Mechanical and thermomechanical stability issues of 96.5SN-3.5AG solder joints in microelectronic packages

Flip chip technology is the ultimate solution for high performance and high density chip level interconnection. This thesis describes the investigation of using eutectic 96.5Sn-3.5Ag solder for flip chip applications. The principal components of the research include mechanical characterization, bumping process development, and finite element simulation for solder joint reliability. A novel solder bumping process was developed for wafer level fabrication of 96.5Sn-3.5Ag solder bumps. As a baseline process, an electroplating method was applied to fabricate the micro-scale solder bumps with 125-{dollar}mu{dollar}m diameter, 250-{dollar}mu{dollar}m pitch and approximately 80-{dollar}mu{dollar}m height. Pre-deposition of solder bumps was carried out by electroplating over a fine-pattern photoresist mask. Rapid dissolution of Ag into Sn was accomplished during reflow and chip joining process. Nickel was selected as the diffusion barrier and wetting layer in the under-the-bump metallurgy (UBM). Microstructural and compositional analyses were performed using SEM and EDS.; Three different mechanical testing techniques including tensile creep, lap shear creep, and automated ball indentation tests were used to characterize the mechanical deformation behavior of 96.5Sn-3.5Ag solder and solder joints. Constant-load creep tests on bulk specimens revealed a dislocation climb mechanism with a relatively large stress exponent of n = 10 for creep strain rates ranging from 10{dollar}sp{lcub}-9{rcub}{dollar} to 10{dollar}sp{lcub}-3{rcub}{dollar} and at temperatures ranging from 298K to 453K. The apparent activation energy for creep was found to be 0.57 ev. Lap shear creep tests on 96.5Sn-7.5Ag solder bumps also revealed a dislocation climb mechanism with a stress exponent of n = 10 for creep strain rates ranging from 10{dollar}sp{lcub}-7{rcub}{dollar} to 10{dollar}sp{lcub}-4{rcub}{dollar} at room temperature. In general, the solder joints are more creep resistant than the bulk specimen due to the inclusion of solder/base metal intermetallics. The intermetallic compounds may form precipitates or dispersoids in the solder matrix and thus hamper the creep deformation rate. The aspect ratio may also influence the creep deformation rate.; A constitutive model is established which describes the mechanical deformation behavior of 96.5Sn-3.5Ag as a function of stress, time, and temperature. The constitutive equation was implemented in a two dimensional finite element model to study the solder joint reliability under a typical thermal cycling condition. The strain range values can be used in the Coffin-Manson equation for estimating the accelerating factors and thus to predict the life of a flip chip package. The methodology used in this investigation can be applied in the areas of solder mechanics, solder processing, and electronic packaging technologies.