Study Of SAC/Cu Micro Solder Joints Plastic And Creep Performance

The microelectronic packaging technology has been developing in the direction of the high density. This makes the solder joints in microelectronic products became more and more small. The board package of micro solder joints has reached 200?m to 760?m in diameter, and precast convex point on the chip reach to 20 microns in diameter. At this point, in terms of plasticity and creep properties of the solder joints, there is a big difference between large size solder joints and micro solder joints. In this paper, several typical SAC/Cu BGA solder joints(400?m in diameter) were used as the carrier. With the help of nanoindentation method, we studied in situ micro plasticity and creep properties of solder joints.The plastic deformation in micro bumps was studied by using the nanoindenter. The effect of loading rate on plastic deformation behavior in micro solder joints was also studied. With the increase of loading rate, the hardness increased in micro solder joints. On the base of traditional plastic stress-strain constitutive model, the plastic deformation stress-strain constitutive equation was established by analyzing the nanoindentation load-depth curve. The ratio of plastic strain generated in the process of indentation deformation and total strain was used as the plastic factor to characterize the plastic of micro solder joints.The relationship between plastic factor and temperature(25? to 85?) was established to study the micro indentation. The results revealed that there is a size effect in plastic factor when the indentation depth is small(less than 1000 nanometers). Based on the strain gradient theory of solid mechanics, the relationship between size factor and the geometry required dislocation was established.Based on the traditional viscoplastic creep constitutive mode and with the help of nanoindentation method, the creep constitutive equation of SACBN/Cu and SAC305/Cu was established, respectively. The creep stress indexes in micro solder joints under different temperature(25? to 85?) were also obtained. The creep properties of SACBN/Cu and SAC305/Cu were obtained under varying stress by using ladder load and unload method. With the sudden increase of applied stress, the creep rate increased rapidly first, then became slow gradually, and eventually stabilized. After ladder loaded, stress-hardening became more obvious in micro solder joints, also the creep strain rate decreased and stress index increased.The high temperature plastic deformation behaviors of interfacial intermetallic compounds of Cu6Sn5 and(Cu, Ni)6Sn5 were studied. The hardness and elastic modulus of intermetallic compounds(Cu, Ni)6Sn5 were both higher than Cu6Sn5 at room temperature. With the increase of temperature, the hardness change of Cu6Sn5 and(Cu, Ni)6Sn5 follows a linear regression equation and their elastic modulus decreased with the increase of temperature. The relationship equation between plastic factor and temperature of Cu6Sn5 and(Cu, Ni)6Sn5 was established, respectively. With the increase of temperature, the deformation ability of interface intermetallic compound Cu6Sn5 and(Cu, Ni)6Sn5 increased.The effect of Ni addition on the morphology of IMC and the shear performance were analyzed. With the addition of Ni element, the constituent, morphology and distribution of the IMC were changed. The shear performance of the solder joints was improved. There was a better match between the IMC inside the micro solder joints and the bulk solder when the bulk solder had a high strength. In the case that the solder joints had a good plasticity, the shear strength can be further improved.