solder matrix. After aging, the fracture toughness was decreased due to the coarsening of interfacial IMC. Brittle fracture along IMC was discovered, of which the shear strength was reduced compared with that of solder matrix under the same aging condition. The influencing mechanisms of the interface are listed as follows. The thickness and grain size of interfacial IMC only affected the type of fracture. The defects in interfacial Cu6Sn5 and the higher elastic modulus of interfacial IMC would promote the brittle fracture. The influence of the crystal structure transformation of interfacial Cu6Sn5 and the roughness of solder/IMC interface was small.(5) When the deformation temperature in shear test decreased from room temperature to-40℃, the difference of elastic modulus between the solder matrix and IMC was reduced and the mechanical mismatch was alleviated. The joint inclined to be controlled by the behavior of solder matrix. When the strain rate increased, the difference of the elastic modulus was increased and the mismatch became serious. Under this condition, the brittle behavior of interfacial IMC was more dominant.