abstract: Pb-free solders are used as interconnects in various levels of micro-electronic packaging. Reliability of these interconnects is very critical for the performance of the package. One of the main factors affecting the reliability of solder joints is the presence of porosity which is introduced during processing of the joints. In this thesis, the effect of such porosity on the deformation behavior and eventual failure of the joints is studied using Finite Element (FE) modeling technique. A 3D model obtained by reconstruction of x-ray tomographic image data is used as input for FE analysis to simulate shear deformation and eventual failure of the joint using ductile damage model. The modeling was done in ABAQUS (v 6.10). The FE model predictions are validated with experimental results by comparing the deformation of the pores and the crack path as predicted by the model with the experimentally observed deformation and failure pattern. To understand the influence of size, shape, and distribution of pores on the mechanical behavior of the joint four different solder joints with varying degrees of porosity are modeled using the validated FE model. The validation technique mentioned above enables comparison of the simulated and actual deformation only. A more robust way of validating the FE model would be to compare the strain distribution in the joint as predicted by the model and as observed experimentally. In this study, to enable visualization of the experimental strain for the 3D microstructure obtained from tomography, a three dimensional digital image correlation (3D DIC) code has been implemented in MATLAB (MathWorks Inc). This developed 3D DIC code can be used as another tool to verify the numerical model predictions. The capability of the developed code in measuring local displacement and strain is demonstrated by considering a test case. / Dissertation/Thesis / M.S. Mechanical Engineering 2011

The constitutive mechanical behaviour with a focus on creep of Sn-Pb and various Sn-Ag-Cu based Pb-free solders in the 25-150°C temperature range has been studied using nanoindentation and various new meso-scale tests. All alloys have been studied as bulk wave soldering bars, as-received solder balls and solder joints. Ball Grid Array (BGA) solder joints in a typical electronic configuration were manufactured in-house using both Cu and Pd-Ag metallizations. Microstructural characterisation of all configurations used various types of optical and electron microscopy and showed that the solder pad metallization type played a major role in intermetallic compound (IMC) formation. There were comparatively fine and coarse-grained microstructures in both as-received solder balls and BGA solder joints depending on ball diameter. Nanoindentation creep measurements in the stress range 20-500MPa showed that grain boundary sliding occurred together with dislocation glide and dislocation climb in the low temperature (25-50°C) and high temperature (100-150°C) regimes respectively. Smaller grain sizes (<20µm) encouraged grain boundary sliding that enhanced creep. New elevated temperature mechanical tests were developed using the nanoindentation platform to enable testing of entire solder joints in shear and compression, with stresses in the 1E-2 - 3MPa range, more relevant to in-service conditions than those in nanoindentation. Meso-scale spherical indentation creep behaviour in compression on as-reflowed solder balls showed good agreement with that obtained by conventional nanoindentation. However, when BGAs were tested in shear, the solder microstructure had relatively little influence on the creep response, which was significantly less creep resistant than individual phases in the ball obtained by nanoindentation or the ball itself obtained by meso-scale spherical indentation. In shear, the creep conformed to diffusion controlled behaviour and interfacial microstructure was suggested to now control creep response, with the microstructure of the majority of the solder joint playing only a minor role.

abstract: Microelectronic industry is continuously moving in a trend requiring smaller and smaller devices and reduced form factors with time, resulting in new challenges. Reduction in device and interconnect solder bump sizes has led to increased current density in these small solders. Higher level of electromigration occurring due to increased current density is of great concern affecting the reliability of the entire microelectronics systems. This paper reviews electromigration in Pb- free solders, focusing specifically on Sn0.7wt.% Cu solder joints. Effect of texture, grain orientation, and grain-boundary misorientation angle on electromigration and intermetallic compound (IMC) formation is studied through EBSD analysis performed on actual C4 bumps. / Dissertation/Thesis / M.S. English 2013

The factors controlling the reliability of Pb-free solders when subject to thermomechanical regimes relevant to the harsh aerospace environment have been studied. Ball grid array (BGAs) typical of microelectronic devices have been manufactured in-house and subjected to isothermal ageing and thermal cycling. The BGAs comprised both Cu and Ni-Au metallizations, Pb-free Sn-Ag-Cu 400 and 600&mu;m solder balls, FR4 and Al₂O₃ boards, and included circuits to measure resistance changes due to damage in the joints during thermal cycling. Microstructural evolution within the solders balls and complex interfacial reactions were studied in all configurations using various types of electron microscopy. The mechanical properties of the different phases formed within solder joints were studied using nanoindentation at room and elevated temperatures up to 175°C for the first time. Intermetallic compounds (IMCs) were stiff, hard and brittle with very low creep rates, while the softer primary Sn, eutectic regions and Cu metallization readily underwent creep. Two-dimensional finite element analysis (FEA) of nanoindentation was used to understand better the physical meaning of nanoindentation creep data. Reliability experiments comprised both thermal cycling and FEA of BGAs. The difference in coefficient of thermal expansion (CTE) in the BGA materials caused interfacial fatigue damage in the solder joints, which was detected primarily at the solder/metallization interface of the outermost, most strained solder joint. Accumulated creep strain per cycle at this interface was evaluated using 3D FEA of the stress-strain state of the BGA and results calibrated against experimental BGA mean lifetimes using the Coffin-Mason relationship. Nanoindentation combined with FEA has been shown to be a viable route for the rapid assessment of creep performance and lifetime in lead-free solders under aerospace thermal cycles.