A B S T R A C TThe durability and reliability of lead-free solder joints depends on a large number of factors, like geometry, processing parameters, microstructure and thermomechanical loads. In this work, the nature and influence of the plastic constraints in the solder due to joining partners have been studied by parametric finite element simulation of solder joints with different dimensions. The apparent hardening due to plastic constraints has been shown to strongly depend on the solder gap to thickness ratio with an inversely proportional evolution. Due to interaction of several parameters, the macroscopic stress-strain constitutive law of lead-free solder materials should be determined in the most realistic conditions. In order to identify the elasto-plastic constitutive law of Sn-Ag-Cu solders, a sub-micron resolution Digital Image Correlation technique has been developed to measure the evolution of strain in solder joints during a tensile test. Experimental results of the stress-strain response of Sn-Ag-Cu solder joints have been determined for several solder gaps. The measured loaddisplacement curves have been used in an inverse numerical identification procedure to determine the constitutive elasto-plastic behaviour of the solder material. The effects of geometrical constraints in a real solder joint with heterogeneous stress and strain fields are then studied by comparing the apparent (constrained) and constitutive (non-constrained) stress-strain relationships.Once the size dependant constraining effects have been removed from the stress-strain relationship, the scale effects can be studied separately by comparing the constitutive elasto-plastic parameters of joints with a variable thickness. Experimental stress-strain curves (constrained and unconstrained) for Sn-4.0Ag-0.5Cu solder in joints of 0.25-2.4 mm gap are presented and the constraining and the size effects are discussed.

The effects of particle reinforcement of Sn-4.0wt.%Ag-0.5wt.%Cu (SAC405) lead-free solder on interfacial intermetallic layer growth and strength of the ensuing joints through short-term isothermal aging (150°C) were studied. Composite solders were prepared by either incorporating 2 wt.% Cu (3 lm to 20 lm) or Cu 2 O ($150 nm) particles into SAC405 paste. Aggressive flux had the effect of reducing the Cu 2 O nanoparticles into metallic Cu which subsequently reacted with the solder alloy to form the Cu 6 Sn 5 intermetallic. While all solders had similar interfacial intermetallic growth upon reflow, both of the composite soldersÕ growth rates slowed through aging to reach a common growth rate exponent of approximately 0.38, considerably lower than that of the nonreinforced solder (n = 0.58). The nanoscale reinforced solder additionally exhibited the highest tensile strength in both the initial and aged conditions, behavior also attributed to its quick conversion to a stable microstructure.

The current study proposes a combined experimental and modeling approach to characterize the mechanical response of composite lead-free solders. The influence of the reinforcement volume fraction on the shear response of the solder material in the joint is assessed. A novel optimized geometry for single lap shear specimens is proposed. This design minimizes the effect of plastic strain localization, leading to a significant improvement of the quality of experimental data. The constitutive model of the solder material is numerically identified from the load-displacement response of the joint by using inverse finite element identification. Experimental results for a composite solder with 0.13 reinforcement volume fraction indicate that the presence of the reinforcement leads to a 23% increase of the ultimate stress and a 50% decrease of the ultimate strain. To interpret experimental data and predict the elastoplastic response of the composite solder for varying particle volume fraction, a three-dimensional (3D) homogenization model is employed. The agreement between experiments and homogenization results leads to the conclusion that the increase in the ultimate strength and the decrease in ductility are to be attributed to load sharing between matrix material and particles with the development of a significant triaxial stress state which restricts plastic flow in the matrix.